Apple collar rot

Phytophthora species are very destructive organisms belonging to the Oomycetes and are therefore strictly speaking not fungi. They thrive at soil temperatures between 15 and 23 °C and higher water content than is optimal for crop growth.

Phytophthora overwinters in roots, infected tubers and bulbs or in the soil in different stages: as oospores (sexual overwintering spores), sporangia (special structures which can either germinate directly or produce zoospores), chlamydospores (thick-walled mycelium cells which are resistant to desiccation) or mycelium. Zoospores are formed from the overwintering structures, which infect the host. Zoospores are spores with flagellae, which enable them to move through water. Infection is usually from the root to the foot, but direct infection of the foot also occurs. Exudates leaking from growing root tips promote germination and attract the zoospores. The pathogen grows into the stem and on the plant surface and new sporangiophores (structures containing sporangia, a type of spores) with sporangia are formed, that protrude through the stomata. Secondary infection is either by germinating sporangia or by zoospores produced in the sporangia. For germination on above-ground plant parts, leaf wetness is required.

Dispersal by wind and rain causes the pathogen to be returned to the soil from infected plants. Dispersal in the soil happens via water, since the zoospores can readily move through water. This explains why the disease spreads more rapidly when the water content of the soil or substrate is higher than normal or optimal for crop growth.