- 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.
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