Scout for Stripe Rust in Wheat

Don Hershman, Extension Plant Pathologist, University of Kentucky


Be on alert for stripe rust. The disease may have overwintered in your area and there may be some hot spots that could serve as fuel for a rapidly developing problem on a field by field basis. I wrote a KY Pest News article on this a couple of issues ago. It may not be a problem, but it is easy to miss things this time of year unless active crop scouting is taking place. The disease level in this particular field really increased over the past two weeks. Phil Needham said he has seen stripe rust in other fields too, but this one in Oak Grove is the worst so far. Still, there may be fields with a similar level of disease.


For information on management of stripe rust go to Chapter 7 of the Wheat Management Guide.

Figure 1. Stripe rust in wheat.

 Figure 2. Stripe rust in wheat. 

Observations on Corn Diseases

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.

Late Season Pests in Corn and Soybean

Many late season pests are showing up in corn and soybean fields. The latest Kentucky Pest News contains articles on the following topics:

CORN

-Diplodia Ear Rot and Hybrid Trials

-Reasons to Scout Late-Planted Corn in 2009

-Fall Armyworm Reports in Kentucky

SOYBEAN

-Late Breaking - Fall Armyworm on Soybean Making Impressive Appearance in Nearby Southern States!

-Late Season Soybean Pests

Click here to see the August 18, 2009 version of the Kentucky Pest News.

Increased Risk of Foliar Diseases in Late-Planted Corn

Paul Vincelli, Extension Plant Pathologist
Plant Pathology, University of Kentucky

Rainfall patterns have delayed corn planting throughout much of the state. Late planting increases the risk of damaging levels of certain foliar diseases, particularly gray leaf spot, southern leaf blight, and northern leaf blight.

Several factors can contribute to this increased risk:

1. When there is a mix of fields planted early near fields planted late, the early fields can be a source of spores for late-planted field. The early fields act a bit like “Typhoid Mary”.


2. Compared to early fields, late-planted corn is often at an earlier stage of crop development during periods of spore release and leaf blighting. Since leaf blighting early in plant development is more harmful to yields than late-season blighting, the late-planted fields have the potential to be hit harder than earlier fields.

Fields not planted until the last week of May or into June have the highest risk of foliar disease. Producers planting corn late this spring should use hybrids with adequate levels of resistance to gray leaf spot. Selecting hybrids with good resistance to gray leaf spot is especially important if the field is under conservation tillage (30% or more residue cover) and has had corn anytime in the last two years. Also, if a field has a recent history of southern leaf blight or northern leaf blight, consider those diseases in hybrid selection.

Of course, many producers have already purchased seed for this spring. If a field is sown late and the hybrid doesn’t have substantial resistance to the diseases mentioned above, a fungicide application is more likely to be cost-effective. Figure 1 lists the factors that increase the likelihood of getting a positive economic return from a fungicide application in corn. The more of those that are in place, the more likely a corn field is to benefit economically from a fungicide application.

If you do choose to use fungicides, it is always a good idea to leave at least one untreated strip in the field in order to see if the fungicide provided any benefit. Sometimes it will but often it won’t, and getting on-farm evidence helps in making future farming decisions.

Figure 1. The more of these factors are in place, the higher the probability of getting a positive economic return from a foliar fungicide application.