Many plant diseases that are caused by soil borne pathogens can be difficult to predict, detect and diagnose. Investigations into these pathogens are further limited by the nature of the soil environment, which is extremely complex. This makes gaining an understanding of soil borne plant pathogens, and the diseases they cause, a challenging aspect of plant biosecurity.
Soil borne plant pathogens generally fall under the category of virus, bacterium, fungus or nematode. They can be extremely effective at surviving long periods without a host plant and their pervasive nature makes control and treatment particularly challenging. The ability of soil borne pathogens to survive independently in the soil between planting periods varies greatly with each pest.
Pathogens known as saprophytes can survive during growing periods by taking nourishment from plant debris or other organic matter, both on and in the soil.
However, other pathogens, such as fungi and nematodes, can survive in the soil for long periods without the aid of decaying plant matter. Survival is aided by their ability to form robust resting structures (eg cysts, spores and hyphae), which enable them to survive long periods without a suitable host, or when environmental conditions are unfavourable.
Fungal pathogens are perhaps the most common type of soil borne plant pest. Clubroot, caused by the fungus Plasmodiophora brassicae, affects cruciferous crops such as rocket and kale and results in distortion of roots, as well as wilting and stunted growth of the affected plant.
Damping off is a serious plant disease that commonly results in seedling death and is often caused by soilborne fungal pathogens, such as Rhizoctonia solani. Symptoms range from rotting within the seed coat before germination, to decay of the taproot or rootlets following germination. Surviving plants are stunted, and affected areas often show uneven growth.
Bacterial soft rot can result when wound sites of plant roots are infected with bacteria of the Erwinia spp. This results in a slimy rot that can affect any part of the plant, including heads, curds, edible roots, stems and leaves. Both damping off and bacterial soft rot affect a broad range of vegetable crops.
Pathogenic nematode species can be particularly pervasive in soil. For this reason they can pose a serious threat to vegetable crops. Globodera rostochiensis (potato cyst nematode – PCN) infects Solanaceae, such as potato and eggplant. Cysts from this pathogen contain hundreds of eggs that can remain dormant in the soil for up to 30 years.
These cysts are commonly dispersed when infested soil is carried by machinery, footwear or on plant roots to other growing areas.
Due to the invasive nature of soil borne pathogens, biosecurity best practice is important for controlling the risks presented by them.
There are several cost effective biosecurity measures that can be implemented in order to reduce the risks presented by soil borne pathogens.
Acknowledgement: This article was reproduced with permission from AUSVEG.