Sclerotinia sclerotiorum, S. minor and S. trifoliorum particularly affecting legumes such as green, broad and faba beans.
Disease syndrome has differing names that relate to particular crops - celery (causing pink rot), carrot and occasionally parsnip (white mould, cottony rot, watery soft rot), parsley (white mould), brassicas (white mould, sclerotinia rot, watery soft rot), peppers (white mould, sclerotinia rot), green beans (white mould), broad beans (white mould, sclerotinia rot), lettuce (lettuce drop), tomato (white mould, sclerotinia rot).
Soil-borne fungus that perennates as dormant sclerotia. These vary in size with different species of Sclerotinia. Temperatures above 15 degsC and moist soils favour germination, which leads to the formation of small cup-shaped, light-brown sexual fruiting bodies (apothecia) that produce wind-dispersed ascospores. These infect leaves and other soft tissues, a process requiring high humidity.
Sclerotinia minor is capable of secondary spread as vegetative mycelium growing between plants and is less capable of producing apothecia. Sclerotia mature and are returned into soils as diseased crops decay.
Sclerotinia spp. can infect and destroy crops in the field and in store after harvest. Infected brassica and celery seedlings and transplants develop brown leaf blights accompanied by white fluffy mycelium and eventually sclerotial formation. In the field a typical pink-to-brown rot develops at the base of collapsing celery petioles.
Symptoms on carrots are brown water-soaked lesions (5-10mm in diameter) on foliage, crowns and upper root surfaces. White mycelium spreads aggressively in moist conditions, ultimately forming large numbers of hard black sclerotia. Affected carrot roots decay as soft rotting spreads, releasing resting sclerotia. In store, diseased roots are colonised by a white, fluffy mycelium becoming slimy, soft and rotten.
Damaged root systems in crops such as brassicas and parsley result in wilting of foliage and seed stalks. Early invasion of maturing brassicas reduces cauliflower and cabbage heads to soft, brown, water-soaked tissues covered in white or pinkish mycelium carrying black sclerotia. The canopies of infected crops such as beans, peas, lettuce, tomatoes and peppers collapse as a result of water-soaked lesions that destroy the foliage, crowns and lower stems, and may invade developing fruits.
Initially the foliage, stems and pods, especially of legume crops, show small, irregular, water-soaked lesions. White fluffy mycelium and black sclerotia develop from these. Badly affected plants wilt, collapse and die. This is especially the case with lettuce, where infection leads to brown, soft, watery decay, particularly damaging areas of the stem base and upper root.
Diseases caused by Sclerotinia spp. are encouraged by mild (temperatures above 10-15 degsC), moist or humid conditions both aerially and in the soil.
Unbalanced soil nutrition such as calcium or boron deficiencies or the use of excessive amounts of ammonia-nitrogen (NH4-N) predisposes crops to white mould development. Physiological stresses such as blackheart in celery have been associated with the development of white mould.
Integrated disease management
Resistant cultivars No cultivars carrying monogenic resistance have yet been bred. Differences in cultivar susceptibility result from variations in plant architecture. Green beans with upright, open canopies are less prone to these diseases.
Husbandry control measures Crop rotation that requires a sufficient break from susceptible crops, allowing the natural destruction of resting sclerotia by antagonistic predatory soil microbes. Deep ploughing that buries sclerotia below the rooting depth of crops helps to prevent invasion. Crop density should be adjusted, allowing ample air movement between plants and preventing the build-up of high humidity within crops. Similarly, irrigation should be carefully timed or applied from trickle systems so foliage does not stay wet overnight.
Avoid excessive use of nitrogen fertilisers but applications of potassium are beneficial. Micro-nutrient levels, especially calcium and boron, should be regularly monitored and increased when appropriate, preventing stress conditions developing within vulnerable crops.
For some crops, use of biological control with forms of antagonistic fungus Coniothyrium minitans may be an effective control. Stored crops such as carrots should be carefully handled, preventing bruising at all stages of harvesting and washing. All stillages should be effectively cleaned and disinfected before use. Stored carrots should be held at 0 degsC and 95 per cent relative humidity. Regular inspection and the removal of diseased roots is essential. Use only high-grade seed certified free from sclerotia. Avoid farm-saved seed.
Regular crop monitoring is essential so that fungicides, where approved and allowable, may be applied well in advance of epidemic development. This is especially important for seedlings and transplants so that pathogen spread is halted. Growers should avoid cropping near fields of oil seed rape because this is a particularly susceptible host offering ample opportunity for disease development and spread.
Currently allowable chemical treatments include:
- Azoxystrobin (off-label, EAMU - lettuce and celery).
- Cyprodinil + fludioxonil (peas, green and faba beans).
- Azoxystrobin, azoxystrobin (off-label, EAMU), azoxystrobin + difenconazole, azoxystrobin + difenconazole (off-label, EAMU), boscalid + pyraclostrobin, fenpropimorph (off-label, EAMU), prothioconazole, sulphur (off-label, EAMU), tebuconazole + trifloxystrobin (carrots and parsnips).
For use on all fresh produce, vegetable crop growers must in advance of use ensure that a particular commercial agrochemical formulation is legally acceptable for their particular crop/husbandry regime and is also accepted by the intended purchaser's crop-quality standards specification as agreed with the relevant crop technologist.