i) Distance of susceptible plants from the source of primary inoculum:

• Longer the distance from the source of survival of the pathogen, longer will be the time required for the buildup of an Epiphytotic in a susceptible crop.

ii) Abundance and distribution of susceptible hosts:

• Continuous cultivation of a susceptible variety over a large contiguous area helps in the buildup of the inoculum and improves the chances of epiphytotics.

iii) Disease proneness in the host due to environment:

• Susceptibility is genetically controlled but the disease proneness in the plant to get infected can be induced by environment and other factors (Host nutrition, excessive application of nitrogenous fertilizers, etc).

iv) Presence of suitable alternate or collateral hosts:

• These host plants help in the survival of inoculum of different pathogens in off season.
• Presence of grass hosts helps in the survival of Pyricularia oryzae in the off-season.