Weather across Illinois has been variable this year, but a dry June across much of the state has given way to a July weather pattern with frequent rains, high humidity and more moderate temperatures.  These conditions elevate the risk profile for development of foliar disease in corn – at exactly the wrong time, as the crop is approaching or moving through pollination. Timely rains have increased yield potential, while also dampening the futures market for that grain. So how do we make a decision on foliar fungicide in the face of all these competing factors?

Remember, the primary role of foliar fungicide is to protect the upper canopy leaves from disease during the reproductive phase of the crop. Foliar diseases diminish the leaf surface area available for photosynthesis, the process used to convert sunlight energy to sugars that drive grain fill. Simply put, you can’t make as much grain with a damaged photosynthetic factory. 

Most fungicides, even those marketed as ‘curative’, will not reverse plant damage done by disease - they all work better when applied prior to infection. This means that the fungicide decision in corn is all about evaluating and managing the go-forward risk for disease in each given situation. Development of disease in any crop is dependent upon an interaction of three factors, commonly referred to as the disease triangle (susceptible host, infectious pathogen and favorable environment). A fourth component is time – the three factors must coincide in time, and symptoms take time to develop. The disease triangle is a helpful framework for thinking about fungicide risk management decisions, because every field characteristic can be applied against the model. 

Foliar disease infectious pathogens are numerous and abundant in Illinois, but the two most economically significant are gray leaf spot and northern corn leaf blight. Both of these pathogens overwinter in corn residue, so production systems with higher crop residue carry greater pathogen loads and higher risk – specifically, continuous corn and reduced tillage. Pathogens in residue infect the plant via rain splash and gradually move up the plant as spores are released. Other pathogens, like southern rust, overwinter in the south and must be moved into the region on southerly storm fronts. These migrating pathogens often infect the upper canopy as spores and spread downward.

While corn foliar diseases vary in their preferred temperature ranges, nearly all require some period of continuous leaf wetness and humid conditions to infect the plant and reproduce. After initial infection, the reproduction and infection processes cycle each time the weather becomes favorable again.  Expression of plant symptoms as lesions on the leaf may be delayed by 7-14 days after an infection cycle, which means infection may occur long before you can see it. Spores that infected corn over the July 4th weekend may just be starting to appear as symptoms. If the new wetter weather pattern persists through pollination and grain-fill, risk increases substantially for disease-related yield losses. Your FS crop specialist can evaluate each individual field situation to help you make the most profitable decisions for your farm.

Originally published in FarmWeek. Tim Laatsch serves the FS System as GROWMARK insect and plant disease technical manager. He can be reached at tlaatsch@growmark.com .