Southern rust continues to develop across Iowa

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 we have been having. July was warm and extremely wet across the state, and the precipitation has continued through this first 10 days of August.

Favorable conditions for southern rust development

Southern rust thrives in warm (77-82°F), wet (dew, rain) conditions. At least 6 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 (Figure 1). Once we get into the cooler days of fall, southern rust development should slow.

Figure 1. Southern rust development in a late planted (31 May), irrigated field in central Iowa in 2024. Disease severity is mean of three separate trials in the same field. 

Is it too late to apply a fungicide? 

According to this resource from the Crop Protection Network, 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). I would 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.  I’ve heard lots of chatter that southern rust is severe in the lower canopy.  Remember that the ear leaf and canopy above are filling grain. These are the leaves we need to apply a fungicide to and 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, based on my experience. Older products and generics are likely protecting the leaves for 21 days.

Figure 2 shows a field at growth stage R3/R4 that was sprayed with a fungicide in mid-July. Although southern rust is prevalent in the lower canopy (Figure 2A), the upper canopy looks almost disease free (Figure 2B) indicating the fungicide has protected those leaves. Any spore landing on a fungicide-protected leaf would not be able to germinate and infect the leaf. The product used in this field has a long efficacy period (~5 weeks) and thus I would expect the upper canopy to remain clean through end of August.

Figure 2. Southern rust observed in the lower canopy (A) and upper canopy (B) of a field at growth stage R3/R4 that was treated with a fungicide in mid-July.

Is southern rust going to be another disease we need to manage 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.