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Sclerotinia life cycle and biology

Course: Oilseed rape diseases | Last Updates: 7th October 2015

 
Peter Gladders (Dr)
Plant Pathologist
ADAS UK Ltd
Biography >>
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Sclerotinia stem rot of oilseed rape is a common disease, average levels in crops have been 1-2% plants affected in recent years. There are "hot spots" in most regions where yield losses occur regularly and after problems in recent years many farmers and advisers will be reviewing their strategies to manage this disease.

Host range

Stem rot is caused by the fungus Sclerotinia sclerotiorum, which has a wide host range that extends to many broadleaved crops and weeds. Cereals and grasses are not affected.

Oilseed rape, peas, potatoes, sunflowers and borage can be severely affected while linseed and spring beans have shown more limited problems. Vegetable crops including carrots, celery, dwarf and runner beans and lettuce are very susceptible.

Winter field beans are affected by a different species, Sclerotinia trifoliorum that can kill patches of beans in late winter. Winter beans are therefore a useful second break crop where there are sclerotinia problems.

Disease cycle

Sclerotinia sclerotiorum survives in soil by means of resting bodies called sclerotia. These have a black rind and white internal cortex and are irregular in shape, varying from 1-15mm long by 1-5mm wide.

Sclerotia buried within 5-7cm of the soil surface germinate mainly in spring (March-June) to produced small (5-15mm diameter) pale brown saucer-shaped fruiting bodies (apothecia). The apothecia are the sexual stage of the fungus and produce large numbers of air-borne ascospores that germinate on petals, leaves and other plant surfaces. The spores are usually only dispersed over short distances to adjacent fields. These spores require an external source of nutrients to be able to infect the host plant and this is usually provided by petals or other floral parts in flowering crops such as oilseed rape. In carrots and lettuce, ascospores can survive on leaves for several weeks and only cause disease when leaves are damaged or start to senesce.

When infected petals stick to the foliage, sclerotinia is able to infect the plant within a few hours. Pale brown or white lesion on stems are produced about two weeks after infection and gradually increase in size. When lesions have girdled the stem, the upper part of the plant ripens prematurely. New sclerotia form within the stem cavity, within the stem lesions and occasionally on the outside of the stem. These sclerotia fall back into the soil in crop residues at harvest, though some may be harvested with the seed and spread the pathogen into new fields when they contaminate seed.

Spraying

Infection conditions

Infection by ascospores occurs at temperatures between 7 and 22C when there are long periods of high humidity. Conditions that enable petals to stick firmly to the foliage are important for infection to occur. "Petal stick" events occur infrequently and are difficult to predict. Light rain that moistens petals and leaves appears to be required. Heavy rain washes petals off the leaves and reduces ascospore dispersal if apothecia are flooded.

High-risk conditions occur when several dry days that have provided good conditions for ascospore release are followed by a day with light showers. In 2007, there was infection in late-April in some areas followed by further infection more widely in mid- to late-May.

Risk factors

The history of sclerotinia in oilseed rape and other crops on the farm is an important factor. DEFRA-funded monitoring of 300 crops in the 1990's indicated that farms where sclerotinia had previously affected 20% or more plants of oilseed rape were most at risk.

Sclerotia from previous disease outbreaks produce the spore inoculum that infects the crop. Often inoculum is too low to cause major problems and crops show only 1-2% infected plants. Inoculum levels can be identified by testing petals on agar plates. The technique takes about 10 days to give a result and is not suitable for use on large numbers of fields. In future, it may be possible to develop rapid DNA tests for field use.

Other risk factors include short oilseed rape rotations, other sclerotinia susceptible crops in the rotation (eg, peas, potatoes) and lodging. Short crops tend to more severely affected than tall ones. Within fields, areas with pigeon damage or weed problems can be more severely affected that the surrounding crop.

Control

There are no resistant varieties, though differences may be noted and reflect variation in flowering or crop canopy structure. Lengthening the rotation between susceptible crops provides some benefits, as can deep burial of sclerotia by ploughing. After a severe attack of sclerotinia, there is an option to use a fungal biological control agent, Coniothyrium mintans (available as Contans) that kills the sclerotia. This could be applied post-harvest to reduce the return of viable sclerotia to the soil.

Fungicides

Fungicide chart

HGCA fungicide performance at full dose against sclerotinia in winter oilseed rape in 2006.

A wide range of fungicides are available for sclerotinia control in oilseed rape and can give 80-90% control as shown by 2006 results (see chart).

Data on sclerotinia control in 2006 and 2007 is available from HGCA Appropriate Fungicide Dose project (see www.hgca.com). Results to date indicate that fungicides should be used at or close to full label dose to achieve the best results.

Spray timing is very important as fungicides have little curative activity and they must be used as protectant treatments. The optimal timing is usually at early to mid-flowering. In practice, spray timing should be adjusted so that applications precede rain that is expected from early flowering onwards. Manufacturers recommend applications are made in at least 200 litres/ha of water to ensure good coverage and penetration into the crop canopy.

Economics

Average yield losses from sclerotinia are 50% on each affected plant. Wheeling losses at spray application range from 1% at early flowering to 3% at the end of flowering on 24m tramlines. A crop with 10% sclerotinia and a 4t/ha yield potential would have a 5% yield loss (0.2t/ha) and this should enable fungicide and application + wheeling costs to be recovered. There may also be future benefits for rotations from sclerotinia control that are less easy to quantify.

Key points

  • Assess risk based on previous history of sclerotinia and rotations
  • Check weather forecasts and be prepared to apply fungicides prior to rain from early flowering
  • Use fungicides as protectants
  • Ensure dose and spray volumes follow manufacturer's guidelines
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