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Black root rot - Cylindrocladium
The fungus Cylindrocladium Morg. causes black root rot of peanuts and soybean. The losses in these crops can be severe, however in Vietnam, this is not the case. It has been documented, but it does not cause great economic losses.
Yellowing at the top of the plants, perhaps not until the plants are in the early pod stage. Interveinal tissue then turns light brown. Defoliation occurs usually without wilting. Affected plants appear scattered or confined to small areas in affected fields. All underground plant parts will exhibit black rot symptoms, with the tap root first to display necrosis develop a black rot, the pith of the stalk becomes a grey brown, reddish orange perithecia develop on the stem at the soil surface and just above. The stem tissue is sometimes red in the absence of perithecia.
Description of the Pathogen
Cylindrocladium spp. have long cylindrical conidia arranged together and along sterile filament called a stipe attached to one end which leads to a vesicle. The perfect state is Calonectria. Species within this genus are differentiated according to the length and septation of conidia and the shape and nature of the stipe and vesicle.
To isolate the fungus from soil, seed is germinated on the soil surface to act as a bait for the pathogen. The fungus can then be lifted off the seedling roots with a needle under a microscope.
Distribution within Vietnam
This disease has been recorded in the central part of the country and the mid-lands of Northern Vietnam.
Peanuts, soybeans, lychee and coffee are affected by Cylindrocladium.
Microsclerotia are the primary survival and dispersal propagules. They are formed abundantly in infected roots and Rhizobium nodules. As infected tissues decompose, the microsclerotia are released into the soil and disseminated by farm equipment and wind.
Cold periods where the soil temperature goes below 5°C significantly reduces the number of viable sclerotia in the field.
The pathogen is favoured by a soil temperature of 25°C and moisture levels near field capacity. If there are heavy rains early in the growing season, infection is likely to occur. When a period of moisture stress follows, the number of functional roots declines and the aboveground plant parts exhibit symptoms of the disease.
Perithecia may develop on diseased tissue near the soil surface. The ascospores are sensitive to desiccation , so are thought to be short-lived and only important to the disease cycle when dispersed by rain-splash or possibly insects.
Cultural methods appear to be the most feasible in controlling this pathogen as chemical control is not effective. Avoiding a rotation with both soybean and peanut will aid in keeping inoculum levels down. Rotation with non-hosts such as corn, tobacco and cotton are useful alternatives. The exposure of microsclerotia to winter temperatures by tillage, avoiding cover crops and removing hay from the soil surface may also reduce inoculum levels. Machinery should not be taken from an affected field into an unaffected field without cleaning.