Causal Organism and Disease Cycle

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Cylindrocladium black rot (CBR) is caused by the soil inhabiting fungus, Cylindrocladium parasiticum Crous, Wingfield & Alfenas (telomorph Calonectria ilicicola Boedijin & Reitsma). Prior to 1989, the causal organism was called Cylindrocladium crotalariae (C. A. Loos) D. K. Bell & Sobers (telomorph Calonectria crotalariae (C. A. Loos) D. K. Bell & Sobers).

The fungus is clearly visible in diseased fields when it produces reddish-orange perithecia on diseased stems, pegs and pods. These fruiting bodies develop in moist weather on infected tissues near the soil surface. Perithecia represent the only diagnostic sign for positively identifying the disease in the field. Spores produced by these structures are highly sensitive to dessication and thus have limited importance in disease spread, except for possibly localized spread during extended periods of favorable moisture and temperature.

The causal fungus overwinters in soil as microsclerotia, which are clusters of thick-walled cells that collectively are no larger than finely ground pepper. Microsclerotia are produced abundantly in cortical tissues and Rhizobium nodules of infected peanut roots. As infected tissues decompose, microsclerotia are released into soil. Movement of these structures in plant debris and soil represents the primary avenue for disease spread. Transporting infested soil and plant debris on diggers, combines, and tillage equipment probably accounts for a high percentage of field to field spread of the disease. Microsclerotia in crop debris expelled from peanut combines can also be carried long distances by prevailing winds during harvest. Baling and removing peanut hay from infested fields provides another mechanism for disease spread, if the hay is used as livestock feed in fields where peanuts will be grown in subsequent years. Soil temperatures have a profound influence on survival of microsclerotia. Cold winters that freeze soil water in the plow layer and sustained periods of cold at or below 40 F can result in a marked reduction in populations of viable microsclerotia in infested fields.

Seed peanuts from infested fields have been found to carry the fungus at frequencies of less than 1%. Seed treatment fungicides reduce this frequency significantly, but appear to fall short of providing complete eradication.

Studies relating inoculum density and disease incidence have demonstrated that the number of observed infections on roots and the level of symptom expression by plants are directly proportional to microsclerotial densities in soil. Soil temperatures of 20-25C and moisture levels near field capacity are most conducive to infection and rot of peanut roots by C. parasiticum. Irrigation in excess of amounts necessary to alleviate dry weather stress should be avoided early in the growing season and prior to fruit set in late July. Root infection is suppressed markedly at low soil moisture levels and soil temperatures of 30 C and higher. These factors are thought to be responsible for the reduction in disease severity when planting is delayed until May 15 in Virginia.

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Last Updated on March 5, 1998 by Barron Britt Keeling