- The initial infection that occurs from the sources of pathogen survival in the crop is primary infection and the propagules that cause this infection are called primary inoculum.
- After initiation of the disease in the crop, the spores or other structures of the pathogen are sources of secondary inoculum and cause secondary infection, thereby spreading the disease in the field.
- Ex: The fungus (Phytophthora infestans) causing late blight of potato survives in seed tubers or in soil. Infected tubers bring the primary infection in the field while primary inoculum (PI) present in soil causes primary infection of the crop from healthy seed.
- The PI may also be brought by wind from neighboring fields or long distances.
- Then the fungus produces spores on leaves.
- These spores are dispersed by wind and water and reach healthy plant surfaces to cause new infections. This is secondary infection.
- The primary infection initiates the disease and secondary infection spreads the disease.
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