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Stem rot and blight - Sclerotium rolfsii
Sclerotium rolfsii Sacc. causes a stem base rot, also known as southern blight or sclerotium rot, which affects a wide range of crops. It is important on vegetable crops in most parts of Vietnam. Other species of Sclerotium, such as S. cepivorum Berk. on onion, may become important in the future.
Water-soaked areas initially develop on the stem at or near the soil surface. Infections typically arise here because of the increased humidity and immediate contact between disease propagules and the plant tissue. From this point, the pathogen spreads down into the root system or up into the canopy. Leaf petioles may be rotted around the stem base, causing wilting and yellowing of the lower leaves. Infected stem tissue turns light to dark brown and the entire plant may appear to collapse. The pathogen often girdles the stem at the soil line and white string-like mycelium can grow out from the stem base onto the soil surface. Small spherical sclerotia, around 1-2 mm in diameter, first white then brown, develop soon after on the soil and affected plant parts. The plant may die within days. Patches of infected plants often appear.
Description of the Pathogen
S. rolfsii is a basidiomycete and the white hyphae are typically course and thread-like. Sclerotia, produced readily by the fungus are 0.5-1.5mm in diameter, and they are initially white but turn brown with age. S. rolfsii can easily be cultured on potato dextrose agar at 25-35°C.
Distribution within Vietnam and Host Range
Sclerotium rolfsii has been isolated form the following crops from the provinces indicated:
This disease has been observed in all parts of Vietnam on many other crops, such as lettuce, sesame, watermelon, melon, cucurbits, cabbage, cauliflower, broccoli, kohlrabi and medicinal plants.
Warm, moist soil favours disease development. S. rolfsii can survive for many years in the soil as sclerotia in the soil or on host debris. The optimum growth of the fungus is 30°C, decreasing dramatically below 15°C and above 37°C. Volatile compounds produced by senescing plant tissue stimulate sclerotial germination. Hyphae are then able to directly penetrate host tissue due to the production of cellulolytic and pectinolytic enzymes and oxalic acid. The fungus can be spread by farm equipment, soil and water movement or inoculum introduced with transplanted seedlings. THe mode of dispersal may have an effect on distribution patterns within a field.
The increase in the incidence of diseases caused by Sclerotium species in the Red River Delta of Northern Vietnam is thought to be a result of a shift from traditional rice rotations to continuous vegetable cultivation. The traditional two seasons of paddy rice in the vegetable growing rotation may have aided in reducing the survival of sclerotia in the soil between vegetable crops.
There are potential benefits in the inclusion of paddy rice into a vegetable growing rotation, in that the survival of sclerotia may be reduced with the flooding of the soil. The disease cycle can also be broken with tolerant or resistant crops. Yams and sweet potato may be such crops, as they have not yet been found to exhibit symptoms of Sclerotium disease in Northern Vietnam.