Grey rot or southern blight is a disease caused by the fungus Athelia (Sclerotium) rolfsii. It is an important plant pathogen that inhabits the soil, and is responsible for root and stem rot, wilting and damping-off of seedlings. It has a wide range of hosts, around 500 botanical species, including dicotyledons and monocotyledons, distributed in all agricultural regions, predominantly in tropical and subtropical areas, where conditions of high humidity and high temperature followed by periods of drought predominate. It was first observed in tomato crops in Florida in 1892, and it has been regularly reported as causing rotting to the root and stem in tomato plants. Significant losses occur in soils infested by the pathogen, which have a light texture and humidity close to field capacity. In the southern USA, losses can reach 5% of annual production.
Life cycle and appearance of southern blight
The fungus Athelia (Sclerotium) rolfsii is characterized by the production of vigorous mycelium. When the mycelium is young it is white; later it darkens. There are many sclerotia. They are round and their diameter varies from 0.4 to 2 mm. They can be dispersed via water, agricultural tools, and contaminated seeds. The favourable conditions for the development of the pathogen are temperatures between 25 and 30 ºC, relative humidity of over 90%, and a soil pH of under 7. The ideal pH range for germination is between 2.6 and 4.4, but it can occur between 2.6 and 7.7. Very humid soils favour development of the fungus. In the anamorphic form it does not produce spores or fruiting bodies; it only forms hyphae. In addition, germination is induced by the presence of volatile compounds emanating from crop residues in the soil, organic material without suppressive effect, since the fungus needs to grow saprophytically on organic substrate before acting as a pathogen. Injuries make a host more susceptible, but it can invade via direct penetration, generally close to the surface of the stem. In favourable conditions, the sclerotia can germinate in two forms, hyphal or eruptive, causing infection in the plants. The sclerotia can survive in the soil for a minimum of a year. The telemorphic phase corresponds to the basidiomycetes Athelia rolfsii, but this is rarely observed. The fruiting bodies are asexual and there are no spores, forming dark brown or black sclerotia which are rounded, irregular, or compact. The sclerotia have a resistance structure resulting in longevity of more than five years in the soil, in a dry environment. Both the sclerotia and the mycelium are a source of inoculum, as the fungus can survive saprophytically in agricultural debris in the form of mycelium, which then forms the sclerotia.
Symptoms and damage
Symptoms begin with brown, aqueous lesions on the stem. As the damage progresses, it causes darkening and rotting of the stem, resulting in the destruction of the cortex and the main root. As a consequence, leaves higher up in the plant start yellowing. Severely affected plants present girdling of the stem, which causes wilting of the upper part, necrosis, loss of leaves, and death of the plant. Symptoms begin with brown, aqueous lesions on the stem. As the damage progresses, it causes darkening and rotting of the stem, resulting in the destruction of the cortex and the main root. In highly humid conditions, white mycelium growth is observed on the plant’s stem, which may also develop on the adjacent soil. Spherical sclerotia, which are initially white and then darken, also form on this vigorous mycelium. The fungus may reduce the stand of the plants. Damping-off may occur during germination or establishment of the plant (seedling phase), due to the occurrence of fungi inhabiting the soil, or associated with the seeds. In favourable conditions, these fungi cause necrosis of the young tissues and softening of the stem, cotyledons, and roots. The main damage caused by damping-off is reduction of the stand, meaning it is very often necessary to replant, which increases the crop's production costs.