Soybeans

Some growers are noticing damage on their newly emerged soybeans.

Kiersten Wise of Purdue University says the fungicide fluopyram (ILeVO) is currently marketed as a seed treatment to manage sudden death syndrome. The treatment can result in a discoloration on soybean cotyledons that can resemble disease or other abiotic stress such as herbicide injury. The discoloration occurs because the fungicide is moderately systemic within the soybean plant, so it will naturally move to the plant’s sinks, the roots and cotyledons.

This accumulation can result in phytotoxicity, causing the tips of the cotyledons to turn a yellow-brown color. This necrosis is typically uniform and present on every seedling grown from an ILeVO treated seed. However, environmental conditions may impact the frequency, uniformity and severity of the phytotoxicity observed.

The phytotoxicity is not usually found on the unifoliate or trifoliate leaves. Research conducted by several land grant universities and the Ontario Ministry of Agriculture, Food and Rural Affairs demonstrated that this phytotoxicity, also referred to as a halo effect, does not result in long-term soybean stunting or yield loss.

Pre-emergence herbicides can also cause soybean seedling damage, particularly when cool temperatures coincide with rain soon after seedlings begin to emerge from the soil. Pre-emergence herbicides, typically PPO-inhibitors (flumioxazin, sulfentrazone, saflufenacil; group 14) or photosynthetic inhibitors (metribuzin; group 5), can occasionally be injurious to plants growing in cold, wet soils. Soybeans are typically able to metabolize these herbicides, but when metabolism slows due to stress, like cold temperatures, herbicide injury can occur.

Pre-emergence herbicide injury also occurs when heavy rain events splash concentrated droplets of residual herbicide from the soil onto the emerged seedlings. Spotty necrosis can occur on any exposed portion of the plant where the splash event occurred, and metribuzin can cause symptoms similar to the phytotoxicity caused by ILeVO.

Damage from seedling blights such as pythium root rot, pre-emergence herbicides, and ILeVO can look very similar. ILeVO damage is usually only on the surface of cotyledons, so snap a few cotyledons and look for green on the inside to distinguish from other injuries or diseases.

If you are still unsure of the cause of the damage observed, send a sample to a local diagnostic laboratory. Obtaining an accurate diagnosis will allow you to determine the best management strategies for your soybean field.

Leon Ressler is a Penn State Extension educator based in Lancaster County, Pennsylvania.

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