Survival as dormant spores or specialized resting structures:

Plant viruses have no resting stage and are transmitted through a continuous infection chain.

Phytopathogenic bacteria:

• The plant bacteria also do not produce resting spores or similar structures.
• They continuously live in their active parasitic stage in the living host or as active saprophytes on dead plant debris

Nematodes: They survive in the form of active parasitic phase on a living host and also survive through dormant structures, i.e., eggs, cysts, galls, formed in host tissues.

Phanerogamic parasites: They survive in dormant state for many years through seeds.

Ex; Seeds of Orobanchae survive in soil for more than 7 years.

Among plant pathogens, fungi are the only organisms that produce spores, analogous to eggs of nematodes, and other resting structures for their inactive survival.

These dormant structures of survival can be classified in the following categories :

1) Soil borne fungi:

a) Dormant spores : Conidia (Peach leaf curl pathogen, Taphrina deformans), Chlamydospores (Wilt pathogen, Fusarium sp.), oospores (Downy mildew fungi), perithecia (Apple scab pathogen, Venturia inaequalis) etc.

Fig : I) oospore   ii) Chlamydospores    iii) Perithecium

b) Other dormant structures such as thickened hypha, sclerotia (Cottony rot fungus, Sclerotinia sclerotiorum), microsclerotia (Verticillium), Rhizomorphs (Armillaria mellea), etc.

fig : I) Thickened hyphae        ii) Sclerotia    iii) Rhizomorphs    iv ) Microsclerotia

C) Factors affecting the survival of pathogen in the soil are :

a) physical factors (high temperature, irradiation, dessication and anaerobiosis),

b) chemical factors (antibiotics, antagonistic chemicals produced by other microbes) and

c) biotic factors (parasitism, predation by microflora and microfauna).

2) Seed borne fungi:

a) Externally seed borne: Dormant spores on seed coat Ex: Covered smut of barley, grain smut of jowar, bunt of wheat, etc.

b) Internally seed borne: Dormant mycelium under the seed coat or in the embryo

Ex: Loose smut of wheat (Ustilago nuda tritici)

c) Factors affecting the survival of the pathogen on/in the seed are temperature and moisture.

3) Dormant fungal structures on dormant or active host Ex:

• In downy mildew of grapevine, powdery mildew of grapevine, apple etc.,
• The fungus mycelium may be present in dormant state in the affected twigs or its oospores or perithecia may be embedded in the tissues of the affected organs.

4) Survival in association with insects, nematodes and fungi :

• Several important plant pathogens may survive within the insect body and over winter therein.
• The corn flea beetle, Cheatocnema pulicaria carries inside its body, the corn wilt pathogen, Xanthomonas stewartii and thus helps in over wintering.