Autonomous or direct or active dispersal:

• In this method the dispersal of plant pathogens takes place through soil, seed and planting material during normal agronomic operations.
• There is no major role of external agencies like insects, wind, water, etc. in this type of dispersal.

A) Seed as the source of autonomous dispersal:

• The dormant structures of the pathogen (Ex: seeds of Cuscuta, Sclerotia of ergot fungus, smut sori, etc.) are found mixed with seed lots and they are dispersed as seed contaminants.
• The bacterial cells or spores of fungi & Dormant mycelium is transmitted to long distances.

                            There are three types of dispersal by seed, viz., contamination of the seed, externally seed borne and internally seed borne.

a) Contamination of the seed:

• Seed borne pathogens move in seed lot as separate contaminants without being in intimate contact with the viable crop seeds.
• The seeds of the pathogen or parasite and the host are mixed during harvest of the crop.
• Ex: Smut of pearlmillet and ergot of rye. Smut sori and ergots mix easily with the seed lots during harvest and threshing.

b) Externally seed borne:

• Close contact between structure of the pathogen and seeds is established where the pathogen gets lodged in the form of dormant spores or bacteria on the seed coat during growth of the crop or at the time of harvest and threshing.
• Ex: Short smut of sorghum, bacterial blight of cotton, loose smut of barley etc. The spores of Tilletia caries (Stinking smut of wheat) remain viable even after 18 years and those of Ustilago avenae (Oat smut) for 13 years.

c) Internally seed borne:

• The pathogen may penetrate into the ovary and cause infection of the embryo while it is developing.
• They become internally seed borne. Ex: Loose smut of wheat.

Differentiate Seed infection and infestation

2) Soil as a means of autonomous dispersal:

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 the 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.

3) The plant and the plant organs as a means of autonomous dispersal:

• The plants, plant parts other than seed that are used for vegetative propagation, raw field produce and plant debris that accumulates during the course of cropping constitute the third method of autonomous dispersal.
• Ex: Late blight of potato was introduced in North America and in Europe through seed tubers brought from the native source of the in South America.
• Citrus canker was introduced into California from Asia.