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. Dispersal in the soil happens via water, since the zoospores can readily move through water. For germination on above-ground plant parts, leaf wetness is required.ĭispersal is by wind and rain causes the pathogen to be returned to the soil from infected plants. Secondary infection is either by germinating sporangia or by zoospores produced in the sporangia. 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. Late blight, caused by the fungal-like organism Phytopthora infestans, is the most severe disease of potato and tomato. Exudates leaking from growing root tips promote germination and attract the zoospores. Infection is usually from the root to the foot, but direct infection of the foot also occurs. Zoospores are spores with flagellae which enable them to move through water. Zoospores are formed from the overwintering structures which infect the host. 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. They thrive at soil temperatures between 15 and 23 ☌ and higher water content than is optimal for crop growth. Phytophthora species are very destructive organisms belonging to the Oomycetes and are therefore strictly speaking not fungi. Late blight, caused by the oomycete Phytophthora infestans, is a major global disease of potato and tomato.
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