Soybean Seedling Injury Symptoms

The fungicide fluopyram (ILeVO) is currently marketed as a seed treatment to manage sudden death syndrome (SDS). This seed 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 plants “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 Universities 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) or photosynthetic inhibitors (metribuzin), 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 (i.e. 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. Pre-emergence herbicide injury is more likely to occur in sandy low organic matter (OM) soils than in loam or clay soils with higher OM. Also, some varieties of soybean are more sensitive to these herbicides than other varieties. Herbicide sensitivity information is available from some, but not all, seed companies.
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