Sclerotinia Stem Rot [Sclerotinia sclerotiorum]

Symptoms & Diagnosis

Occurance

A wide range of broad-leaved crops and weeds are susceptible including oilseed rape, potatoes, dwarf beans, carrots, celery, lettuce, peas and spring field beans. Note winter field beans are affected by the related species Sclerotinia trifoliorum. A third species, Sclerotinia minor also occurs in the UK but has not yet been reported in oilseed rape.

Pattern of damage

The first leaf and stem lesions are most commonly seen during flowering, usually soon after petals are observed to be sticking to leaves. The leaf lesions are pale brown or white, often centred on an adhering petal and gradually enlarging in size. White stem lesions develop along stems, often originating from leaf petioles. Young lesions have a water-soaked leading edge. More advanced lesions will encircle stems, destroying all underlying tissue and white fluffy mycelium, along with immature white and mature black sclerotia, can be found inside the stem. When conditions are humid, white mycelium and sclerotia are produced on the outside of the stem lesions. The position of lesions on the stem may be a useful guide to when infection occurred. Lesions at the base of the main stem usually occur at the early flowering stage or possibly from leaf infection during the winter. The latter occurs from late autumn onwards and is usually associated with vigorous early sown crops that have large numbers of dead leaves present. Stem lesions may be large and extend from the lower stem downwards into the taproot and upwards so that the smaller branches are affected. The smaller branches may also have discrete white lesions that do not contain sclerotia because of the small volume of rotted tissue. Premature plant death can occur and severely infected stems are brittle and break easily, increasing crop lodging. Severe infection can be seen as “whiteheads” against the backdrop of healthy, green crop from the end of flowering onwards. Plants that die during pod-filling become progressively blackened as sooty moulds develop on them. There are various causes of premature ripening and careful diagnosis is required to distinguish Phoma stem canker, botrytis, phytophthora root rot and verticillium wilt from sclerotinia stem rot.

Life cycle

The pathogen overwinters as sclerotia in soil and crop debris, and can survive for over 10 years when buried in soil. The sclerotia are very variable in size and shape (1-20mm) depending on the size of the host plant and where they are produced. Sclerotia may be introduced into fields in seed. Occasionally plant infection may take place directly from infected crop residues or by mycelial spread from sclerotia. After a period of cold conditioning during the winter, sclerotia mainly germinate in moist soils at temperatures above 10°C to produce saucer shaped fruiting bodies (apothecia). Apothecia are pale brown in colour and 5-15mm in diameter. They liberate air-borne ascospores, during drying conditions. Wet weather may reduce ascospore dispersal if the apothecia become flooded with water. Ascospores are dispersed within the crop or field where they are produced, but may spread to nearby fields if they can escape from the crop canopy. Ascospores land on petals, leaves, pods and stems, but require exogenous nutrients to enable them to germinate and infect oilseed rape plants. Petals and other flower parts are the main nutrient source. Senescent plant tissues and damage (eg hail) may also enable sclerotinia to become established. A key part of the disease cycle is weather that leads to petals sticking firmly to the foliage. Light rain is most likely to give good petal sticking whilst heavy rain can reduce disease risk when petals are washed off the leaves. In Germany, guidance of sclerotinia risk is provided by SkleroPro which uses an infection period requiring at least 23 hours with 80% relative humidity at temperatures above 7ºC. Once petals have stuck to the foliage plants becomes infected within 24 hours, though stem lesions usually become evident 10-14 days later. Stem infection can occur directly in the leaf axil or from leaf lesions that spread down the petiole. Stem lesions increase in size girdling the stem and extending up and down the stem. The diagnostic sclerotia are produced most commonly inside infected stems. These are harvested along with the seed or returned to the ground, providing more inoculum along with those present in stem bases and roots. Secondary spread can occur within crops when an infected plant is in direct contact with other plants. This is most apparent in lodged crops where sclerotinia mycelium quickly spreads from stem to stem. Pods may be affected, particularly in lodged crops. Some spread via the roots may also occur. The sclerotia present in the top 5-7.5 cm of the soil germinate and produce apothecia in the following year. Sclerotia buried deeply can remain dormant for many years and will germinate when brought near the soil surface.

Treatment

Prevention

Epidemics are difficult to predict. Decisions are based mainly on the history of the disease on the farm and the forecasts of unsettled weather at flowering that could lead to petal sticking. Fungicides have almost entirely protectant activity, but can provide very effective control when applied just before infection takes place. Two sprays may be required at high risk sites to protect the crop throughout flowering.

Combat

Sclerotinia may be deeply buried by ploughing down after problems in oilseed rape, but should not be brought back to the soil surface in future years. Biological control agents such as the fungus Coniothyrium minitans can be applied as a spray of spores that parasitise and kill the sclerotinia.

Remarks

Yield losses of over 50% (>2 t/ha) have been reported in severely affected fields. Average losses nationally have been 1-2% in recent years, but widespread epidemics in 2007 and 2008 caused losses of 3- 4%. Once occurring every few years, stem rot has recently started to increase in severity and frequency across the country. Its importance should be considered for the whole rotation because of its persistence in soil and its ability to affect many other crops in the rotation.