Essential components/conditions for an Epiphytotic:

1. Host factors
2. Pathogen factors
3. Environmental factors

1. Host factors :

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.

2. Pathogen factors:

i) Presence of virulent/aggressive isolate of a pathogen:

• For any epiphytotic, rapid cycle of infection is essential, and successful infection can be caused only by virulent isolates of the pathogen.

ii) High birth rate:

• The fungi that assume epiphytotic form invariably have the capacity to produce enormous quantity of spores that are adapted to long distance dissemination in a short time.

iii) Low death rate of the pathogen:

• Epiphytotics is attributed to low death rate of the pathogens in those in which the causal agent is systemic and protected by the plant tissues.

iv) Easy and rapid dispersal of the pathogen:

• The ability of a pathogen to cause epiphytotics is much more dependent on its dispersal rate.
• Ex: Fungal spores disseminated by wind, water, etc.
• Viruses disseminated by insect vectors
• Bacteria dispersed through rain splashes and water.

v) Adaptability of the pathogen:

• Most of the pathogens causing epiphytotics adapt themselves to various adverse conditions.

3. Weather factors:

• Weather conditions such as, optimum temperature, moisture, light, etc., are very essential for the development of an epidemics.
• Science which deals with the relationship between weather and epiphytotics is called metereopathology.