Observations of Paul Vincelli, Extension Plant Pathologist, University of Kentucky as of August 21, 2009.
I (Paul Vincelli) just returned from a trip inspecting corn diseases through western Kentucky as far west as the Mississippi River, and here is a quick summary and comments.
1. Southern rust is prevalent in Kentucky, having been found in Fayette County and every western Kentucky field inspected. See most recent article in Kentucky Pest News, http://www.uky.edu/Ag/kpn/kpn_09/pn_090818.html. This disease can progress very rapidly on corn, since almost all corn hybrids vary from moderately susceptible to highly susceptible. However, incidence and severity vary widely from field to field. In some fields, it is hard to find; in others, many plants show leaf reddening and desiccation in the lower canopy and the upper canopy has a few pustules. It is a little difficult to decide when to pull the trigger on spraying fungicide, if for no other reason than to protect stalk health. Most fields I inspected were at early dent, which in my opinion is too late to treat with fungicides. The very few fields I saw that were in early dough might be candidates for treatment, but only if rust was present, easy to find, and producing abundant reddish sporulation in at least some spots in the field. Cool weather expected over the next few days will slow it down, which is good news. I think the main thing is that growers should scout all fields for stalk health as they mature, and schedule early harvests on those fields with weak stalks. In at least some cases, spending money on propane for grain drying probably makes more sense than a fungicide application.
2. Northern leaf blight is widespread but generally occurring at levels that will not hurt yields. There is also another look-alike out there: Diplodia leaf streak. This disease has lesions that look somewhat like Northern leaf blight, but the edges of the lesions are wavy like Stewart’s wilt. Also, the lesions might follow the secondary views like Stewart’s wilt. Lab diagnosis is the way to confirm this disease. The fungus that causes Diplodia leaf streak (Diplodia macrospora) is distinct from the common one that causes Diplodia ear rot and stalk rot (Diplodia maydis), but Diplodia macrospora will also cause ear rot and stalk rot. Diplodia macrospora may be increasing in occurrence in Kentucky; seed companies will want to keep an eye on this.