The Vegetable Growers News

Microdochium blight lesions on pumpkin vine are caused by a soil borne fungus.

Microdochium Blight Common in Ohio

By Laura Wutz and Mac Riedel Ohio State University

Microdochium blight caused 50-60% losses to pumpkins in Ohio this summer where rainfall was plentiful. Until recently microdochium blight had been regarded as a relatively minor fungal disease. It seems to be increasingly common in Ohio, however. Microdochium blight is caused by a soil borne fungus, plectosporium tabacinum, formerly known as microdochium tabacinum, or fusarium tabacinum. It affects a wide variety of plants in other countries, but has mainly been found to be a pathogen of vine crops in the United States. Researchers in Tennessee published the first report of microdochium blight in 1988. It was found the next year in Ohio on pumpkins in Ross County. Since then, it is reported to have caused damage to pumpkin crops in Wayne, Highland, Miami, Brown, Champaign, Green and Franklin counties. Microdochium blight is now commonly found across the eastern United States. Symptoms are characterized by the development of sunken, tan, spindle-shaped lesions. These lesions form on all aerial parts of the plant. In warm and humid locations where microdochium blight is prevalent, lesions may develop in as little as five days after fungal spores come in contact with the plant. Infected stems become dry and brittle, breaking easily. In severe infections, complete defoliation may occur in as little as two weeks. For growers, the main impact of microdochium blight is on fruit appearance. Lesions on fruit appear as white, tan or silver russeting on the upper surface. These lesions resemble severe mite damage. Once pumpkins are affected with the disease, they are virtually unmarketable. Most literature reports that the microdochium blight fungus survives in more porous, well-drained soils in the top two to three inches. Survival of the fungus is most likely on infested crop residue in the soil. Fungal spores are presumably spread by splash dispersal through rain or irrigation. Disease incidence has been reported to be highest during peak production for intensively-managed fields where irrigation and pumpkin harvesting often occur on a daily basis. Current disease control programs have not given good control of microdochium blight. Unfortunately, none of these fungicides have demonstrated adequate economic control of the disease in Ohio under severe conditions. Fungicide applications for pumpkins in Ohio typically begin the last week of July. Observations in Ohio in 2001 indicate that spray applications for control of microdochium blight must begin early, perhaps in early June. A two to three year crop rotation with a non-vine crop may help; although we are not certain of the complete host range of microdochium blight. If irrigation is used, water plants early in the day so that leaves are dry by evening. Removal of crop debris is also a way of lowering the amount of inoculum in the field. One way of accomplishing this task could be deep plowing. Disking the field may reduce the amount of fungus on the top layer of soil. Non-chemical methods of control for microdochium blight would be more economically sound since three to four early fungicide applications could be eliminated. Protective mulches could possibly delay development or eliminate the disease entirely by preventing dissemination of inoculum from the soil surface to the host plant. There is a current interest in mulches for weed control and improvement of soil tilth. Disease control could offer an additional advantage. Straw mulch was one of the mulches tried in Ohio this summer. Vermicompost was also tried as a protective mulch in 2001. These solids are precomposted at temperatures high enough to reduce weed seeds and pathogens. These non-chemical methods, combined with traditional control methods could give more adequate control of this new disease that has become very destructive in the eastern United States.