Soy Capital News

2018 Spring Weather Conditions Ideal for Fungal Infections in Soybeans

June 29, 2018

2018 Spring Weather Conditions Ideal for Fungal Infections in Soybeans

By: Mark R. Smith
June 21, 2018

Many growers throughout Central Illinois have begun to notice areas in soybean fields where plants are damping off. The cool, wet conditions of early to mid-April, followed by a period of above normal temperatures with both wet and/or dry conditions, have provided an environment conducive to the growth and development of fungal pathogens. While there are now few management controls to limit the result of these spring conditions, understanding the problems may provide better planning for future plantings. The following paragraphs explain conditions that brought on the disease pressures we have already seen in 2018, and the conclusion offers suggestions on what to look for between now and the end of the growing season.

The primary soybean disease we are seeing in areas with recent, extended periods of hot, dry conditions is Rhizoctonia root and stem rot (Rhyzoctonia solani). This fungus usually attacks young plants, and infections can be further escalated with herbicide injury and the presence of soybean cyst nematode populations. Reddish-brown cankers that form on the main root and basal stem are characteristic symptoms of this disease. In many cases, the secondary root system is destroyed. Often, infections show in the form of 4-10 foot circular patches, but can reduce a stand up to 10%. Where crop row spacing allows, cultivation (specifically plowing soil into the row) of infected areas may allow plants to form new roots and recover in-season. Cultural practices, including crop rotation, tillage (along with adequate drainage) to reduce soil compaction, seed treatments, and the selection of tolerant varieties can be effective management strategies.

In instances where field conditions are hot and wet, Phytophthora sojae is the organism more likely causing plants to damp off. Characteristics of this disease include a soft rot of root and crown tissues, or of stem tissue near the ground. Noticeable patches of dead plants will occur in fields, and this oomycete will persist in soils from season to season. Utilizing resistant varieties of new, superior grade seed, is the first step in controlling this disease in fields. Other management strategies include utilizing seed treatments specifically formulated to protect against oomycetes, crop rotation, and planting at the correct depth and time. This organism favors wet, saturated soils, therefore, enhanced field drainage could also be an effective control practice.

Given that we are already seeing the above diseases, it is all the more important that we continue to scout fields for additional foliar diseases. The presence of the above mentioned diseases may make stands more susceptible to other diseases as we progress. The fungi that cause frogeye leaf spot, Septoria brown spot, and soybean rust, thrive in warm, humid environmental conditions. Rainfall and winds will continue to disseminate these diseases. If recent rains and high humid conditions persist, we could see foliar diseases reach levels where foliar fungicide applications would produce an economic return.

An additional resource and photos on these diseases can be found on the June 15, 2018, University of Illinois Pest Management and Crop Development Bulletin at this link: http://bulletin.ipm.illinois.edu/?p=4264. If you have questions on these topics, please contact Soy Capital Ag Services farm manager Mark Smith at msmith@soybank.com or (309) 665-0053.