Sclerotinia rot (white mould)

Symptoms

On the stems, at the base of the plants and on parts in contact with the soil, a watery, soft rot appears, covered with a dense, white, cottony fungal mycelium. Within the tissue and on its surface, black, hard resting bodies (sclerotia) the size of a grain form, which are a diagnostic feature. Infected plants wilt, snap and die; the disease attacks many species of vegetables and ornamentals. The infection usually begins as a pale, watery, almost translucent spot that quickly enlarges into a soft rot; in humid air it becomes overgrown with the characteristic, cotton-white mycelium. As the disease progresses, the infected stem becomes girdled and constricted and loses its rigidity, so that the plant above the point of infection wilts and dies, while the tissue, once dried out, bleaches and turns dry, stringy and frayed. The inside of affected stems (the pith) is often hollow and filled with black sclerotia the size of cereal grains — this is the most reliable feature distinguishing this disease. Sclerotinia infects a very wide range of hosts (including lettuce, beans, cucumbers, carrot, brassicas, sunflower, oilseed rape and numerous perennials and ornamentals), and the rot also develops after harvest, in stored roots and heads. In spring, small, cream-brown, cup-shaped fruiting bodies (apothecia) grow from the sclerotia lying in the soil and release ascospores, which infect plants most often through spent petals and weakened tissues.

Treatment

Removing and destroying infected plants together with the surrounding soil, in which the sclerotia survive for many years. In field and protected cultivation, fungicide treatments and limiting humidity are used; infected plant residues should not be composted. A single infected plant can no longer be cured — the key is to remove it as quickly as possible together with its root ball and the adhering soil, before the sclerotia mature and replenish the soil for years to come (the resting bodies usually remain viable for several years, and in extreme cases even up to 8–10 years). At the same time, humidity must be lowered immediately: ventilate the covers, thin out the plantings, limit leaf wetting and improve drainage, because without free water the mycelium does not develop further. Fungicide treatments are preventive, not curative — they are effective only when they protect the tissues and spent petals against ascospore infection, so they are applied within the flowering window of susceptible plants, not after the rot has appeared. Systemic fungicides from various groups with differing modes of action are used (SDHI, strobilurins/QoI, triazoles), rotating them to limit the risk of the pathogen developing resistance. Biological support comes from introducing into the soil the antagonistic fungus Coniothyrium minitans, which parasitises the sclerotia and gradually reduces their reservoir — but it acts preventively and over the long term and does not replace the removal of diseased plants.

Prevention

A wide crop rotation with no susceptible plants for several years, airy spacing, avoiding excessive moisture and density, weeding, deep ploughing to bury the sclerotia and good ventilation under cover. Because the pathogen infects several hundred species of dicotyledonous plants, the rotation should rely on species that are not its hosts — cereals and grasses — and maintain at least a 3–4 year break between susceptible crops (brassicas, legumes, lettuce, carrot, sunflower, oilseed rape). Deep ploughing moves the sclerotia below the zone from which they could produce apothecia and effectively infect plants. Irrigation is carried out by drip or at the base, avoiding leaf wetting, and if overhead — in the morning, so that plants can dry off before nightfall. Moderate nitrogen fertilisation also helps (lush, dense stands hold moisture longer and are more susceptible), as does careful removal of crop residues and host weeds, and under cover — soil solarisation and efficient ventilation that lowers the relative humidity. The reservoir of sclerotia in the soil can be further reduced by the preventive introduction of the fungus Coniothyrium minitans.

How to tell it apart from look-alikes

Control — organic and chemical methods

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