Management of Soil Borne Diseases in Plants

In this article we will discuss about the management of soil borne diseases.

Plant diseases result when a susceptible host and a disease-causing pathogen meet in a favorable environment. If any one of these three conditions were not met, there would be no disease. In contrast, the threat of disease epidemics in crop production is constant. Microorganisms are important component of soil.

Soil bacteria and fungi mediate soil processes such as decomposition, nutrient mobilization and mineralization, storage and release of nutrients and water, nitrogen fixation and denitrification. In the frame of agriculture, the micro flora is of great significance because it has both beneficial and detrimental influence upon man’s ability to feed himself.

All the environmental factors which influence the distribution of bacteria and actinomycetes also influence the fungal flora of soil. The quality and quantity of organic matter present in soil have a direct bearing on fungal numbers in soil since most fungi are heterotrophic in nutrition.

Fungi are dominant in acids soils because acidic environment is not conducive for existence of either bacteria or actinomycetes, resulting in the monopoly of fungi for utilization of native substrates in soil. Isolation of fungi from different horizons of soil profiles shows that these organisms exhibit selective preference for various depths of soil.

Those fungi which are common in lower depths are rarely encountered on the surface of soil which may be explained on the basis of the availability of organic matter and the ratio between oxygen and carbon dioxide in the soil atmosphere at varying depths. Farm practices including crop rotation and fertilizer or pesticide applications influence the nature and dominance of fungal species.

A soil is considered suppressive when, in spite of favorable conditions for disease to occur, a pathogen either cannot become established, establishes but produces no disease, or establishes and produces disease for a short time and then declines.

Suppressiveness is linked to the types and numbers of soil organisms, fertility level, and nature of the soil itself (drainage and texture). The mechanisms by which disease organisms are suppressed in these soils include induced resistance, direct parasitism (one organism consuming another), nutrient competition, and direct inhibition through antibiotics secreted by beneficial organisms.

The level of disease suppressiveness is typically related to the level of total micro­biological activity in a soil. The larger the active microbial biomass, the greater the soil’s capacity to use carbon, nutrients, and energy, thus lowering their availability to pathogens.

In other words, competition for mineral nutrients is high, as most soil nutrients are tied up in microbial bodies. Nutrient release is a consequence of grazing by protozoa and other microbial predators: once bacteria are digested by the predators, nutrients are released in their waste.

High competition—coupled with secretion of antibiotics by some beneficial organisms and direct parasitism by others makes a tough environment for the pathogen. Our goal is to create soil conditions with all three of these factors present.

Therefore, we want high numbers and diversity of competitors, inhibitors, and predators of disease organisms, as well as food sources on which these organisms depend. The food for beneficial organisms comes either directly or indirectly from organic matter and waste products from the growth of other organisms.

Limiting available nutrients is a key for general suppression. With an abundance of free nutrients, the pathogen can prosper. Virtually any treatment to increase the total microbial activity in the soil will enhance general suppression of pathogens by increasing competition for nutrients.

So, how does the plant survive without readily available nutrients? It does so through microbial associations with mychorrhizal fungi and bacteria that live on and near the roots. These microbes scavenge nutrients for the plant to use.

In return the plant provides carbon in the form of sugars and proteins to the microbes. This symbiotic system supports the beneficial organisms and the plant, but generally excludes the pathogens that would attack the plant.

It should be noted that general suppression will not control all soil-borne diseases. Rhizoctonia solani and Sclerotium rolfsii, for example, are not controlled by suppressive soils—their large propagules make them less reliant on external energy or nutrient sources, and, therefore, they are not susceptible to microbial competition.

With these two pathogens, “specific” beneficial organisms such as Trichoderma and Gliocladium will colonize the harmful propagules and reduce the disease potential. The various ways to manage the soil borne diseases are dis cussed as under.