Over the past week, a substantial number of soybean acres appear to have been affected by seedling disease causing organisms. Fields planted around the week of Easter (April 14 to 19) and treated with residual herbicides or herbicide tank mixtures either immediately prior to or following planting have experienced an extremely stressful environment. The majority of the soybean fields in question are located in Bolivar, Coahoma, Sunflower, and Tallahatchie counties. General symptoms include what appears to be postemergence damping-off, characterized by brown, dead seedlings. Where seedlings have emerged, injury on the underside of the cotyledon leaves is also common. The pitted plant tissue along the edge of the leaves appears burned as a result of herbicide splash from the soil surface. In a few cases, the soybean stems appear pinched and pitted at the soil line. All of the outlined symptoms can be associated with general soybean seedling diseases such as Pythium damping-off (both pre- and postemergence) and Rhizoctonia root and stem rot.
Environmental conditions immediately following the planting period included extremely cool air temperatures, marginal soil temperatures, and excessive rainfall in some parts of the state. The combination of stressful environmental conditions that delayed soybean seedling emergence as well as herbicide injury contributed to the symptoms observed throughout several counties similar to the situation that occurred last season (see: http://www.mississippi-crops.com/2013/05/14/soybean-seedling-issues-seedling-disease-versus-herbicide-injury/). Seedling disease causing organisms in this type of scenario are likely a secondary infection as the result of wounding from a residual herbicide splashing onto the stem and/or underside of the cotyledon.
Additional agronomic practices have contributed to the outlined observed symptoms. In one particular case, the soybean seed was planted quite deep and the seedlings struggled to emerge. Excessive planting depth can contribute to additional stress that results in uneven stand emergence as well as exposure to herbicides.
The specific residual herbicides in question have tended to be those that contain a PPO herbicide (e.g., Authority products, Fierce, Sharpen, Sonic, and Valor SX). However, fields treated with a herbicide mixture containing metribuzin (e.g., Boundary,Intimidator), have exhibited symptoms, but to a lesser degree. In specific situations where symptoms occurred following application of a herbicide mixture containing metribuzin, the fields were planted with a metribuzin-sensitive soybean variety. To verify the sensitivity of a specific soybean variety to metribuzin consult:
and more recent data available from the University of Arkansas:
In the future, there are management practices that can reduce the likelihood of seedling injury (see: http://www.mississippi-crops.com/2013/04/26/chase-the-sprayer-with-the-planter/ ). Preemergence residual herbicides are extremely important due to widespread herbicide-resistant Palmer amaranth. Applying herbicides 7 to 10 days prior to planting will reduce the likelihood of soybean injury resulting from the chemistries used as preemergence herbicides.