Sclerotinia

Pathogen

Fungus

Hosts

Oilseed rape, peas, potatoes, carrots, lettuce, beans and other vegetable crops

Symptoms

Bleached/fawn stem lesions and hard, black resting bodies (sclerotia) within the central stem cavity or bleached stems in stubble after harvest.

Development

Leaf infection occasionally occurs during the winter and this can lead to early stem infection but crops are most at risk of Sclerotinia infection from the onset of flowering. During flowering initial infection comes from mainly airborne ascospores landing on petals. The disease develops when petals fall and stick to leaves or stems with moisture from light rain.

Decaying petals provide sufficient nutrition for the fungus to penetrate the leaf cuticle resulting in pale brown or white lesions. Stem lesions develop where the pathogen spreads by mycelial growth from leaf lesions and where initial infection is in the leaf axil.

Stem lesions develop, spread up and down the stem and may be covered in white fungal growth in humid weather. When infection occurs towards the end of flowering, more lesions tend to develop on the smaller branches than on the main stem. When lesions have girdled the stem, the stems and pods beyond the lesions ripen prematurely. In lodged crops, Sclerotinia can spread rapidly by plant to plant contact.

The fungus completes its lifecycle by forming irregular black resting bodies known as Sclerotia in the stem cavity and in the roots when lesions are at stem bases. Sclerotia can also form on the outside of the lesion following suitably humid weather.

With severe infection plant stems are weakened by lesions. This can cause lodging and stem splitting releasing Sclerotia back into the soil to become the source of inoculum for future crops.

Crop are most at risk where acospores are produced within an oilseed rape crop, however some spores are dispersed more widely from other fields into surrounding crops. Acospores land on petals and infect the plant when temperatures are above 7°C and there are long periods of humidity.

Favourable Factors

Sclerotia occur within the soil profile and those in surface layers germinate in spring when soil temperatures are above 10°C to form apothecia. These trumpet-like structures are 5mm to 15mm in diameter and produce acospores which are discharged into the air. Soils must be moist for sclerotia germination to occur.

Warm, wet conditions during flowering, long periods of leaf wetness and proximity to other susceptible brassica or legume crops.

Importance

Sclerotinia is an important disease of oilseed rape causing lodging and pod splitting leading to yield losses of 30% to 50% in severely affected crops. Attacks are very variable from year to year and between fields on the same farm in the same year.

Control

• There are no known resistant varieties

• Adopt a rotation of at least four years between oilseed rape, pea and bean crops

• Destroy or bury infected stubble

• Ploughing a field after oilseed rape buries sclerotia and impairs their ability to germinate if they can be kept deeply buried

• Avoid excessive nitrogen applications so that crop canopies are not too dense or likely to lodge.

• Foliar fungicide treatment. Optimum application timing is usually prior to mid flowering, ahead of the main period of petal fall. A repeat application will be required 3 weeks later if the crop is still flowering.

• Soil incorporation of biological control agents such as Contans WG.