Wallaces Farmer

Should you use a fungicide seed treatment on low-quality soybean seed?

December 20, 2018

4 Min Read
hands holding soybean infected with fungicide
QUALITY CONCERN: Seed quality is lower across most Midwest states than in recent years. Fungicide seed treatments improve germination scores to acceptable levels on all but the most badly infected lots.

With the 2018 soybean harvest delayed beyond what is considered an ideal window of time, the opportunity for diseases to infect seed pods and, in some instances, the seed itself greatly increased.

Across Iowa and the north-central U.S., seed suppliers have reported that crops of soybeans grown for seed in 2018 are this winter frequently testing positive for the Diaporthe fungus (phomopsis seed decay), which is resulting in lower-than-normal germination rates for seed. Seed decay is characterized by a cracked, shriveled seed with white chalk-colored mold on the seed surface.

Large amounts of rain that fell throughout August and September set up the soybean crop to be at a disadvantage to the Diaporthe , which typically infects soybean pods between R5 (early seeding stage) and R6 (fully seeded pods) growth stages, says Darren Mueller, Iowa State University Extension plant pathologist. This is important because only infections initiated in the pods can infect seeds and cause seed decay.

As soybean plants reach the R7 growth stage (beginning to mature and dry out), pod colonization declines drastically, Mueller says. Seeds will not become infected once moisture is below 19%. However, during periods of wet and warm weather, seed infection and coloniza­tion can continue or resume if seed moisture increases to more than 19%.

Germination scores lower
Infected soybean seed will have a lower probability of germination, when planted in 2019, Mueller says. If soil conditions are wetter and cooler than normal this coming spring, that could drastically impact both the survival and stand count of plants. Dimin­ished seed quality and reduced seed vigor, germination and emergence are all consequences of seed decay. Seed decay can also reduce seed test weight and oil content.

Many seed dealers will want to have their seed treated with a fungicide to increase the chances of germination and prevent seedling diseases, Mueller says. The ISU Seed Lab can test the germination rate of seeds to verify if and what percentage of seeds are infected with the Diaporthe fungus. Depending on the germination rate and incidence of infection, the use of a seed treatment on beans may be warranted.

According to past ISU research, appropriate seed treatments can increase germination rates by 10% to 15%. Given the progress and development of fungicides, germination rates could be further improved, given the right mix.

“However, winter storage of seed can also increase germination rates, because under dry storage conditions, the mycelium of the fungus will die, improving the seed quality,” Mueller says. “A suggested practice is to dry and store low-germination seed over winter and retest the seed in February.”

Which treatment?

When deciding on which fungicides will be the most effective, consult the Crop Protection Network's guide on seed treatments, which includes a chart rating the efficacy of fungicides for several diseases (consult the Phomopsis section for combating Diaporthe).

It is recommended to not use seed lots with more than 20% Diaporthe infection because severely infected or moldy seed will fail to germinate even after being treated, Mueller says.

To be proactive with seed production next year, foliar fungicide applications to protect from seed infection between R3 (beginning pod) and R5 (beginning seed) growth stages may reduce seed infection, especially in seed production fields. Although fungicide applications may reduce disease and improve seed quality, yield may not be impacted, he notes.

Source: Iowa State University

 

Avoid soybean germination issues

Soybean seed lots evaluated by independent seed organizations, such as the Iowa State University Seed Testing Lab and the Crop Improvement Association, are running at lower germination scores so far this year compared to the previous three years.

Disease damage appears to be the culprit. Ramifications range from ordering and locking in specific varieties early to insisting on seed treated with fungicides.

There is a wide range in germination scores for the samples tested. Is low-testing seed an isolated problem in certain areas of the Midwest? No, it appears widespread from areas in Iowa through Illinois and Indiana into Ohio. Very wet conditions in September favored disease development in fields.

What is the take-home message for farmers? First, you will want to check seed germination tags or labels. You may be used to seeing germination scores in the low 90% range, but this year you may be in the mid-to-high 80% range. It may be necessary to adjust the seeding rate at planting time to account for lower germination.

Second, check with your seed sales representative. There could be shortages of certain varieties if they were affected more than others.

Third, this is a year when you will likely want seed treated with a fungicide.

Does fungicide improve germination score? Yes, it may raise scores to a respectable level, so seed can be sold and planted successfully. However, agronomists also point out that if seed lots are in the 40% germination range or lower, fungicide isn’t going to salvage it. — Rod Swoboda

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