Fungicide seed treatments in soybean

Fungicide seed treatments have proven invaluable to corn production and planting treated seed is a standard practice. What about soybean production – should fungicide seed treatments be standard practice? Why use a fungicide seed treatment? Fungicide seed treatment fungicides are used for managing seedling diseases which are caused by fungal pathogens. Seedling diseases result in lower plant populations and reduced plant vigor and as a result, lower yields in corn. In contrast a reduced stand of soybean may not always mean a lower yield.

Several fungi are able to infect germinating and cause damping off. These fungi are most often soilborne, e.g., Phytophthora sojae, Pythium, Rhizoctonia, and Fusarium spp, however seedborne fungi, e.g., Phomopsis, can also affect germination and cause damping off. Each of the pathogens is favored by different environmental conditions at and soon after planting. Very, wet conditions are required for infection by Phytophthora and Pythium, where as infections by Rhizoctonia and Fusarium is favored by drier soil conditions. Soil temperature is also important. Pythium prefers cool soils (<60F), while Phytophthora prefers warmer soils (60 – 75 F) and Rhizoctonia prefers even warmer soils (>75 F). 

Seedling blights caused by Pythium and Phytophthora are characterized by a soft, wet rot of the hypocotyl and root tissues (Figure 1). Only laboratory tests are able to distinguish between these two pathogens. Symptoms associated with infection by Rhizoctonia are easily recognized as red, sunken lesions on the plant stem at soil level and poor lateral root development (Figure 2). Light brown lesions on the lateral roots are symptomatic of Fusarium infection.

Some production practices increase the risk of seedling blights
A recent change in soybean planting date in Iowa is likely to lead to an increase risk of seedling blight. The yield potential of soybean is increased if soybean are planted early. Soybean planting should be initiated on April 25 for the southern 2/3 of Iowa and May 1 for the northern 1/3 of Iowa, if seedbed conditions are good. However, cool and wet soils are frequently encountered at these planting dates and these will slow seedling growth and make them more vulnerable to seed rot and seedling blight caused by plant pathogenic fungi that may reduce stand.

In addition to planting early, many growers should also reduce their seeding rate.  Soybean seed prices have increased a lot over the years and will continue to increase in the future. For that reason many growers have started to reduce their seeding rate to save dollars. A lower seeding rate does not always mean a lower yield since the soybean plant is able to compensate for space. However, the risk of a lower yield is increased at lower seeding rates, especially if low seeding rate is used in combination with early planting date. Numerous experiments conducted in Iowa over the last 4 years with the soybean checkoff and the Iowa Soybean Association, show that a uniform stand of 100,000 plants per acre at harvest is enough to maximize yield. 

It is difficult to determine pre-planting what the risk of seedling blight might be in any season. There are no methods currently available to quantify pathogen populations in a field. In addition, environmental conditions soon after planting greatly influence disease risk. Who can accurately predict what weather conditions are likely to occur within the next couple weeks? In addition, most fungicide seed treatments are active only for 2-3 weeks. Thus, when germination and emergence are slowed due to cool soil temperatures, seedlings are still vulnerable to infection by pathogens. This is particularly true for those varieties with partial resistance to P. sojae, since this type of resistance is only expressed from growth stage R1.

Growers can consider the following categories to determine if the risk of stand loss due to disease warrants using fungicide seed treatments in Iowa:

1. History of stand establishment problems
2. Planting early (late April/early May) using minimum tillage or no-tillage practices in poorly drained fields
3. Planting in poorly drained fields using a low seeding rate (~125,000 plants per acre)
4. Planting moderate to poor quality seed
5. Have to replant because of stand establishment problems

Choosing a fungicide seed treatment
Not all seed treatment fungicides are equally effective against all fungal pathogens. Products that contain the active ingredients metalaxyl or mefenoxam, e.g. Allegiance, and Apron XL are effective against Pythium and Phytophthora. Other active ingredients, e.g., azoxystrobin, captan, carboxin, fludioxonil, PCNB, thiram and thiabendazole are effective against Fusarium, Phomopsis and Rhizoctonia. Obviously, choosing a fungicide seed treatment will depend on knowledge of what disease problems are prevalent in a particular field. Combination seed treatments are available, and can be used when this information is not available. A nice summary article on current seed treatments available was written by Dr. Laura Sweets at University of Missouri.

Fungicide seed treatments in soybean are often regarded as insurance in Iowa. We have conducted more than two dozen fungicide seed treatment experiments across Iowa over the last four years, and have not seen any negative impact on either stand or yield.  We have not consistently seen a lot of positive affect either. As with all types of insurance, regular returns are rare, but when you do need it you are thankful to have it. Although we can not predict what the odds are of getting your money back, chances are if your production field falls into one of the five categories mentioned above, you will be glad you invested in a fungicide seed treatment.

This article was published in the Iowa State University Integrated Crop Management Newsletter on March 26, 2007 by Palle Pedersen, Department of Agronomy and Alison Robertson, Department of Plant Pathology.