G1852 · Index: Crops, Crop Production, Field Crops

Issued March 2017

Seed Treatment Fungicides for Soybeans

Loren J. Giesler, Extension Plant Pathologist

Joshua J. Miller, Doctor of Plant Health

Seedling diseases frequently cause economic yield losses for soybean farmers across the Midwest. These most often manifest as damping off and seed rot, which result in poor emergence, loss of stand, and reduced yield. The most common pathogens that cause seedling diseases are species of Fusarium, Phytophthora, Pythium, and Rhizoctonia. All four are capable of killing soybean seedlings or at least causing damage sufficient to reduce the yield potential of infected plants.

The chance of seedling diseases increases when conditions are cool and wet. Planting date recommendations continue to move earlier in the season to maximize yield, and this increases the risk of favorable conditions for pathogens to cause seedling diseases. When growers plant soybeans early in the season, seed treatment fungicides may be an effective way to reduce the risk of early seedling diseases. However, as soybeans are planted later in the season and conditions are less likely to be cool and wet, the chance of an economic benefit from using a seed treatment fungicide declines.

In addition to planting date, field history is a key component of the decision-making process for managing soybean seedling diseases. In most cases, problem fields will have a history of seedling emergence or post-emergence problems. The distribution of the problem area within a field, however, can increase significantly in a wet year and may have a substantial impact on stand throughout the field or particularly in low, poorly drained or compacted areas in the field.

An early season disease that manifests later in the season is sudden death syndrome (SDS) caused by F. virguliforme. This fungus infects the soybean host early in the season during cool, wet conditions. Typically, after mid-July the fungus infects the vascular system of the plant and releases toxins. Symptoms first appear after heavy rains when plants increase water uptake from the soil and, consequently, move the toxin up the plant, as well. Until recently, the only management options for SDS control were delayed planting, extended rotations out of soybeans, and tolerant variety selection. However, in 2014 Bayer CropScience released ILeVO(R) seed treatment for SDS control. Field trials have shown positive yield responses when significant amounts of SDS were present, but more research is needed to conclude what percentage of the field must be infected and how severe the disease must be for the seed treatment to be economically beneficial.

The correct fungicide seed treatment is critical for effective disease control. Fungicides used as protectants (contacts) are effective only on the seed surface, while systemic fungicides are absorbed by the emerging seedling and inhibit or kill the fungus inside host plant tissues. Contact fungicides usually have shorter residual activity than systemic fungicides. Contact fungicides used for soybean seed treatment include captan, fludioxonil, PCNB, and thiram. Systemic fungicides used for soybean seed treatment include azoxystrobin, caroboxin, mefenoxam, metalaxyl, trifloxystrobin, and thiabendazole. When selecting a fungicide, make sure to use products with activity against the pathogen in question as activity will not be the same across all pathogen types. Fusarium and Rhizoctonia are both fungi, but Phytophthora and Pythium are actually water molds referred to as fungus-like.

Fields with a history of Phytophthora require additional management. The compounds labeled for control are ethaboxam, metalaxyl, and mefenoxam, and increased rates may be required (Table 3). Mefenoxam is one of the chemical compounds that has been isolated from metalaxyl and is the most active part of metalaxyl products. As a result of isolating the active component, products containing mefenoxam are typically used at half the active ingredient rate of metalaxyl products.

Seed treatment fungicides are available in a variety of formulations. Some products are labeled for commercial use in slurry- and mist-type seed treaters. Other products are labeled for on-farm application and commonly are referred to as hopper-box or planter-box treatments. Thorough seed coverage is required for maximum benefit in all cases. To attain good seed coverage with on-farm application, most seed treatment companies recommend adding half of the seed to the planter box, then adding half of the seed treatment product and mixing thoroughly. Repeat this with the remaining seed and fungicide. Good coverage can also be obtained by mixing the seed and treatment in a suitable container before adding the treated seed to the planter box. Always read and follow label directions before making any chemical applications. A list of common single active ingredient (Table 1) and multiple active ingredient (Table 2) seed treatments and application information, as well as disease ratings for each active ingredient component, are provided below.

If you need assistance with seedling disease identification, samples can be submitted to the Plant and Pest Diagnostic Clinic at UNL. Refer to http://cropwatch.unl.edu/plantdisease/pest-samples on how to prepare and submit samples.