(a) When the pathogens, such as bacteria, fungi and viruses get favourable conditions for growth and propagation, they spread and infect the crop plants causing crop diseases.
(b) Depending upon their mode of occurrence, crop diseases are of the following types:
Seed-borne diseases: These diseases are spread through seeds, e.g. leaf spot of rice.
Water borne diseases: Pathogens of these crop diseases are transmitted by water, e.g. Bacterial blight of rice.
Air borne diseases: These crop diseases are transmitted through the air, e.g. rust of wheat.
Soil-borne diseases: These diseases are spread through the soil mostly affecting roots and stems of crop plants, e.g. smut of bajra.
(c) All the seed and soil-borne diseases can be controlled, by treating the seed or soil and the airborne diseases are controlled by spraying fungicidal solution, on infected parts.
