Foliar Diseases and Fungicides in Soybeans

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Each day this week we are releasing information delivered at the Soybean Field Day at the NCDA&CS Piedmont Research Station in September for those who could not attend.

One of the strongest predictors of high soybean yield in recent yield contest analyses was foliar fungicide use, providing a 5.4 bu/A yield advantage on average. Foliar fungicides have also been documented to provide considerable yield protection in small-plot research when disease is present. The most robust mechanism to manage foliar disease is varietal resistance, however if this is not available foliar fungicides are an effective disease management option.

There are many different types of diseases that can affect soybean plants. Growers understand the importance of correctly identifying soybean maladies as diagnosis is the first step towards an effective disease management strategy.

Soybean diseases show up at different, often predictable times during the growing season. In general, scouting for diseases is based mainly on crop development stages. The table below highlights some common soybean diseases found in North Carolina.

Disease Causal organism Growth stage to scout Best time to scout  Where is it likely to occur? Symptoms
Pythium and Fusarium seedling blight Pythium spp.Fusarium spp. Before V2, ideally two to three weeks after planting. Early plantings – April/ early May or when soil temps are cool (<60 F). Low-lying areas of the field with prolonged soil saturation. Check for pre- and post-emergence damping off. Dig seedlings up and check for lesions on hypocotyl and root rot/discoloration.
Phytophthora and Rhizoctonia seedling blight Rhizoctonia solaniPhytophthora spp. Before V2, ideally two to four weeks after planting. Optimum to late plantings – May/early June or when soil temps warm to 70-80 F. Low-lying areas of the field with prolonged soil saturation. Dig seedlings up and look for lesions on hypocotyl (girdling) and root rot/discoloration. 
Frogeye leaf spot Cercospora sojina R1 to R6. Mid July through October Anywhere in the field. Primarily foliar but other plant parts can be affected. Foliar lesions are circular or angular with dark margins and lighter centers. 
Cercospora leaf blight and purple seed stain Cercospora kikuchii Predominantly during late reproductive stages (R3 to R6). July through October. Anywhere in the field. Purple, irregular blotches with a leathery aspect on upper leaves. Seed discoloration varies from pink or pale purple to dark purple.
Sudden death syndrome  Fusarium virguliforme, and others Although infection occurs early in the season, foliar symptoms are expressed around R3 to R6. Late July through October. Cool soil temperatures (<60 F) and wet conditions right after planting. Foliar symptoms include interveinal chlorosis, premature defoliation, and wilting.
Soybean cyst nematode Heterodera glycines 6 weeks after planting up until 3 to 4 weeks before harvest. July through October. Field entryways, along fence lines, low-yielding areas. Stunted plants and chlorosis. Carefully dig roots from the soil, search for small pale/yellow lemon-shaped cysts.

Table adapted from Soybean Research & Information Network. 

For help identifying soybean diseases, contact your local NC State agent and/or submit a sample to the NC State Plant, Disease and Insect Clinic.

Fungicide Research in 2022 Across North Carolina:

  • Locations: Beaufort, Camden, Edgecombe, and Rowan Counties
  • Planted AG59XF0 from late April through early May
  • Revytek, Stratego YLD, Delaro, Lucento, Miravis Top, Priaxor, Aproach Prima, Quadris Top SBX
  • Various rates and timings

– Vinicius Garnica, PhD student and Dr. Rachel Vann, Soybean Extension Specialist.