Although early planting can provide many benefits for maximizing corn yields, it can also bring some risks associated with slow crop growth, herbicide injury and vulnerability to disease infection. Planting in cool wet soils may result in seed rotting and poor or no germination. Sometimes soil conditions will be good enough for the seed to germinate and for the seedling to start growth but if a period of cool weather follows, growth will be slowed down again. Root and stem pathogens find this to be ideal conditions for infection.  In fact, any type of stress that slows growth or diverts resources away from growth can result in a higher risk of disease infection. 

For corn, planting in cool wet conditions can favor infection by Pythium. Affected corn plants may be slow to develop, appear yellow or wilted, and result in low stand counts (Image 1). For soybeans, planting in cool wet soils can be particularly favorable for disease infection by pathogens like Pythium, Phytophthora and Sudden death Syndrome (SDS). Phytophthora and Pythium are oomycetes, fungus-like organisms that produce a swimming spore that moves to, and infects, soybean roots and hypocotyl. Both Phytophthora and Pythium can cause seed rot, seedling damping off and taproot plus stem infection. Initially, Pythium and Phytophthora cause a mushy tan or brown rot of root and stem tissue and lateral roots will be missing in larger plants. Active Pythium infection usually stops as soils warm and dry out, while Phytophthora can continue to affect soybean plants throughout the growing season. Soybean plants that survive seedling Phytophthora infection, may have taproot and lower stem brown discoloration. In the case of SDS, although foliar symptoms don’t show up until after flowering, root infection by the SDS pathogen occurs shortly after planting when cool temperatures and moist soils prevail. SDS can cause premature defoliation and reduced photosynthetic surface area for pod fill, which leads to fewer and smaller seeds.  

How do we protect our newly planted crops from early season disease infection? A few helpful recommendations would be to wait until the soil is dry and warm enough for the crop to emerge promptly. However even when we wait for the right conditions to plant, the truth of the matter is that weather can sometimes be unpredictable. Fungicide seed treatments and variety selection can help minimize the risk and potential yield losses caused by early season disease infection. Fungicide seed treatment packages containing metalaxyl, mefenoxam and ethaboxam can be efficacious against Pythium and Phytophthora. Unfortunately, many different species of Pythium exist and some of the seed treatments available have reduced efficacy against some of those Pythium species. For SDS, specialized seed treatments such as fluopyram and pydiflumetofen are effective in fending off disease infection and delaying the onset of foliar symptoms and minimize yield impacts. For more detailed information on soybean seed treatments visit: https://www.fssystem.com/Products-Services/Resource-Center/Resource-Detail/seed-treatments-for-soybeans

Regarding variety selection, different varieties will have different levels of resistance or susceptibility to at least some of the most common diseases. This resistance rating information will be provided by the seed company on a numerical scale, or in the case of Phytophthora, the resistant gene may be listed. Varieties with a “good” resistance rating will still get infected but to a lower degree of severity than susceptible ones. Understanding how resistance works for pathogens like Phytophthora is a bit complicated as there are several races of Phytophthora, and there are varieties that may have a resistance gene effective against one specific race. Then there’s partial resistance (tolerance) where varieties have multiple resistance genes that can slow disease development and result in fewer rotted roots.   However, this resistance does not kick in until plants begin to produce trifoliates and is not effective against early season seedling infection. For more information on management of early season diseases of corn and soybeans contact your local FS crop specialist.