Life cycle and appearance of Phytophthora blight
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 is 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.
Phytophthora causes root rot, seedling damping off and rot of stems, bulbs, tubers, leaves and fruits.
Late blight or Phytophthora blight (Phytophtora infestans f.sp. infestans) causes water-soaked lesions with vague borders and white fluffy fungal growth on the leaves. The centre is grey-brown with a light-green edge. On the stem, elongated brown lesions occur, usually circling the stem. On potato tubers, blueish lesions appear, that can be seen through the skin.
The Great Famine was a period of mass starvation caused by this disease which ravaged potato crops throughout Europe during the 1840s.
How to prevent Phytophthora blight
- Apply hygiene measures, like cleaning machinery and tools, destroy waste heaps, remove remaining roots, infested leaves
- Choose less susceptible or resistant cultivars when available
- Prevent over-irrigating and be careful with overhead irrigation since both a high water content of the soil and water splash promote disease. Ensure fast drying and wound healing, by allowing air circulation
- Disinfect recirculating nutrient solutions
Prevent plant diseases by optimizing plant potential and crop resilience.