**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

#### *Earth's Energy Budget Impact on Grassland Diseases DOI: http://dx.doi.org/10.5772/intechopen.99971*

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.
