Sclerotinia sclerotiorum on Rape
Sclerotinia rot affects a wide range of plants particularly non-woody species. Sclerotinia rot is caused by S. sclerotiorum. Sclerotinia rot can affect plants at any stage of production including seedlings, mature plants and harvested products. Plants with senescing or dead tissue are particularly susceptible to infection.
The infected area of a plant initially takes on a dark green or brown water-soaked appearance, then may become paler in colour. Dense white cottony mycelium usually develops and the plant begins to wilt and eventually dies. Resting or survival structures (sclerotia) are produced externally on affected plant parts and internally in stem pith cavities. The sclerotia are hard, black, irregular in shape, mostly 2-4 mm in size, and difficult to see once incorporated into the soil.
Disease sources and spread
The life-cycle of S. sclerotiorum includes both a soil-borne and an air-borne phase. Sclerotia of S. sclerotiorum can survive in the soil for ten years or more. They germinate to produce small funnel-shaped fruiting bodies (apothecia) that are approximately 1 cm in diameter. Apothecia produce air-borne spores, which can cause infection when they land on a susceptible host plant, either via flowers, or by direct germination on leaves. Occasionally, infection of stem bases can occur when fungal strands (mycelium) develop directly from Sclerotia near the surface. New sclerotia develop in infected plant tissue and when the plant dies they remain on the soil surface or may become incorporated during subsequent soil cultivation.
Conditions for Infection
After a period of cold conditions in winter, sclerotia, overwintering in the top 5 cm of the soil, germinate from spring onwards to produce apothecia, when soil temperatures are 10°C or higher and the soil is moist. Sclerotia do not germinate in dry soil or when the soil temperature is above 25°C. Sclerotia buried below 5 cm in the soil are less likely to germinate. Once apothecia are fully formed, spore release can occur in the light or dark but is temperature dependent, so tends to peak around midday. Apothecia can last about 20 days at 15 to 20°C, but shrivel after less than 10 days at 25°C. For flowering herbs, spores landing on petals and stamens germinate rapidly (germination within 3-6 hours and infection within 24 hours) in optimum conditions of 15-25°C, continuous leaf wetness and high humidity within the crop. Subsequent infection of leaves and stems depends on petals falling and sticking on leaves. The risk of infection is increased if the leaves are wet because this causes more petals to stick. Infected dead or senescing petals provide nutrients for the invasion of the fungus into leaves and stems. For non-flowering herbs, infection is mainly by air-borne spores landing directly on leaves. Spores can survive on leaves for several weeks until conditions favourable for leaf infection occur. Spore germination and infection depend on the presence of nutrients on leaves, either from plant wounds or senescing plant material. As for flowering herbs, the optimum spore germination and infection conditions are 15-25°C with continuous leaf wetness and high humidity. Once plant infection has occurred, rapid disease progress is favoured by warm (15-20°C) and moist conditions in dense crops.
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