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Southern rust has continued to spread across Iowa and increase in severity since the disease was first reported in mid-July.

This is not surprising considering the weather Iowa has been having. July was warm and extremely wet across the state, and the precipitation continued through the first 10 days of August.

Southern rust thrives in warm (77-82 degrees), wet conditions. At least six hours of leaf wetness is required for infection to occur. Under these conditions, the time between a spore infecting a leaf to the production of a pustule filled with new spores is 7 to 10 days.

Compare this to tar spot (14-21 days) and gray leaf spot (14 days).

Consequently, southern rust epidemics can develop very quickly. Once we get into the cooler days of fall, southern rust development should slow.

Is it too late to apply a fungicide?

Applying a fungicide through R3 (milk) is likely to be beneficial. Only with severe disease pressure would a fungicide application be recommended at R4 (dough).

Consider severe southern rust disease pressure to be southern rust present on the ear leaf or in the upper canopy on most plants in the field at approximately 1% disease severity.

Chatter suggests that southern rust is severe in the lower canopy. Remember that the ear leaf and canopy above are filling grain. These are the leaves needed to apply a fungicide to protect yield.

For folks who have already applied a fungicide, a second application is likely unnecessary.

Your decision should depend on the product you used, the amount of disease in the upper canopy and the growth stage of your crop.

It goes without saying that at today’s grain prices, a second application of fungicide input is unlikely to result in an output (ROI).

How effective are fungicides against southern rust?

Fungicides vary in their efficacy against southern rust based on observations from corn pathologists across the U.S. Newer products with VG-E efficacy will protect leaves from new infections for 35-42 days. Older products and generics are likely protecting the leaves for 21 days.

Is southern rust going to be another disease that must be managed every year?

The southern rust pathogen is different from the pathogens that cause tar spot, gray leaf spot and northern corn leaf blight in that it will not survive the winter in Iowa. All rust pathogens are obligate parasites, which means they can only survive on a living host.

Rusts are, in a way, “snowbirds.� They survive on living hosts in Central America during our winter.

Then each growing season, their spores are blown up to Iowa on southerly winds. Consequently, each new growing season starts with a clean slate when it comes to southern rust.