Phyllosticta leaf spot on soybean

Eastern Iowa has received more rain than western Iowa, and some foliar diseases, such as bacterial leaf spot and brown spot, are developing in eastern Iowa soybean fields. A disease relatively new to Iowa is Phyllosticta leaf spot, caused by Phyllosticta sojicola. Limited information is available on Phyllosticta leaf spot; most reports are from southern states. It is considered a minor disease throughout soybean production areas in the United States. A certified crop adviser detected this disease in Muscatine County, and his finding was confirmed by the Iowa State University Plant Disease Clinic. This disease also was found in several fields in Jefferson County.



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Leaves with Phyllosticta leaf spot.

Symptomatic plants are scattered throughout a field. Symptoms may look like those caused by Phytophthora root rot, but close examination reveals leaf scorch symptoms without root rot. New lesions are round or oval. Infected leaves often have lesions starting at the leaf margin and progressing inward. As the disease progresses, lesions have an irregular or V-shape pointing inward. When severe infection occurs, diseased tissues extend through petioles to stems, killing the plants and causing stand reduction. Sometimes, canker-type symptoms can be observed on lower portions of stems in severely infected plants. With cool and moist conditions, pods and seeds can be infected, causing seed discoloration.



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Lesion of Phyllosticta leaf spot on soybean.

The fungus produces numerous small spores, which can spread to healthy leaves and plants, thereby causing new infection. The fungus can survive on seeds and can be spread with infected seeds. Research on survival of this pathogen in and on soybean residues has not been conducted; however, it has been recommended to use rotation and plowing for disease control, indicating the infested residues are possible sources of inoculum.

For disease management, plant pathogen-free seed. If Phyllosticta leaf spot is prevalent in seed fields, conduct a seed health test before saving the beans for seed. If disease is severe in a production field, consider the use of rotation and tillage to reduce infested residues for the next soybean crop.

This article originally appeared on page 146 of the IC-488(18) -- July 22, 2002 issue.

Updated 07/21/2002 - 1:00pm