Depending upon the occurrence and transmission, plant diseases are classified into four categories:

(i) Seed-borne diseases: These diseases spread through seeds, e.g., “ergot of bajra” or of pearl millet; “leaf spot of rice”; “loose smut of wheat”; red rot of sugarcane; - all are caused by fungi.

(ii) Soil-borne diseases: These diseases spread through the soil and mostly affect roots and stems of crop plants, e.g., “smut of bajra,” “tikka disease of groundnut”.

(iii) Air-borne diseases: These crop diseases are transmitted by the air, e.g., “rust of wheat”, “blast of rice”, etc. Air-borne diseases attack all aerial parts of the plants, e.g., leaf, flower, and fruits.

(iv) Water-borne diseases: Pathogens of these crop diseases are transmitted by the water, e.g., “bacterial blight of rice”.