MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

As growers strive for higher yields, there is a tendency to push planting dates to capture yield increases that are associated with early season planting. Seed treatment for soybeans is recommended to control seed-borne infection, to improve stand emergence, and when seed quality is poor. Many seed treatment fungicides are offered for sale, but the efficacy of most of them have not been evaluated in university research trials in Michigan.

A two-year project funded by Michigan Soybean Promotion Committee (MSPC) to evaluate some of the newer products available to growers was completed in 2008. The project evaluated the effect of fungicidal seed treatments on stand establishment, control of common seedling diseases, yield and seed quality. Pythium, Phytophthora, Rhizoctonia, and Fusarium spp. are pathogens that are commonly responsible for seedling diseases of soybeans. Pythium is active when soil temperatures are less than 60ºF, and soils are wet, conditions that may be prevalent during early plantings. Fusarium spp. are active over a broader temperature range (57-73°F), depending on species.

During early planting, cool temperatures and wet soils create favorable conditions for seed or seedling rots that may be caused by either Pythium or Fusarium. As soil temperature increases, the risk of Pythium damping-off decreases. Phytophthora and Rhizoctonia are most active when soil temperatures are warmer, around 70-80ºF, and tend to be associated with late-planted soybeans.

In our study, we evaluated six commonly available fungicidal seed treatments on MG 2.4 Roundup Ready soybeans at two planting locations in Michigan. Treatments included Apron MAXX at a high rate ((recommended for Phytophthora), 2.5 fl. oz fludioxanil + mefanoxam, + 0.48 fl. oz mefanoxam/cwt), Apron MAXX at a low rate (fludioxanil+mefanoxam, 1.5 fl. oz/cwt), Kodiak HB (Bacillus subtilis, 4.5 oz/cwt), Yield Shield (0.1 oz Bacillus pumilus /cwt), Trilex AL(0.32 fl oz trifloxystrobin + 0.20 fl oz metalaxyl/cwt), Trilex AL + Yield Shield (0.32 fl oz trifloxystrobin + 0.20 fl oz metalaxyl + 0.1 oz Bacillus pumilis /cwt). The “high rate” of Apron MAXX was the only treatment labeled for use against Phytophthora in addition to Pythium spp., Fusarium spp. and Rhizoctonia. The “low rate” of Apron MAXX, Trilex AL and Trilex AL + Yield Shield are labeled for use against Pythium spp., Fusarium spp.and Rhizoctonia. The two biological controls in the study, Kodiak and Yield Shield, are both labeled for use against Fusarium spp. and Rhizoctonia.

Seeds were planted at a depth of one and a half-inches in 30-inch rows, with eight seeds per foot at the Bean and Beet Farm on May 12, 2008. The soil temperature at planting (two-inch depth) was 58.2°F. Seed emergence was recorded on May 27, May 30, June 3, and June 11. Frost damage to some seedlings was observed on May 30, but regrowth of seedlings was evident by the next evaluation date. The percentage of seed emergence ranged from a low of 75.3 percent for the high rate of Apron MAXX, to a non-significant 81.2 percent for the low rate of Apron MAXX. There were no significant differences in emergence counts for any rating date or treatment at BBF. After the initial planting succumbed to seed corn maggot, soybeans were replanted at Southwest Michigan Research and Extension Center (SWMREC) on May 28 at a depth of one and a half inches in 30-inch rows with eight seeds per foot. Soil temperature at planting (two-inch depth) was 75.3°F. Emergence counts were made on June 5, June 9, June 11 and June 16. The percentage of seed emergence ranged from a low of 68.7 percent for the untreated control, to a non-significant 75.7 percent for Trilex-AL. There were no significant differences in emergence counts for any rating date or treatment at the SWMREC although several treatments fell below the 100,000 plants/A benchmark established as adequate for achieving full yield.

Stand, yield, test weight and 1000 seed weights at the Bean and Beet Farm (2008)

Stand, yield, test weight and 1000 seed weights at SWMREC (2008)

In general, seedling disease pressure in the test plots was low. There were very few gaps in rows in any of the plots. Frost may have affected emergence ratings and yield at the Bean and Beet Farm, but the majority of damage seemed to be evenly distributed among the treatments, and many plants recovered. Under the field conditions present in our studies, there did not seem to be a compelling advantage to using fungicidal seed treatments. There was a slight yield advantage (3.85 bu) to using Yield Shield over the untreated control at the SWMREC, but no real yield advantage at the Bean and Beet Farm.

However, under different environmental conditions, this could change. The Pythium isolates we recovered were very consistent at colonizing and killing seeds. Using a seed treatment when planting into fields with a history of disease, especially under cold, wet conditions favoring Pythium would seem to be a worthwhile precaution. In our study, soil temperatures at planting were not favorable for either Phytophthora or Rhizoctonia. There were no yield or quality advantages conferred by using the higher rate of Apron MAXX recommended for Phytophthora control over the other treatments. Further study is needed to determine how much Fusarium spp. contribute to seedling death.

In our study, we isolated several species of Fusarium, but the predominant species was F. oxysporum. F. oxysporum tended to cause moderate levels of disease in germinating seeds without killing them outright, although there were some isolates that did kill seeds and some that left them untouched. They likely contributed to stand reduction, but pathogenicity of isolates collected varied widely, even within plots, so it is difficult to determine how much they reduced yield. In the absence of environmental conditions that cause seeds to remain in the soil for extended periods of time without germinating, and without a field history of seedling disease, fungicidal seed treatments do not seem to provide significant improvements to soybean yield or quality. Research in other states to test the efficacy of seed treatments against the Fusarium that causes soybean sudden death (SDS) has not shown consistent control, but work is ongoing.