What’s Wrong With My Soybean Plants?
DR. HEATHER MARIE KELLY, ELIAS ZUCHELLI AND SHIKSHA SHARMA
JACKSON, TENNESSEE
As the earliest planted soybeans in Tennessee push into beginning seed development and beyond, there are some soybean diseases that are becoming more notable. The good growing conditions most have experienced promote good yields and diseases – correctly identifying these diseases are crucial to implementing the best management practices to protect yield.
As with disease in any crop, there are multiple factors that influence if disease will impact yield and be a PEST – Pathogen present, Environment conducive for infection and disease development, Susceptible host/variety, and Timing of infection/disease development at a growth stage that impacts yield. The sufficient rainfall throughout the season and early plant dates of many soybean fields have the potential to have multiple diseases showing up. Specifically, the frequent rain events that occurred during June and July in the middle and west TN have accumulated 2 to 5 inches more than historical averages. Such conditions are favorable for multiple foliar diseases as well as soilborne diseases that exhibit foliar symptoms. It is very important to distinguish between foliar and soilborne diseases, as foliar diseases may require a fungicide while soilborne diseases do not, as well as to identify what is not disease but injury or nutrient deficiency – as all of these can cause soybean plants to look “wrong”.
Specifically in Tennessee, many soybean fields were planted in March and April which had cooler soil temperatures and with the good moisture, we are now seeing in susceptible varieties Sudden Death Syndrome (SDS). Symptoms of SDS include yellowing (chlorosis) between leaf veins that can progress to brown, dead (necrosis) tissue between the leaf veins and lead to defoliation, with petioles staying on the plant (Images 1-3). Similar leaf symptoms can also be caused by other diseases (such as stem canker, taproot decline, red crown rot, Phytophthora – which are all soilborne pathogens but cause leaf symptoms) and chemical injury (burn caused by fungicide, surfactants, etc.).
Always examine the newest growth on the plant – if it is green and not exhibiting the same symptoms, then it is likely injury you are dealing with and will need to look back over your spray records – if the newest growth is exhibiting symptoms further investigation is needed to determine the exact cause (disease or nutrient deficiency).
Sudden death syndrome (SDS) is a soilborne fungal disease caused by Fusarium virguliforme. While the disease expresses its foliar symptoms during the reproductive growth stages, it has infected and colonized the roots earlier in the season. SDS is favored by wet soil conditions, making disease severity greatest in irrigated fields and in saturated soils, particularly in low-lying areas, transition zones, and areas prone to waterlogging and soil compaction. Moderate to high populations of soybean cyst nematode (SCN) (Heterodera glycines) are frequently associated with SDS and can increase disease severity.
A key diagnostic feature of the disease is observed by splitting the lower stem: although the roots maybe severely rotted and the outer cortex is brown, the central pith remains white (Image 4). While there are no management options for SDS after planting occurs, one should take note of which field and variety the disease is present, so following seasons a more resistant/tolerant variety can be planted and considerations for seed treatments investigated. Furthermore, this fall soil samples should be submitted (after harvest and before first frost) for nematode analysis – some of the most severe SDS is due to associate with SCN and to protect yield both SDS and SCN need to be addressed. Information on how to take soil samples and where to submit them for nematode analysis in Tennessee can be found here (UTcrops.com). Additional disease symptomology and management information can be found at guide.utcrops.com.
Foliar diseases we have been observing in the field include Septoria brown spot, Cercospora leaf blight, target spot, and frogeye leaf spot (Images 4-7). While these diseases can be managed with foliar fungicides, to have the best efficacy and return on investment application should occur before beginning seed (R5). See fungicide efficacy table at UTcrops.com and/or Crop Protection Network for more information. ∆
DR. HEATHER MARIE KELLY, ELIAS ZUCHELLI AND SHIKSHA SHARMA
UNIVERSITY OF TENNESSEE