What to expect from fungicide applications to corn

Most years at this time, our corn crop looks good, but precipitation prospects start to decline. At the same time, we can see high humidity during the day and even heavy dews overnight that keep the crop’s canopy just wet enough to provide the potential for disease pressure. It can make our decision to apply a fungicide – or not – more difficult than we’d like.

If you’re on the fence about a fungicide application, start with a look at hybrid disease susceptibility as well as the previous crop and weather outlook. Southern rust (confirmed thus far only in a few counties in Georgia-Florida) likes nights above 80 degrees and high humidity. Tar spot likes cooler temperatures and prolonged leaf wetness. Gray leaf spot and Tar spot both survive on corn residue, making previous crop important, whereas Southern Rust has to blow in each year.

If you scout knowing these factors, it can make the decision-making process a little easier. For example: if a susceptible hybrid is showing disease symptoms on the third ear below the ear or above on 50 percent of the plants, it’s probably good to at least consider a fungicide, whereas many resistant hybrids may not require anything at all (depending on disease).

If you do elect to apply, do so in as timely a manner as possible. Tassel to R1 applications are typically the “sweet spot,” but make timing decisions in collaboration with scouting. If little to no disease is present, waiting until R1 might be worth it. Fungicide efficacy will begin to decline in three to four weeks, with later applications providing better potential to protect against later season pressure (some data suggests we might even be able to go later).

Will it pay? University of Illinois corn fungicide trials have shown that if at least five percent of the ear leaf was affected by disease at season’s end, a fungicide application at VT to R1 would likely have been beneficial. You can help make plans for next year by doing end of season evaluations to see how much disease pressure was actually present.

Want to track disease movement, visit https://corn.ipmpipe.org/. It’s useful to monitor Southern Rust and Tar Spot. Individual field scouting is likely a better option for Gray Leaf Spot.

Bagworm Hatch

If you’ve ever fought bagworms, you know how quickly they go from hatch to damaging to uncontrollable. While they’re relatively easy to control when young, they’re difficult to find. Further compounding the management process is the fact they may hatch over several weeks.

Control efforts will depend a lot on potential pressure from previous year’s bags as well as the health of your trees. With that in mind, scout now. Young bagworms were confirmed this past week almost as far north as the state line, with larger larvae likely further south.

Lots of products can control bagworms (and on smaller shrubs, hand picking may be as effective as anything), but all require good coverage (top to bottom and inside to outside) when bags are small. If you’ve got evergreens, start scouting now to prevent bigger issues later.

For information, see our bagworm resource at https://bookstore.ksre.ksu.edu/pubs/MF3474.pdf.


David Hallauer56 Posts

David Hallauer is the Meadowlark Extension District agent in the areas of horticulture and crops and soils.


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