Stem and head rots - Sclerotinia

>> Differentiating S. sclerotiorum from S. minor

Introduction

The two key pathogens within this genus are Sclerotinia sclerotiorum (Lib.) de Bary and Sclerotinia minor Jagger. S. sclerotiorum is the most common and causes a soft wet rot in host tissue on which white mycelium and black sclerotia are produced. The disease occurs all year in the Da Lat region of the Lam dong Province and the Sa Pa region of the Lao Cai Province. The disease can also occur in the winter and spring crops, especially crucifers, grown in the Red River Delta. There are reports of the occurrence of S. minor in Vietnam but this is yet to be properly documented.

Symptoms

Symptoms of Sclerotinia infection begin with the rapid wilting or flagging of the tips of infected branches. The infection sites are characterised by small light green to light-brown watersoaked lesions, which later appear sunken and become elongated and tan. These appear either at ground level or on the lower branches. Older lesions are dark brown with a distinct division between healthy and diseases tissue. Foliage becomes greyish green and chlorotic as necrosis begins, then turns dark brown and withers. Once the stem is girdled, the plant dies. White fluffy mycelium develops on the diseased tissue, especially during periods of high humidity. Numerous sclerotia on and in infected plant tissue are a characteristic sign of the disease. Sclerotia are borne superficially and have a black outer rind and a white inner cortex. The sclerotia of S. sclerotiorum are 0.5-1cm long, but are sometimes larger. In S. sclerotiorum infection of soybean, pod development and fill may be affected, as with sunflower, where infected seeds shrivel and may be replaced by the large irregular shaped sclerotia. Sclerotia may develop inside the pith cavity of diseased stems.

The Pathogen

Sclerotinia is an ascomycete genus characterised by the production of a sexual stage known as an apothecium and black sclerotia. The sclerotia of S. sclerotiorum are large and irregular in shape whilst those of S. minor are small and round. The apothecia are produced from the sclerotia and produce asci and ascospores on the upper surface of the apothecia. The ascospores are easily dispersed in air currents. S. minor does not produce apothecia.

Distribution within Vietnam

Sclerotinia sclerotiorum can be found all year in the Da Lat region of the Lam Dong Province and the Sa Pa region of the Lao Cai Province. The pathogen can also occur in the winter and spring crops grown in the Red River Delta.

Host Range

The host range of Sclerotinia sclerotiorum includes crucifers, pea, carrot, celery, soybean, corn, bean and cucurbits.

Epidemiology

Sclerotinia survives in the soil as sclerotia which accumulate in the soil where an infected crop has been grown and viable sclerotia are found in the plough layer of fields that have not grown a susceptible crop for up to four years after the disease occurred. Plant tissues in contact with the soil and adjacent to a sclerotium are infected by the fast growing white mycelium and senescent or damaged leaflets are easily colonised by Sclerotinia. Cool (18°C) moist soils and high relative humidity (95-100%) favour infection. The optimum pH for germination is 6.5. In these conditions, white fluffy mycelium can be seen in infected plant tissue and on the soil surface, and sclerotia are produced abundantly on infected plant parts. Ploughing the soil disseminates the sclerotia throughout the soil. Infection is usually scattered throughout a crop, but is more concentrated in low-lying areas or where the crop density and humidity is greater and nitrogen levels are high. One sclerotium per 100g of soil is sufficient to cause severe disease.

If the soil remains damp for several weeks, if there are excess weeds increasing humidity around the crop, or if there is a pattern of drying and rewetting, those sclerotia of S. sclerotiorum found in the top 5cm of soil will be stimulated to form apothecia. The apothecia expel ascospores into the air and air currents can carry them to a healthy host where infection may occur. The fungus will attack living tissue, but often infection is initiated via a senescent flower or leaf. S. minor does not form apothecia, and therefore can only infect tissue in direct contact with the soil containing sclerotia.

One of the hazards of Sclerotinia stem decay of sunflower and soybean is the contamination of seed with sclerotia. This may form the basis of trade restrictions when the shipment is intended for human consumption or may facilitate the dissemination of inoculum when the seed including the sclerotia is used for sowing.

Control

Including paddy rice in the rotation may be beneficial in reducing the level of sclerotia in the soil and therefore decreasing the potential for disease. Avoid planting susceptible crops in affected areas for 3 seasons, especially common bean and sunflowers.
Some treatments such as fungicides or fumigants may reduce the incidence of disease, but some may even exacerbate it. As yet, there are no efficient chemical control measures available. Benomyl and Prodione may be effective in controlling this disease. Remove and burn all infected plant material prevent sclerotia accumulating in the soil.
Eliminate weeds that may act as alternate hosts and may also contribute to the humidity required for apothecia germination. Try to keep the canopy open to maximise air flow and minimise humidity.
Infected produce may develop a rapid postharvest rot covered in white mycelium. Only healthy produce should therefore be packed for transport.

Differentiating Sclerotinia minor from S. sclerotiorum S. sclerotiorum and S. minor in culture

Symptoms and Epidemiology

While the symptoms of infection by S. minor are similar to those caused by S. sclerotiorum, there are distinct differences between the two diseases. The sclerotia of S. minor are smaller (0.5-3mm) than S. sclerotiorum, round and black. Sclerotia of S. minor do not produce apothecia , so they are not dispersed by wind currents and are unable to infect upper plant parts. Characteristic Sclerotinia symptoms are therefore only seen at ground level, where host tissue is in direct contact with disease propagules.

Host Range of S. minor

The host range of Sclerotinia minor includes peanut, Brassica spp., potato, sweet potato, lettuce, tomato, tobacco, lettuce, celery, carrot and green bean.