Andrew Kness, Agriculture Agent
University of Maryland Extension, Harford County
As corn and soybeans begin to enter reproductive growth stages, foliar fungal diseases can have a serious impact on yield if growing conditions favor their development. However, with the hot and dry weather, disease pressure is low and will remain low if these weather patterns persist. Weather patterns in the Mid-Atlantic can switch rapidly and in general, fungicides need to be applied as a protectant, so spray decisions need to be made before the onset of disease. Coupled with the fact that many new fungicides on the market today are promoted to help plants cope with stresses, such as drought and heat, can complicate the decision.
I like to remind folks that fungicides are designed to do one thing—and that is to manage fungal diseases; they do this job very well. Hundreds of university trials have demonstrated that the most likely yield response and economic return occurs when fungicides are applied at the correct time and used when disease pressure is high. When disease pressure is low, yield responses sometimes occur, but are far more inconsistent than when disease is present.
There is also limited university data regarding fungicide utility to help plants manage drought stress. In the lab, strobilurin and triazole fungicides have been shown to regulate stomatal conductance and photosynthesis intensity in some corn hybrids, which improves the plants response to drought. However, when conducted at the field level, results are inconsistent and do not occur predictably. More research is needed to determine if fungicides can improve yields in drought conditions, and if so, when the application should be made. Existing studies indicate that the fungicide needs to be applied before drought stress occurs in order to stimulate the appropriate response in the plant; which again, makes spray decisions difficult without a crystal ball in hand to see into the future.
Another consideration, and perhaps the most important, is economics. While a fungicide application may provide some measurable differences in appearance and/or yield, is it enough to cover the application cost? Again, this is a tricky question to answer, but the body of research indicates that a single fungicide application in corn around VT-R1 and R1-R3 in soybean is the most likely to provide an economic return on investment when disease is present. In the absence of disease, the probability of an economic return is very low. Here are some additional factors that you should account for when considering a fungicide:
- Crop history and tillage: Many fungal pathogens of corn and soybean are residue-borne. If growing corn-on-corn or soybeans after soybeans, those fields will be prone to higher disease risk. Conservation tillage fields are also at higher risk.
- Irrigation: In dry years, fields that are overhead irrigated will be at higher risk of developing fungal diseases than dryland fields.
- Disease progression: Scout your fields and see if and where fungal lesions are present on the plants. Many of the most important diseases of corn and soybean start low in the canopy and progress up the plant if environmental conditions are conducive (Fig. 1). For soybean it is critical to keep the top ⅓ of the plant clean of disease from flowering to pod fill; and for corn, it is critical to keep the ear leaf clean. If you notice fungal pathogens encroaching on these upper plant parts as they begin to flower and tassel, a fungicide application may be beneficial to protect yield.
Figure 1. Two economically significant diseases of corn and soybeans: grey leaf spot lesions on corn (left) and frogeye leaf spot lesions on soybean (right). - Hybrid and variety resistance: Pay attention to your corn hybrid and soybean variety foliar disease ratings (resistance ratings are disease-specific); those that are more susceptible will have the greatest chance of an economic return on investment to a fungicide application than resistant varieties. Planting resistant varieties and hybrids is one of the most effective disease management tools.
- Resistance management: It is generally considered bad practice to spray a fungicide when disease potential is low, as you are exposing pathogens to unnecessary chemistry, and with each exposure you drive the population towards resisting that chemical. This is why it is important to rotate modes of action and use full label rates, as cut rates can accelerate the development of resistance. The Take Action website (https://iwilltakeaction.com/) can help you choose products with differing modes of action.
The take-home message is to realize that you have the greatest chance for an economic return on your investment with a fungicide application when disease actually develops. Applying a fungicide to help plants cope with drought stress will likely not help to a degree that covers the cost of the application; as replicated field trials have yielded inconsistent and unpredictable results.
