a) Dispersal in soil:The following are the three stages of dispersal in soil
i) Contamination of soil: Contamination of the soil takes place by gradual spread of the pathogen from an infested area to a new area.
ii) Growth and spread of a pathogen in soil:Once the pathogen has reached the soil it can grow and spread based on its ability to multiply and spread.
On the basis of this competitive saprophytic ability the pathogens in soil can be of three types.
Specialized facultative parasites (Saprophytes) can pass their life in soil in the absence of host plants, but they depend more on the residues of the host plant.(ex: Armillariella mellea, Ophiobolous graminis etc.).
Unspecialized facultative parasites can pass their entire life in the soil (Pythium sp., and Phytophthora sp.).
The soil borne obligate parasites such as Plasmodiophora brassicae, Synchytrium endobioticum require the presence of active host.
iii) Persistence of the pathogen in soil: The pathogens persist in the soil as dormant structures like oospores (Pythium, Phytophthora, Sclerospora etc.), Chlamydospores (Fusarium), smut spores (Ustilago) and sclerotia (Rhizoctonia, Sclerotium).
b) Dispersal by the soil:
- The pathogen is dispersed by the soil during cultural operations through the agricultural implements, irrigation water, workers feet etc.
- Propagules of fungi and the plant debris containing the fungal and bacterial pathogens thus spread through out the field.
- Eg : Pythium aphanidermatum (causal agent of stem or foot rot of papaya) can introduce the pathogen in new pits for transplanting the seedlings.