Experience with early soybean plantings in the Mid-South has shown that excellent soybean yields can be reduced in value by seed rot or decay that occurs before harvest. This has been used by some as an excuse to stick to conventional planting dates. The fault is not early planting per se, but the time when seeds in early plantings mature in relation to weather patterns that occur during and after maturity.
When poor seed quality occurs, it is highly correlated with seed diseases, especially Phomopsis. Many years of testing seeds from early soybean plantings in the Mid-South show that seeds harvested from early plantings always have the potential for poor quality. That is, disease organisms that cause seed rot are always present on the seed, and will reduce quality when weather conditions that promote prominent infection occur before harvest.
Seed rot of the magnitude that occurred in 2001 in the Mid-South was the result of disease infection that mushroomed because of hot, wet conditions that persisted during the latter phase of seed development and after seed maturity.
This problem will persist in the early planting system. It can also occur in seed of conventional plantings, as it did in 2002. Thus, the problem is what to do to prevent seed rot and poor seed quality that may reduce net yield regardless of planting date.
Presently, the only way to ensure the effect of preharvest seed rot will be minimized is to apply a fungicide that prevents seed infection so pathogens will be minimally present if weather conditions that promote their development occur before harvest. Applying foliar fungicides between R3 and R5 (or later if weather conditions become favorable for increased infection) may prevent significant seed rot occurrence.
Keep in mind that yield may not be increased. Rather, the fungicide application is purely a preventive measure to guard against yield reduction that may occur if weather patterns increase pressure from seed rot pathogens that were not controlled earlier.
Producers must decide if this approach is worth the cost because there is no guarantee of a return in years when seed rot is not yield-reducing. The problem is that this will not be known until after the fact, and weather conditions at maturity, such as occurred in 2001 and 2002, are not normal.
Recent research indicates fungicides applied to prevent infection by seed rot pathogens will at least pay for themselves.
Even seed of good visual quality may be low in germination because seed rot organisms are present on the seed and can cause seed rot during germination tests or when planted. This aspect of poor seed quality can be remedied by applying a broad-spectrum seed treatment such as ApronMaxx or Stiletto at planting (these also control problematic soilborne seedling diseases).
The unknown is what effect the increased spraying of fungicides to prevent or control Asian soybean rust will have on preharvest seed rot. Many of the fungicides used to prevent rust may also be active against seed rot fungi. Since spraying for rust prevention is timed to the same stages (R3 to R6) that are thought to be appropriate for fungicide applications to control seed rot, a positive side effect could be a reduction in the potential for seed rot.
I encourage producers, seedsmen, and researchers to keep accurate records of time, rate, and plant stage of fungicide applications, and then conduct seed quality tests soon after harvest in 2005 to determine if fungicide sprayings for rust prevention also increased quality of harvested seed.
The Section 18 labeling of many fungicides for rust control and their likely application over a large acreage in the Mid-South in 2005 offers a unique opportunity to assess their effect on seed quality.
Larry G. Heatherly is a retired USDA-ARS Research Agronomist and current crop consultant. e-mail [email protected]