• Although early planting can bring yield benefits, planting in cool and wet soils can increase the risk of early season disease infection.
• For soybeans, planting in cool wet soils can favor disease infection by pathogens like Pythium, Phytophthora and Sudden Death Syndrome (SDS).
• For corn, planting in cool wet conditions can favor infection by Pythium
• Fungicide seed treatments and variety or hybrid selection can help minimize the risk and potential yield losses caused by early season disease infection.

While 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, the 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 decisions 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 or hybrid selection can help minimize the risk and potential yield losses caused by early season disease infection. But not all seed treatments are created equal. There are very specific fungicide active ingredients that are effective against Pythium, Phytophthora and even SDS that will not be effective against other early season pathogens like Rhizoctonia and Fusarium. Fungicide seed treatment packages containing metalaxyl, mefenoxam and ethaboxam for example 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, a specialized seed treatment like fluopyram is very effective in fending off disease infection and delaying the onset of foliar symptoms and minimize yield impacts. However, keep in mind that seed treatments usually provide protection against these early diseases for about 2-3 weeks, depending on product and dosage. Diseases can still attack crops if environmental conditions are favorable after the residual has faded.

Regarding variety/ hybrid selection, different hybrids and 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. In some cases, hybrids or varieties with a “good” resistance rating may 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. The key point to understand is that there are several races of Phytophthora, and there are varieties that may have resistance genes that are effective at preventing infection against certain races.  This is called race specific resistance.  Consider selecting varieties with stacked or multiple resistance genes to multiple races of the pathogen as most likely you will not know which race (s) you have present in your field.  Some soybean varieties also have additional resistance, called partial resistance, which is effective at slowing disease development of all races of Phytophthora.   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 ontact your local FS crop specialist.