CORN Newsletter: 2009-35, October 12, 2009

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C.O.R.N Newsletter 2009-35
October 12, 2009 - October 20, 2009

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Check for Phomopsis Seed Rot
by Anne Dorrance


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Delayed harvest due to rain can set the stage for Phomopsis seed rot. This is a fungal disease of soybean seed which is characterized by chalky white, shriveled, shrunken soybeans. For more information, visit The Ohio State University soybean disease web site at the following address: http://www.oardc.ohio-state.edu/ohiofieldcropdisease/soybeans/phomopsis.htm. There are three fungi that are associated with this disease, however the seed rot is predominately caused by Phomopsis longicolla. The other two fungi involved are better known for the black dots that they form on pods and stems, Diaporthe phaseolorum var. sojae and D. phaseolorum var. caulivora. All three fungi can be isolated from poor quality seed, but Phomposis is the one most commonly recovered.

Seeds can also be infected and not show symptoms, but many of these when put through the germination tests may die. On a more positive note, the mycelium in some of the infected seeds will die when the soybeans are dried for winter storage, and germination tests may improve somewhat over the course of the winter. Thus it is critical to harvest affected fields as quickly as possible (yes, I have read the forecast for this week and it is going to be miserable).

Both seeds and soybean debris are sources of inoculum for Phomopsis. Fungicide seed treatments can greatly help with the seed born inoculum. Fludioxonil is the only fungicide that will have a measurable effect in managing this disease for seed lots with low levels of infection (20-25%). The strobilurins and metalaxyl/mefenoxam are not effective against this fungus on the seed. There is also resistance to this pathogen. Studies from Missouri definitively showed that Phomopsis can readily be managed by having resistance in the varieties one is planting. If you have a field that has a history of high levels of the disease, be sure to work with your seed dealer to get the best resistant variety possible. Fields that are heavily impacted by Phomopsis should be harvested as quickly as possible to prevent further colonization of more seed and tilled to bury the infected residue. For no-tillers, rotate this field away from soybeans for a couple of years to drop the inoculum level.

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C.O.R.N. is a summary of crop observations, related information, and appropriate recommendations for Ohio Crop Producers and Industry. C.O.R.N. is produced by the Ohio State University Extension Agronomy Team, State Specialists at The Ohio State University and Ohio Agricultural Research and Development Center. C.O.R.N. Questions are directed to State Specialists, Extension Associates, and Agents associated with Ohio State University Extension and the Ohio Agricultural Research and Development Center at The Ohio State University.

Information presented above and where trade names are used, they are supplied with the understanding that no discrimination is intended and no endorsement by Ohio State University Extension is implied. Although every attempt is made to produce information that is complete, timely, and accurate, the pesticide user bears responsibility of consulting the pesticide label and adhering to those directions.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Keith L. Smith, Director, Ohio State University Extension.

TDD # 1 (800) 589-8292 (Ohio only) or (614) 292-1868


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