No. 2 Article 6/April 4, 2008

Fungicide Seed Treatments and Poor-Quality Soybean Seed

Soybean seed with lower-than-normal germination, especially in certain maturity groups, is very common this year. This low germination is apparently due to thin seed coats, which make seeds more prone to mechanical damage. I have received a few questions about how fungicide seed treatments may impact germination on these seeds. Fungicides will not increase germination of poor-quality seeds when the poor quality is due to mechanical damage. If it is due to fungal infection, fungicide seed treatments could increase germination in some cases.

Increased risk of seedling blight could be associated with planting seeds with damaged seed coats. Research conducted in the 1970s at Ohio State University indicated that the quality of soybean seedlings that emerged from seeds with cracked or scarified seed coats was poorer than of seedlings that emerged from seeds with intact seed coats. Exudation of nutrients from seed with damaged seed coats can be greater than from seed with intact seed coats. This nutrient exudation can stimulate Pythium, a soilborne pathogen that can cause seedling blight of soybean. For this reason, treatment with a fungicide of seed with damaged seed coats could provide some protection against Pythium and other soilborne pathogens that can cause seedling blight.

A confounding factor is planting date. When planting into cool soils, fungicide seed treatments may have a bigger impact than when planting into warmer soils. Under cool soil conditions, seeds germinate more slowly and may be more at risk to some Pythium species that do well in low temperatures. With soils presently saturated across the state and forecasts for additional rain, it is unlikely that much "early" planting will occur. Later-planted fields will likely mean planting into warmer soils, where fungicide seed treatments may not provide a benefit.

Some seed companies have recommended not treating the "poor germination" soybean seed this year because of fear of increasing mechanical damage that could happen in the treating process. In these cases, a planter box treatment could be used. If the decision is made to treat soybean seeds or to use a planter box treatment, it is important to use a product that contains either mefenoxam or metalaxyl, both of which provide specific protection against Pythium. In addition, the product should contain at least one other active ingredient (fludioxonil, trifloxystrobin, carboxin, and others) that will provide protection against other pathogens.--Carl A. Bradley

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