White mould (Sclerotinia sclerotiorum)

White mould: biology

White mould overwinters as sclerotia (black, hard, irregular resting structures) in the soil or in potato plant debris. Sclerotia are produced either inside or on infected stems and can survive for at least three years in the soil. Under conditions of high fertility, dense canopies and high humidity, sclerotia near the soil surface germinate to form structures called apothecia which develop in the shade of the crop canopy and release spores. The spores infect leaves and blossoms when plant surfaces are wet for an extended period of time (e.g. 48 hours or more). Infected petals that fall and stick to stems or leaves cause more infections. In dry conditions, sclerotia germinate by producing mycelium that penetrates the stems at ground level.

Once white mould has infected lower stems it saps nutrients from the plant, limiting yield. It can infect almost any broadleaf plant, so it has many alternate hosts, including weeds, canola and soybeans. White mould spores can also be transported great distances over winds. White mould flourishes between temperatures of 15°C -25°C and humid conditions.

Early in the spring, the trumpet shaped fruiting bodies, or apothecia, are visible to the naked eye on the soil.

Symptoms of white mould can appear on both leaves and stems. Low lying areas are the first place to look for symptoms. Specifically:

• On the surface of infected stems, the fungus produces a dense white mycelium. As infection progresses, lesions may girdle the stems, causing the leaves to wilt above the infected area. When the weather is dry, lesions dry out and turn tan or bleached white in color. Severely affected stems become hollow and break easily.
• On leaves, infections usually start in spots where infected blossoms fell or at the base of the leaflets. Young lesions have a water-soaked appearance and are pale green and of irregular shape. Later a white fungal growth develops on lesions.