Earth’s Energy Budget Impact on Grassland Diseases

Abstract

The change in climate have caused different biotic and abiotic factors to be more prominent when management plan is executed. The increase in temperature have then cause frequent drought that may attract alien species of vectors to spread novel diseases among the native plants. However, the change in climate varies in different countries. Thus, common diseases that threatens food security such as Xanthomonas spp., Pseudomonas spp are in limelight of research. Vectors lifecycle may cause plant diseases to by cyclative. Therefore, to find the break in the vector’s lifecycle will be a method to eradicate harmful population in grassland. Modern days will then call for innovative method and limitations should be considered. Climate change have also impacted pathogens migration and mating pattern. The need for innovative management is constantly on the rise.

1. Introduction

The change in climate have impacted the grassland in many ways. Grassland have the capability to buffer climate variability. They provide many other services to the ecosystem as well. The change in earth’s energy budget calls for innovative methods to manage the loss of grassland. Understanding the importance of the presence of grassland, the need to manage loss and be economically efficient is crucial as well.

Atmospheric warming and climate change have the potential for significant effects on agriculture systems and their productivity. Crops and forage systems have display significant vulnerability as the change in temperature and precipitation will then impact cultivation, sowing, growth and utilisation [1]. Farmers will then have to innovate and have other management methods to counter the effects of climate change.

Climate variability have caused frequent droughts. This have impacted the grassland by increasing plant mortality and limiting the geographic distribution of plant species, accelerating grassland degradation [2]. In addition to the observable change, there are other biotic and abiotic factors that will be affected as well. The microorganisms that live in the soil biota changes. The biodiversity may decrease and alien species may increase. With the change in biodiversity in grassland, novel diseases in plant may arise. Grassland diseases are a major part of grassland management. To understand the underlying physiology of pathogens and mode of transmission will be crucial, as intercepting at the point of weakness of pathogen’s lifecycle can reduce damages to vegetation and other costs for management [3].

Different type of grasslands across the globe will have different management requirements as the difference in pathogens differ as the environmental factors differs. The imbalance of Earth’s energy budget will further complicate the understanding and requirements for grassland management. Therefore, this chapter aims to cover and understand how did climate change impact the components that cause plant pathogens to continue to cause damage to grassland. In addition, the chapter covers the common types of grassland diseases that have been a recurring problem in various grasslands and its causative agents.

2. Earth’s energy budget

Energy cannot be created nor destroyed. Earth will require solar energy in order for the basis of life to continue. This can be evidently observed by plants requiring sunlight for photosynthesis to occur and to produce oxygen for living organisms. Earth would freeze without sunlight. The ideal balance of Earth’ energy budget can be explained with the guidance of the diagram below.

In summary, the incoming solar energy is being used, reflected and radiated back to space. To achieve the ideal earth’s energy budget, the incoming solar energy will be equal to the outgoing solar energy (which includes energy that have been reflected back into space).

The earth’s energy is constantly changing as the energy flows through the system. The changes in earth’ energy balance have been contributed by the components human activities. This causes changes to the composition of the atmospheric layers. As such, this could lead to the increased absorption of radiation or decreased absorption of radiation by reflecting those energy back into space as there is high albedo in the atmospheric layer. Albedo is an elaborate word that has a simple physical concept. Lighter surfaces on earth reflects more heat than dark surfaces. Earth’s energy budget in the past was balanced by the long wavelength that is being absorbed and the short wavelength that is being reflected back into the solar system. The reflection of short waves energy could be emitted by earth’s surfaces, clouds, atmosphere, conduction and/or convections and, evapotranspiration. With the imbalance of absorption and reflection it could cause a positive energy imbalance, Earth system is said to be gaining energy causing global warming. With the continuous gain in energy, the albedo in earth would decrease as the ice caps and snow starts melting.

Global warming increases not only the global temperature. The concentration levels of greenhouse gases and of those gases increase, carbon dioxide is of interest to a lot of scientist. The increase in carbon dioxide have been contributed by human activities such as deforestation and burning of fossil fuels (just some to name). The five carbon pools that will cycle the concentration of carbon in the Earth were lithosphere, oceans, soil organic matter, atmosphere and biosphere. Oceans are the biggest carbon pool in the Earth. However, deforestation has contributed to the global temperature rise as deforestation will cause a decreased in absorption of carbon dioxide. With the increase in carbon dioxide in the atmosphere have caused sun’s radiation is being reflected back to earth rather than back into space. Hence, as the Earth loss the ability to release energy, the global temperature increase. Apart from carbon dioxide, the increase in concentration for other atmospheric gases will allow different wavelength of light to pass through.

Thus, greater the amount of atmospheric gases that absorb thermal infrared radiation from the Earth’s surface, the greater the proportion of radiation emitted from the atmosphere towards the Earth’s surface [4]. This would then result in the Earth’s surface being less negative. More energy is then available for sensible and latent heat flux at the surface. Thus, the increase in air temperature.

The change in earth’s energy budget does not impact solely on the plants in Grassland. It would also impact those that are living in the grassland. The impact of global warming stresses the ecosystems through several changes that could already be experienced: rise in global temperature, water shortages, drought and intense storm damage. In addition to those that have been experiences, salt invasion is a rising problem. The influx of salt into the soil and water can change the ionic concentration of an area. The sudden change in soil environment will give little time for underground organisms to adapt.

3. Methodology

The measurement of the Earth’s energy budget can be conducted through remote sensing. A review by Liang et al. [5] has mentioned that there are several components to be calculated. The first formula was to get the surface energy balance and this is the sum of soil heat flux (G), sensible heat flux and latent heat flux. The latent heat flux is derived from the product of latent heat evaporation of water and the rate of evaporation of water.

$${\mathrm{R}}_{\mathrm{n}}=\mathrm{G}+\mathrm{H}+\lambda ET$$E1

R_n is the representation of all-wave net radiation.

However, remote sensing has presented another perspective, where the net radiation is the sum of shortwave net radiation and long wave net radiation (which is represented in Figure 1).

Figure 1.

The diagram depicts an overall movement of solar energy where the energy dissipates into space, being retained on the surface of earth and those reflected by the atmosphere. Courtesy of the NASA global precipitation measurement education.

$${\mathrm{R}}_{\mathrm{n}}={\mathrm{R}}_{\mathrm{n}}^{\mathrm{s}}+{\mathrm{R}}_{\mathrm{n}}^{\mathrm{l}}+\left(1-{\alpha }_{sw}\right){\mathrm{F}}_{\mathrm{d}}^{\mathrm{s}}+{\mathrm{F}}_{\mathrm{d}}^{\mathrm{l}}-{\mathrm{F}}_{\mathrm{u}}^{\mathrm{l}}=\left(1-{\alpha }_{sw}\right){\mathrm{F}}_{\mathrm{d}}^{\mathrm{s}}+\mathcal{E}{\mathrm{F}}_{\mathrm{d}}^{\mathrm{l}}-\sigma \epsilon T_{\mathrm{s}}^4$$E2

The equation above will then include all the other factor that will affect the energy balance. The net radiation is simply the sum of shortwave net radiation ($R_n^s)$ and long wave net radiation ($R_n^l)$. However, the incoming waves will then be affected by the albedo on earth.

Hence, the second part of the equation WHERE the product of the difference in surface shortwave broadband albedo $\left(1-{\alpha }_{sw}\right)$ and the shortwave downward flux incident on the surface ($F_d^s)$, in addition to the difference between the longwave downward ($F_d^l)$ and upwelling radiation ($F_u^l)$ will give the all-wave net radiation.

The third part of the equation will then include the Stefan-Boltzmann’s constant ($\sigma )$. The product of $\left(1-{\alpha }_{sw}\right)F_d^s$ is then added to the product of surface longwave broadband emissivity ($\mathcal{E})$. The product of and the skin surface temperature $\left(T_s^4\right)$ will be deducted and this gives the all-wave net radiation.

The remote sensors on the satellite have been used to measure the Total Solar Irradiance. The sensors from previous studies have allowed scientists to estimate solar constant. Remote sensing on the satellite has the ability to sense the net radiation at the top of the atmosphere. The data recorded includes both spatial and temporal scales. Remote sensing have been used to record the amount of energy that is received at the top of the atmosphere. The conserved energy can then be calculated and be accounted. Different surface of the Earth will then have different rate of energy exchange. Therefore, the change in energy balance will affect the climate.

The loss of grassland have been measured by the proportion where it covers the globe. Grasslands that have been lost regionally will then be measured by various units such as kilometres square (km²) and hectares (ha) on a larger scale. To understand further on how the grassland is affected by climate change and other factors, it can be measured with the annual changes in carbon stocks in grassland. Therefore,

$$\Delta {\mathrm{C}}_{GG}=\Delta {{\mathrm{C}}_{GG}}_{LB}+\Delta {{\mathrm{C}}_{GG}}_{\mathrm{soils}}$$E3

The annual change in carbon stocks is measures in tonnes of carbon per year and is derived from the sum of annual change in carbon stocks in living biomass ($\Delta {C_{GG}}_{LB})$ and annual change in carbon stocks in soils ($\Delta {C_{GG}}_{\mathit{soils}})$ in grassland. However, with this use of formula, to calculate the change of carbon stocks in different region will then take into account of the specific grassland type (i), the climatic zone (c) and the management regime (m). Since the $\Delta {C_{GG}}_{LB}$ can be affected by different factors, regional grassland carbon stock can then be calculated with:

$$\Delta {{\mathrm{C}}_{GG}}_{LB\left(\mathrm{c},\mathrm{i},\mathrm{m}\right)}=\left(\Delta {\mathrm{B}}_{\mathrm{perennial}}+\Delta {\mathrm{B}}_{\mathrm{grasses}}\right)\times CF$$E4

CF is at the default of 0.5. where the change is the product of carbon fraction of dry matter (CF) to the sum of change in above- and belowground perennial woody biomass $\left(\Delta B_{\mathit{perennial}}\right)$ and below ground biomass of grasses ($\Delta B_{\mathit{grasses}})$.

Therefore, to accurately place the equation with the inclusion of the type of grassland, the climatic zone the grassland is in and the management regime that the grassland have been placed under:

$${\mathrm{C}}_{GG}=\left[\left(\Delta {\mathrm{B}}_{\mathrm{perennial}}+\Delta {\mathrm{B}}_{\mathrm{grasses}}\right)\times CF\right]+\Delta {{\mathrm{C}}_{GG}}_{\mathrm{soils}}$$E5

Inventory system could also be set up to record clear data of the plants present, the climatic patterns and the management regime where animals that are grazing or being managed by humans efforts to conserve grassland.

4. Grasslands

Grassland is an area where various grasses dominate. Vegetation in grassland will grow no taller than the height of a shrub nor a tree. Little do people know that grasslands are one of the major ecosystems that covers close to one-third of Earth’s terrestrial surface [6, 7]. In the last century, there is a decline in grasslands area worldwide to convert for arable land for production of animal feed crops and conversely, lack of management and abandonment [8]. Grasslands have been categorised as natural, semi-natural and improved grasslands [6, 9]. Natural grasslands are those that have been formed through processes that are related to the climate, fire and wildlife grazing. Semi-natural grasslands are those with human interventions. Scheduled grazing and hay-cutting are required for maintenance. Lastly, improved grasslands are pastures from ploughing and sowing agricultural varieties or non-native grasses with production value (Figure 2).

Figure 2.

Differences in richness and ecological processes were larger between the two perennial grasslands and maize than between prairie and switchgrass. Standardised effect sizes (Hedge’s D) are shown for differences in richness and key ecological processes between grasslands and maize (A and C) (effect is difference between average of the two grasslands and maize) and prairie compared with switchgrass (B and D). Error bars show 98% confidence intervals. Asterisks indicate statistical significance at α = 0.02. Courtesy of Werling et al. [10].

Grasslands across the globe are managed for a variety of purposes. They are valued for basic goods such as timber and water. They also provide forage, fishes and wildlife, and recreation resources. Grassland is a functional landscape that provides feed for grazing livestock. The landscape provided by the grassland has often been perceived as free and limitless. The table below illustrated that the increase in plant richness will increased pollinators and diversity in the grasslands. With the increase in biodiversity, there was also an increase in pest-egg removal by the arthropod predator. Aphid pressure that is present in a grasslands have also decreased. All these meant that a healthy and well diversified grassland is able to strive and continue to prove various services indefinitely. This has highlighted the importance to conserve and improve grasslands health (Table 1).

Vector taxa	Vector group	Virus groups				Total	%
		Icosahedral particles RNA genome	Rod-shaped particles RNA genome	DNA genome	Enveloped particles RNA genome
Hemiptera	Aphids	26	153a	13	5	197	28
	Whiteflies	—	13	115b	—	128	18
	Leafhoppers	8	—	15	3	26	4
	Planthoppers	10	4c	—	4	18	3
	Other hemiptera	—	8	5	—	13	2
Thysanoptera	Thrips	2	—	—	14	16	2
Coleoptera	Beetles	50	1	—	—	51	7
Acari	Mites	10	9	—	—	10	1
Nematoda	Nematodes	45	3	—	—	48	7
Mycota	Fungi	8	16	—	—	24	3
	No identified vectors	84	60	19	3d	166	24
	Total	253	268	167	30	697
	%	33	39	24

Table 1.

In Hogenbout et al. () study, they have discovered the types of genome that affects the diseases of grassland. The table then further explains that types of virus each insect group have been found as vector. However, this information may be limited to the area of experiment conducted and not in other continents [11].

^a

Includes 110 virus species of the genus Potyvirus, family Potyviridae.

^b

Virus species of the genus Begomovirus, family Geminiviridae.

^c

These are all tenuiviruses that have multiple shapes.

^d

These viruses probably have insect vectors.

Courtesy of Hogenbout et al.

The climate of grassland will be ideal for the growth of grasses only as low precipitation rate is not sufficient to sustain woody plants. Other maintaining factors of grasslands are fires and grazing animals. Grasses are well adapted to grow back after a fire as they have a complex root system and a resilient physiology. Grasses need not grow by seeds. Different part of the world will have different grassland climates. Therefore, they are differentiated by the Berkeley Biome Group.

With reference to the Berkeley Biome group, grasslands are categorised into two main types. These two types are differentiated according to their climate – tropical and temperate. Grasslands are sensitive to the change in climate as they have a strong seasonal climate. This suggests the possible changes that may occur to the characteristics of the grassland with long term exposure to climate changes. Other evidence also supports the hypothesis as there is a phenological and vegetation shifts¹ even before grasslands were impacted by climate change. Grasslands have provided a regulating services by providing climate regulation, carbon sequestration, erosion control, water regulation, air quality regulation, soil formation, pest control, waste treatment and pollination services. These natural environmental services are essential to keep the Earth’s energy balance.

Climate change have since impact grasslands through increased seasonal, annual, minimum, and maximum temperature and the change in precipitation patterns [12]. Depending on the location of grassland, the climatic experiences can vary and theses variations include, increased temperatures, reduced rainfall and prolonged periods of drought. Grasslands are often bordered by forests, deserts seas and mountains. The change in earth’s energy budget then have a vegetation shifts of either having rainforest encroaching into savanna and arid deserts being projected into arid grassland ecosystem. The slightest change in temperature, precipitation could alter the distribution, composition and the abundance of species in grassland. This would then result in the shift of products and services being provided. With the change in energy can also affect the geographical and elevational boundaries [13].

4.1 Adaptations of grasslands

Grassland then adapt to the change in climate by controlling on the opening of the stomata to optimise the balance between photosynthesis and transpiration. With the extended period of change in climatic condition, C3 plants² will no longer have the ability to flourish in such an environment and dormant C4 plants³ seeds or any vegetative parts that is in the soil will flourish and take over the area. Hence, the vegetation shift. The shift in vegetation may not be just the end of the story. As the carbon dioxide levels in the atmosphere continue to rise, carbon, water and nitrogen cycles would also be affected in the grasslands. Gas exchange in the plants is a key player in these cycles. The reduced transpiration level will lead to a reduced mass flow from the soil to the roots and leaves, causing reduced nitrogen uptake and feedback to weaken photosynthetic capacity. The increase in carbon dioxide in the atmosphere has reacted to the change in climate by exhibiting their decreased nitrogen nutrition status [1]. Hence, the change in Earth’s energy budget has impacted grassland by causing reduced stomatal conductance and significant reduction in yields.

For a single plant to strive in the environment, it requires certain criteria to grow. Without the criterion, the plant could have stunted growth, slow growth, discoloration and every other possibility. Apart from the observable morphology signs or poor health, they would also display poor yield. The application of chemical and/or organic fertilisers may not be effective as some plants just simply require concentrations that are readily available in the atmosphere. Thus, the instance stated above, where C4 plants are affected by the change have demonstrated that even if there is a shift in vegetation, if the area is deem inhabitable, the area will continue to remain dry and arid. Therefore, it is a relatively simple concept to comprehend – plants with poor health would then be highly susceptible to other diseases. The complexity of plant physiology is affected by the change due to the imbalance in earth’s energy budget and to ensure the continuity of its own species.

As mentioned earlier, climate change has affected grasslands with the change in temperature and precipitation patterns. In tropical grassland, the change of 1 Undercounter Temperature (uC) to 4 uC, will have grasslands experiencing increased aridity which reduces the productivity of soil organic carbon. It would also have plants experiencing increased water stress, therefore, altering the distribution pattern of grassland communities. There would also be decreased palatability of herbage and increased flammability. Drought tolerance species would then dominate and potentially lead to the extinction of other plant species. The carbon cycle in the tropical grassland would be affected as it will no longer be a carbon sink but it will be a source of emitting carbon dioxide. Grassland that is dominated by the C4 grasses will have enhanced biomass production because of the increase in soil moisture. When the grassland have low nitrogen availability, the response to elevated carbon dioxide will be suppressed. This would cause both long and short term effect. Impact of elevated carbon dioxide can be neglected during a short period of time with the increase in efficacy of nutrient-use and increased nutrient uptake due to higher root biomass at the elevated carbon dioxide.

Temperate grassland have similar changes experienced by the tropical grassland. Similar conditions such as increased drought due to the change in seasonal water regimes. The climate variability has then caused water stress. The water stress would cause reduced forage which then affects grazing livestock and also other animals that graze on the grassland. Grasslands have the capability to provide a buffer against climate variability [14]. The changes in temperate grassland will affect agriculture and grazing animals as compared to tropical grassland at the temperature variability is greater (Figure 3).

Figure 3.

Scenarios of biodiversity change for different biomes for the years 2020 and 2050. Bars represent relative losses of biodiversity of vascular plants through habitat loss for different biomes for two scenarios: (a) order from strength and (b) Adapting mosaic. Losses of biodiversity would occur when populations reach equilibrium with habitat available in 2050 and are relative to 1970 values. Darker bars represent scenarios for 2020 and lighter bars for 2050. Adapted from original Figure 10.6 in [15].

4.2 Impact of climate change

The model above was predicted on what will likely to happen in the year 2050. In this present day, the grassland ecosystem is already under serious threat. In the next 50 to 100 years, grasslands have been predicted to have lose between 8 to 10% of the vascular plant (the differences between these estimates are driven by different socio-economic scenario). The effect of the loss of biodiversity in grasslands and its effects on the carbon cycle would be an example of the synergism of global change drivers where biodiversity loss would constrain grassland ability to cope with the effect of other stressors such as climate change and ozone pollution [10]. The drastic change in grassland would be impacted by human activities the greatest as the soil conditions are suitable for agriculture and the mild climate in the biome. However, this would then be driven by the increase in food security as the climate changes and the socio-economic status of natis widens. Hence, climate change do not only have direct threats on the grassland ecosystem, it also causes the change in mindset and management regimes.

5. Diseases mode of transmission

The ecosystem of grassland is diverse and complex. Apart from the climatic changes that the plants have to face, there are other threats that are threatening the peaceful existence of the C3 and C4 plants aside from being a food source to herbivores. Diseases in grassland can spread like wildfire depending on the mode of transmission. Several viruses or viroids could spread extensively in the field just by contact between healthy leaves. Viruses are spread systemically and can be transmitted through natural grafting. Root graft can also transmit viruses or even parasitic plants. Common transmission modes of plant diseases are often by vegetative propagation. There are also plant diseases that transmit through seed. For instance, sour-cherry yellows in the Prunus spp. The disease is caused by the Prune Dwarf Virus that causes young leave to have chlorotic yellow rings or mottle. The virus can be transmitted by infected pollen grains or infected seeds when pollinated by bees or during propagation process (Table 2).

Vector taxa	Vector species	Modes of transmission				Totals	%
		NPVa	SPVb	PCVc	PPVd
Hemiptera	Aphids	161e	19	12	5	197	49.4
	Whiteflies	5	9	115f	—	129	32.3
	Leafhoppers	—	4	13	10	27	6.7
	Planthoppers	—	—	—	18	18	4.5
	Other hemiptera	2	9	1	—	12	3.0
Thysanoptera	Thrips	2	—	—	14	16	4.0
	Totals	170	41	141	47	399
	%	42.6	10.3	35.3	11.8

Table 2.

The table illustrates the major group of insect that cause the virus spread through vector. The viruses have also been categorised to persistency in the environment [11].

^a

NPV, nonpersistent stylet borne viruses.

^b

SPV, semipersistent foregut-borne viruses.

^c

PCV, persistent circulative (mostly nonpropagative) viruses.

^d

PPV, persistent propagative viruses.

^e

Includes 110 virus species of the genus Potyvirus, family Potyviridae.

^f

Virus species of the genus Begomovirus, family Geminiviridae.

5.1 Diseases transmission via vector

Plant diseases can be caused by various factors. Such factors could be abiotic such as nutrient deficiencies, soil compaction, salt injury or sun scorch. Biotic causes of plant disease transmission are caused by living organisms and they are collectively named as pathogens. Understanding pathogens life cycle, living requirements, movements and disease they carry can allow effective implementation of management regime to intervene the cyclative transmission.

5.1.1 Insects

Diseases in plants can also be transmitted via vectors. Aphids (28%) and whiteflies (18%) have been studied extensively over the years as they have been identified as common pathogenic vectors alongside with beetles (7%) and nematode (7%). However, the dense forests and every different area of land would always lead to a new discovery of a new species. In recent years, there is a new species of wasp, Allorhogas gallifolia. This wasp would make use of other wasp’s gall as nests. Larvae that hatch would then feed on caterpillars that consume gall tissues. A caddisfly, Potamophylax coronavirus, has also been discovered. This moth has eggs and larvae that thrive in the environment near rivers and lakes. With new insects emerging, there are various study opportunities apart from just their life cycle. They can potentially be a reservoir host for all kinds of diseases to humans, animals and plants. The type of insects would often carry similar viroid.

Insects as vectors are relatively tricky to have a proper management to fully eradicate the population. This small, hardy population has found itself thriving through different ages of the earth by having the capability to populate through laying multiple eggs. These eggs laid by one are then sufficient to replace more than one adult in the population. With the rapid replication ability, insects are able to adapt to the changing environment through different mechanisms. Insects can transmit in a cyclative manner where the pathogen is ingested before passing on to the new plant host. Calculative plant pathogens will often induce physiological changes in their plant hosts. Vectors who then feed on the infected plant will then have behavioural changes to optimise the spread of pathogens to other plants. Their ability to invade a new area is dependent on the insects’ ability to adapt to the environment besides the food availability.

5.1.2 Herbivore as vector

Herbivores are essential to the plant communities as grazing removes substantial quantities of biomass and promote plant species diversity [16]. In addition to herbivore vertebrate, insects have been shown to promote species richness by feeding competitive dominants. An instance to display such phenomena would be molluscs. They are a major group of invertebrate in temperate grasslands. These principal forb feeders would contribute negative impacts on species richness. These invertebrates are more well studied compared to soil-borne fungus. The complexity of the pathogen physiology is hard to comprehend as the environment will cause compensatory and additive interactions. Apart from infecting plants. Pathogens have displayed that they have the ability to increase plant susceptibility to herbivores to feed on the plant. Pathogens could also decrease it’s susceptibility as it makes the plant less palatable to grazing animals.

5.2 Soil-borne pathogens

Besides insects as vectors for pathogens, they could be soil-borne as well. They are capable of spreading via swimming spores of primitive and soil-inhibiting pathogenic fungi. Fungal pathogens have been categorised into three functional groups: biological controllers, ecosystem regulators and species participating in organic matter decomposition and compound transformations. Fungal pathogens are dispersed by spores. Their successful inhabitants of soil is accredited to its high plasticity and their ability to adopt various forms in response to adverse or unfavourable conditions [5]. Biological controllers can improve soil health by regulating diseases, pests, and the growth of other organisms. Fungi as ecosystem regulators are responsible for soil structure formation and medication. This will enhance the habitat of other organisms through the regulation of the dynamic aspect of the physiological processes in the soil environment.

Infected grasslands were observed to have increased species richness. However, This would have a negative impact on the dominant species in the grassland. In addition, the affected grassland will have decreased biomass. In a study by Allan et al. (2010), the biomass of the grassland will increase in an increasing exponential manner over the years. Fungal pathogens are not harmful to all. Despite the harm the bring to some of the species, fungi actively participate in nitrogen fixation, biological control of root pathogens, production of hormone and protection against drought. For instance, fungi can be beneficial to some leguminous crop via the improvement of plant uptake of nutrients and provide some form of protection to pathogens.

Nevertheless, bacteria that cause diseases in plants. Bacteria could be transmitted via the similar route as viruses by having physical contact on the health leaves or introducing plant materials that have bacteria. The simple transmission mode carries a huge load of impact on the environment as it alters the plants’ physiology.

6. Grassland diseases

Plant diseases are generally cause by three categories – (i) microscopic organisms like the fungi, bacteria and nematodes, where they have the ability to penetrate and infect more than one type of host, (ii) the sub-microscopic organisms such as viruses as they enter and infect the plant host systematically, (iii) parasitic higher plant that feed off their host.

Grassland diseases have been commonly affected by rust or having a weak rooting system. Disease have also been mentioned previously that transmission could be affected by vectors. Vector survival in the environment will then be crucial to understand the potential and possibility of having new diseases emerging. Studies have managed to display and explain different sources of colonisation for aboveground and belowground microbial communities and different drivers of community assembly. Grassland diseases are also dependent on the type of grasses. Diseases have affected the three main categories – grasses, legumes and cereal crops. Generally, they are all in a similar situation where they have insects as vectors (mainly aphids, beetles and soil-borne larvae), having mycosis and bacteriosis. Mycosis are caused by fungal pathogens where they destroy the plant tissue directly or through the potent toxins, this could be fatal to the host plant and can lead to ergotism in animals when they consume. Bacteria in plants can cause different kind of symptoms such as galls, overgrowth, wilts, leaf spots, leaf specks and blights. Occasionally, soft rots, scabs and cankers can be observed. In comparison to plant viruses, plant bacteria are not invasive and plant often occur as secondary infection through a vector.

6.1 Microscopic organisms in grassland

The soil biota often have bacteria, algae, fungi and soil invertebrates. The diversity of these microorganisms have been underestimated and under-researched. Microscopic organisms can have a symbiotic or a mutualistic relationship with the plant depending on the nature of the micro-organisms. Biological soil crusts have a biological community that is living on the soil surface. They perform several vital functions in grassland such as retention of moisture, stabilises surface soils, enriching soils with nitrogen and carbon, and even providing a favourable microclimate for seed germination The mosses and lichens will have rhizines, the gelatinous sheathe of mobile cyanobacteria, and fungal hyphae can bind surface soil particles to reduce soil and wind erosion. A well-developed biological soil crusts is an important factor in successful post-fire revegetation as they retain the integrity of the soil surface to provide a sanctuary for seeds propagules.

6.1.1 Bacteria

In the soil biota, the grassland have a diversity of bacteria. Bacterial communities were more spatially structured than fungal communities. Those bacteria can be an advantage to the grassland. Similarly, they can also be harmful to the host. Rhyzobium spp. are bacteria that live in the nodules of the roots of the leguminous plants.

In a study in Eastern of Czech Republic, bacterial sequences belonged mainly to Proteobacteria (50%) and Actinobacteria (20%). In shoots, the most abundant bacterial genera were Vibrio, Pantoea and Pseudomonas, all of which belong to Gamma-proteobacteria. In roots, the most abundant bacterial genera were Vibrio, Chthoniobacter (phylum Verrucomicrobia) and Paeniglutamicibacter (phylum Actinobacteria). In soil, the most abundant bacterial genera were Chthoniobacter, Gaiella (phylum Actinobacteria) and Paenibacillus (phylum Firmicutes). With such different bacteria that are found above and below ground, it supports the statement where different environment drivers can drive the formation of different colonies to be form or even the presence and/or the absence of some bacteria above and underground.

Every grassland may have different colonies of bacteria present. Those that are harmful to the host are mainly Xanthomonas spp., Pseudomonas spp. and Aphrodes spp.

Xanthomonas translucens pv graminis is infamous for bacterial wilt in forage grasses that have reportedly caused an outbreak in Europe, Australasia and United States of America (USA). The infection starts from a wound site and will eventually lead to necrosis starting from the infected site. The progression would be made towards the leaf base or the host plant. When the bacteria reached the vascular tissue of the host, the bacteria will colonise rapidly throughout the plant causing wilting of leaves. The plant will eventually be killed within a number of days. Severe yield loss have been experienced in the temperate region. However, translucens pv graminis is not the only specie in Xanthomonas to have caused such massive disruption.

The other three species of Xanthomonas are translucens pv arrhenatheri, translucens pv. poae and translucens pv. phlei. These species have presented distinct host adaptations to the plant species and have been successfully isolated. The strains display low genetic diversity. These host specialised parthovar strains will allow insight into distinct virulence factors where host-specific adaptation at molecular level with reference to Xanthomonas translucens pv graminis in future studies.

6.1.2 Fungus in grassland

Fungal pathogens are strongly influenced by the diversity and composition of the plant community. As such, they have a return effect on plant growth through mutualism, pathogenicity and their effect on nutrient cycling and availability.

Grassland grasses have been observed to be commonly affected by crown rust. Crown rust is common on swards often when bulk of material has been built up for autumn grazing or a late silage cut. The disease is more prevalent when grass depletes its nutrients. The increased temperature have then encouraged crown rust to have increased occurrence. The severity of the infection can result in reduced yield and the palatability is adversely affected. The disease is favoured by warm, moist weather with tropical temperatures. The extreme differences in daily temperature are even more highly favoured. The transmission of disease is through wind and precipitation.

Crown rust were affected by a plant pathogen Puccinia coronata. This fungus have affected plants like oats, barley and most specifically ryegrasses. The orange pustules on the leaf blade produces uredospores that could spread long distances to other plants in the grassland. Black pustules will then produce teliospores and will remain on plant debris over winter. The spores will then stay dormant till spring. Teliospore will then later produce basidiospores that infect secondary hosts. Basidiospores will then further produce aeciospore that will repeat the infection process.

Plant disease mechanisms in specialist and generalist pathogens can promote unwanted diversity of diseases if the dominant species that is susceptible is present in the community. This has a similar concept as the maintenance of the coexistence between herbivores and the plant communities. In a community that have pathogens that attack on less competitive species will cause and adverse effect.

6.2 Sub-microscopic organisms in grassland

Viruses have always exits and have remain its unpredictable virulence, creating havoc in grassland. The most Barley and Cereal Yellow dwarf virus (BYDV) have been one of the most complex and threatening to both food security and the ability for the plant of this species to continue to survive. This virus have then play an important role in the competitive dynamics of native and invasive grasses in non-managed system [17]. As the name of the virus suggests, it is highly infectious among barley and cereal. However, in recent studies, the virus have been discovered to have infect invasive species, Venenata dubia, in grassland habitats. Aphids have been positively identified to have been the vector of this virus. However, no two aphids are the same. Non-colonising aphids have been suggested to be responsible for the expensive spread of the virus [17].

Apart from BYDV, the cocksfoot streak virus (CSV) has been at a rise as the virus turn pastures into hay-like texture. CSV is aggressive and have reduce the quality of hay. Plants that were infected have also reduced ability to withstand frequent defoliation [18]. The virulence of CSV is not as aggressive as BYDV as progenies of infected plant do not have strains of CSV unlike BYDV.

7. Climate impact on vectors

Grassland diseases are highly affected by the availability of the mode of transmission in the environment. The knowledge on entomology is required to understand the point of interference in the life cycle in order to have successful management strategies.

7.1 Aphids

Aphids belong to a superfamily of Aphidoidea, which belongs to the Hemipteran sternorrchyna with whiteflies, jumping plant lice, scale insects and mealybugs. This superfamily is then separated into two sub groups of primitive “aphids” and a group of new world aphids [19].

The studies on aphids’ ability to transmit viruses have become more complex as they are capable of switching between sexual and parthenogenetic reproduction. Aphids are a vector of Ribonucleic Acid (RNA) grass pathogens and such viruses include barley and cereal yellow dwarf viruses. The severity of grassland diseases are dependent on the areas located. For instance, in California, United States of America (USA), the pathogen-mediated invasion in grasslands is the result of competition between native and exotic plants where aphids have higher fecundity on exotic plants compared to that of the natives [20]. This factor has potentially led to the increase in pathogens transmission rates throughout the community. The life cycle has then been studied closely to understand and in hopes to discover a point of interference which would break the reproduction rate.

A single host plant species is often observed to be utilised throughout the year. In response to the decreasing daylight, sexual morphs are produced in the fall. Genetically recombinant eggs that are reproduced by the male and his oviparae⁴ mate would overwinter on the host plant and often experience a high mortality rate. Fundatrix⁵ that emerges from the eggs in spring will proceed to reproduce to live births parthenogenetically. These nymphs would be viviparae⁶ and will continue the lifecycle in summer. The parthenogenetic mode of reproduction has ensured a rapid population build-up by ensuring that there are eggs available on the plant year round. The rapid increase in aphids population in a single host plant could quickly lead up to overcrowding. This would allow the future offspring of the aphids would be switched to those with wings to have efficient dispersal of feeding opportunity and ensures the genetic survival.

Aphids (49.4%) are transmitters for the majority of the mosaic virus and leafhoppers (6.7%) are transmitters for yellow-type viruses. There are many other insects of interest such as whiteflies (32.3%), thrips (4.0%), mealybugs, plant hoppers (4.5%), grasshoppers, scales and beetles. Aphids are sap-sucking insects and have piercing, sucking mouthparts. They strive generally well in regions with cold winters. The use of their mouthparts include a needle-like stylet that assist aphids to have access and feed on the contents of plant cells. The insect’s feeding habits will weaken the plant and cause metabolic imbalance. In addition, aphids secrete honeydew. This is an ideal medium for a variety of fungi to populate. As such, sunlight would be blocked out as the fungi populate, building a barrier for the plant to photosynthesize. During the process of feeding, their stylet has created a point of entry for the pathogens to enter the system of the plant host. The plant, if infected with secondary infection, would then be infected and display disease symptoms.

Aphids have been covered in a relatively large proportion in this chapter. This insect has eggs that are cold-hardy to survive winter. The efficacy to have population build-up is only possible when the temperature is optimum. Every species of aphids have different optimal temperatures. However, the minimum range is said to be at 4 degree Celsius. Acrythosiphon pisum is an aphid that reproduction is dependent on the temperature. The overall increase in global temperature by 2 degree Celsius would have an approximation of generations increased from 18 to 23 generations per year (based on a study in the United Kingdom). However, the generation time of a female differs between species and this could potentially be shorten by the decrease in temperature due to global warming. France has a mean temperature of 10 degree Celsius (in the north) and 15 degree Celsius (in the south) which place the aphids in suboptimal temperature conditions. However, this is an alarming increase for entomologists to study aphids further. Apart from the temperature being favourable to the rate of reproduction. The temperature increase is also favouring the mobility of the aphids. The winged aphids have a threshold of 13 to 16 degree Celsius and an upper threshold of approximately 31 degree Celsius [18].

7.2 Soil dwelling organisms

Apart from aphids that are attacking above ground, there are also vectors attacking below ground. Larvae of Cerapteryx graminis, a moth from the Lepidoptera, are soil-dwelling and the larvae can cause Charaeas graminis on grasses. Larvae of Tholera decimalis Poda (from Lepidoptera) are soil-dwelling and can cause disease to a plant by feeding on its roots. Other larvae that are soil-dwelling can cause large amounts of damage to the plant as the larvae mainly feed on the roots of the host plant and some adults may continue to dwell in the same plant causing more harm. Larvae feeding at the root system may invite secondary infection causing more complications. Nematodes and soil-dwelling borers are also a vector for infection in plants as they could create entry for bacteria to cause further complications to plant health.

These soil-dwelling organisms will be impacted by the decrease in moisture in soil (regions where desertification occurs). Increase in flooding will also be a concern to these organisms as they may not survive if the soil moisture increases too drastically. Their living conditions are also affected by the temperature. The adaptation to the changing climate is similar to that of other insects living aboveground.

7.3 Soil bacteria

Bacteria can be transmitted naturally through exudation out of the host plant and when contact is made between injured plants, they can infect the plant through the wound site. Insects that come in contact with the exudates that infected host plants produce, they can also transmit to other plants as secondary infection. Insects are often attracted to the sugars in the bacterial exudates. During the process of consumption, the mouthparts of the insects will then carry the strains of bacteria. Upon travelling and feeding on other plant, they will create an entry for these bacteria they carry and the plant will now be infected.

Bacteria being microscopic organisms will be sensitive to the change in environment conditions. Depending on the types of bacteria, some may strive in the areas of higher temperature like the Xanthomonas spp. are at advantage but not for Puccinia spp. The virulence of the bacteria have been studied to have been affected by the change in temperature. Agrobacterium strains have their virulence gene amplified as temperature increases but they will have a loss of phosphorylation activity [21]. Pseudomonas have increased production of phytotoxins as there is an increase in temperature to maintain its virulence. Therefore, the change in climate will affect the physiological functions of the bacteria differently and ensure the continuum of bacteria in the environment. The effect of bacterial virulence of some effectors may become apparent under specific environment conditions such as humidity.

7.4 Soil fungus

Soil fungus has different roles in the soil which then serves different ecosystem services. They are also bioindicators of soil health. However, as mentioned earlier, crown rust is a genus of fungus (Puccinia spp.). Some of the Puccinia spp. are considered as parasites of plants. The presence of harmful plant pathogens indicates poor soil quality. The factors that cause changing soil fungal biodiversity are mainly due to the management practices, chemical fertilisation, application of herbicides and fungicides, biochemical amendments of the soil, soil degradation, soil contaminants and soil properties such as salinity and drought conditions.

Global warming can influence the host plant associations through alteration of interactions between plant and mycorrhizal fungi. This group of fungi have the role of having direct influence on individual plant function and the indirect impact processes such as plant dispersal and community interactions. However, to mediate and to survive, they have ways to mediate the current changes of climate. Such methods involve varying in hyphal exploration type liked to root density [5, 22]. Climate change does not seem all that bad when the essential fungus in the soil required are still able to survive.

The change in climate has created new environmental pressures that results in novel fungus diseases. The effect of climate change on the emergence and re-emergence of fungal pathogens have raised concerns on food security, human and animal health, and wildlife extinction due to the report worldwide. There are new virulent fungal lineages with adaptations emerging and they have been suggested to have evolved alongside with the increased pressure of climate change. One such fungus is the Puccinia striiformis, commonly known as rust fungus (same genus to the current crown rust pathogen). Stripe rust has affected wheat crops worldwide. There were records that indicate the preference for cooler regions but has recently invaded to warmer regions. The ability to disperse to warmer regions has allowed the emergence of three novel strains. These strains have been described as being more aggressive with increased thermotolerant [23]. The spread of novel strains have been hypothesised for having the ability to replace older strains and expanding the spread of disease. Through microsatellite genotyping and virulence phenotyping on the novel strains, it has been demonstrated that the evolution can potentially be ongoing alongside with the change in climate.

Another fungal concern would be the emergence of Fusarium head blight in wheat and other cereal crops [14]. The infection can reduce crop yield and quality. Thus, threatening food security. Outbreaks have been reported specifically with years that experience warmer and humid weather. The economic loss during outbreaks could be up to 75 percent [4]. The shift in temperature due to climate change have allowed the fungi to be more aggressive and able to expand the spread of territory. The change of favourable weather has been observed in two species of the Fusarium genus. Fusarium graminearum and Fusarium culmorum are two of the species that display prominent change towards the shift in temperature over the regions. They have very contrasting weather preferences. Thus, these fungus can expand through larger areas that do not adapt. The increase in environmental stress due to the change in climate have also evidently shown some of the species to react by producing more mycotoxins. As such this has a rising concern not only to food security but also human and animal health.

Apart from the changes that the fungi have evolved to ensure the survival of its kind, the spread of spores have then been extensive through the rising disastrous events. Frequent flooding and strong winds causing dust storms are two such extensive transmission methods. Soil-borne fungal pathogens have been speculated to have increased frequency or range due to climate change [6]. They are found out of their normal range and at times can be challenging to have a first diagnosis [24].

8. Future perspective

There might be results that the resistant plant type is achieving ideal suppression of damage done. However, all living things have the ability to change and adapt to the environment they live in. Plants that as antibiosis may achieve ideal results when planted in Region A. However, when planted in Region B and C, the result may vary. Assuming that the soil conditions in all three regions are the same. However, abiotic factors cannot be controlled and that may be the factor that causes the difference in results. Therefore, grassland management has to be very specific to a particular location and changes that occur through the years can be used to study closely to have a more effective management plan.

Disease in grassland have been affected by the Earth’s energy imbalance due to the change in living environment and transmission mode. The change in climate have affected the population of the vectors. Having vectors in the environment is essential for transmission mode in the ecosystem. The reduced availability vectors in the population will ideally have a decreased in the extensiveness of the spread of disease. However, if the disease, in particular consideration to viroids, where to mutate, the mode of transmission could change to either, air-borne or even have a longer dormancy capability to ensure sustainability of its existence.

8.1 Innovative management

The different kinds of vectors will require different methods of surviving as the climate changes. The increase in certain greenhouse gases in the atmosphere makes it complex to understand the change that the vectors are going through. However, there are a few significant points that could be brought across in this chapter. Vectors such as insects have expanded their distribution to regions where it will be more habitable to them. This can be supported by the pink bollworm (Pectinophora gossypiella), an infamous cotton pest that has expanded towards the central of California and away from the South. Other vectors such as the Olive fly (Bactrocera oleae) have demonstrated migration behaviour. They would travel southwards during winter to experience summer in other areas. However, this would increase competition of insects for the availability of food for the population. This will potentially lead the insects to have a change in diet if available and adaptable.

Apart from migration behaviour, vectors that have remained have increased in overwintering survival. They will produce eggs that are more hardy to withstand the change in temperature and environmental damages. Insects have also adapted to migrate as mentioned earlier. Vectors that are freeze-tolerant have physiological adaptation to be diapause⁷. They could be obligate or facultative. Regardless of which they are, the insects will be hormonally mediated to a state of having low metabolic activity. This will suppress development, suspend activities and increase resistance to adverse environmental factors change. Insects will also display aestivation or hibernation. The ability for the insect to synchronise with the changing environment will be the most ideal situation where the expansion and the spread of diseases are still highly plausible.

The change in ambient temperature have accelerated reproduction rates. This has caused an increase in population size. As such, it can lead to the number of species having dynamic equilibrium. To understand the phenological shifts caused by climate variability, it has been measured with growing degree days (GDD). The GDD will then aid in determining the minimum and maximum temperature threshold. Insects of multivoltine, such as aphids, are at the advantage of the rising temperature. Increase in 2 degree Celsius in temperature could have an estimate of additional five generations. Other insects demonstrated having earlier flight as the ambient temperature increases.

The change in climate has also brought about the change in precipitation patterns. High rainfall will have insects such as aphids being washed off and will decrease the opportunity of the insect or pathogens overwintering. However, the insects can migrate further up the soil horizons or deep down. Soil-dwelling wireworms have adapted to the change in precipitation pattern by populating on the upper soil horizon and migrate as they grow as an adult.

8.2 Potential limitations

The cruciality of understanding vectors in plant diseases in relation to that of climate change is complex and underestimated. The importance of vectors’ movement and traceability have yet to be identified clearly as they are showing signs of evolution with rapid reproduction rate. As such, plant diseases can be said to have spread as rapidly as the vectors expand their area of infection. Plant diseases cause secondary infections which are mainly facilitated by insects to allow entry to a pathogen by creating a wound site on the plant organ regardless of it being above or underground. Pathogens that are vectored by insects can also overcome survival to adverse environment factors through the maintenance of over-seasoning in the body of the insect.

There are different management methods innovated in order to suppress the damage incurred. Having a plant that is resistant to specific insects or pathogens is an innovative way or management. For example, antibiosis in host plant resistance is a primary mechanism that works against aphids [17]. The process occurs at the utilisation phase of the interaction between the plant and the insect. It is the result of action of plant-biochemicals in the biological processes of herbivorous insects. Antibiosis would then be expressed in terms of larval mortality, decreased larval and pupal weights, prolonged larval and pupal development, reduced fecundity, prolonged generation time and overall effect on insect survival and development.

9. Conclusion

Climate change has created a new ecological niche and opportunities are provided for vectors to continuously expand their geographic region. Hence, the migrating behaviour. Microscopic plant pathogens and vectors who spend most of their life underground have a comparatively greater advantage of surviving climate change as soil is a thermal insulating medium, buffering temperature change and reducing impact. Apart from the focus of climate change being the increase in global temperature, the change in climate is contributed by human activities where the atmospheric composition changes. The significant gas that all scientists are studying is the carbon dioxide concentration. The increase in carbon dioxide concentration has driven vegetation shift. However, it has also driven the susceptibility of pathogens of these vegetation. High carbon dioxide levels can encourage plant growth. However, it will encourage the feeding for insects as vegetation increases in palatability.

The change in the ecosystem is tied in closely to that of insects and pathogens. Therefore, the change in climate will strongly affect the survival of the vectors of diseases rather than the diseases itself. This can be supported by the expansion of diseases as the vectors expand their movement through migration. The behavioural changes in vectors are significant and as they strive and adapt to the change in climate, so will the plant diseases in grassland continue to cause more damage.