Mississippi Wheat Disease Update and Fungicide Decisions

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist March 24, 2011 20:54

Mississippi Wheat Disease Update and Fungicide Decisions

At present there is little to no disease in the Mississippi wheat crop. On Monday, low levels of powdery mildew were detected in a research plot in Stoneville. In addition, low levels of stripe rust were detected near Shaw, MS on Wednesday. Both Arkansas and Louisiana are reporting leaf rust and stripe rust in several areas of the state. As our wheat crop matures and more heads emerge we will likely encounter additional rust.

Even though disease incidence could be considered low at this time, there are some other issues in the Delta’s wheat crop. Numerous fields have received herbicide drift, more than likely from off-target glyphosate (see http://www.mississippi-crops.com/2011/03/23/identifying-glyphosate-drift-injury-on-wheat-plants/). Since Monday I’ve encountered some fields with minimal spray drift, indicated by consistent brown spots with purple margins throughout the upper canopy in localized areas within fields. Weeds on the turn row had similar symptoms. One particular field received excessive drift with clearly defined areas between a part of the field that received excessive drift and the rest of the field that received almost no drift. Many times I receive a call regarding the presence of a particular wheat disease only to determine that herbicide drift was the culprit. Keep this in mind when scouting wheat fields. Patterns throughout the field as well as lesions that develop on weeds outside of field borders can be indicative of spray drift rather than disease.

Wheat fungicide decisions
Timing a fungicide application in wheat can be difficult and ideally should be based on four main criteria:

1) The wheat variety planted
2) The particular disease present
3) The severity of the disease present
4) The wheat growth stage

Within those criteria, the single biggest decision maker is the particular variety. The majority of wheat varieties planted in MS have some level of tolerance to the most severe wheat diseases. If I were ranking wheat diseases based on the level of yield loss that could be received as a result of infection I’d likely put stripe rust at the top of the list, followed by leaf rust. However, with the varieties that we have planted across MS, and throughout the southern U.S. in general, the majority of the varieties have some level of tolerance to rust diseases. I say the majority of the varieties planted because there are instances where public varieties are planted. In most cases public varieties are planted for a specific purpose such as seed production. Over the past 4 years the main difference between public varieties (i.e. Gore, Roanne) and private varieties (i.e. any of the Coker, Pioneer, Progeny, Terral varieties) has typically been the particular disease present and disease severity within the given field situation. The number one problem that I’ve encountered in the public varieties has been either leaf or stripe rust. In almost every case the public variety likely would have benefited from a fungicide application. Varieties that may benefit from a fungicide application are typically more susceptible to the disease present. Fungicide trials conducted throughout the Mid-south suggest that the most economical response to a fungicide will be observed on varieties with some level of susceptibility. Applying a fungicide to a wheat variety with tolerance to either of the rust diseases will likely not result in an economic benefit at harvest.

Additionally, every season I discuss fungicide options with producers or consultants. A wheat field has to be managed to its full potential. Applying all of the necessary nitrogen is one of the more important things to be done. Expecting a fungicide to produce the full potential of the crop’s yield if nitrogen applications have been skipped is expecting too much out of the fungicide. A fungicide should only be used for yield loss prevention when yield limiting diseases are encountered. Expecting the fungicide to “rescue” the crop from nutrient deficiency, lack of moisture, excessive moisture, herbicide injury, frost injury, or other abiotic malady is expecting too much out of the product.

In addition, every year I get questions concerning the yellow spots or halos that can be observed on wheat leaves if you hold the leaf up to the sun. Many think this is indicative of a rust infection that will develop if the environmental conditions change. There are many things that can occur on wheat leaves that will produce this symptom. Insect feeding, herbicide drift, powdery mildew, or the plant’s reaction to a rust infection if the plant is tolerant to rust can all occur and be observed as yellow flecking, spots, or halos. However, this is NOT indicative of a rust epidemic that will require a fungicide application.

Fusarium head blight or scab
Head scab is a disease that has been encountered infrequently in MS. Fungicides are available and effective at reducing the infection of the fungus that causes head scab. However, this particular disease requires an incredibly specific environment to occur. Moreover, when I have heard of an instance of head scab in MS it has been in single, scattered plants and typically from the northwestern part of the state south of Memphis. It is true that the fungus that causes head scab (predominantly Fusarium graminearum) is also a concern in corn and could be a greater concern in a situation where wheat is planted following corn; however, wet, mild weather will need to occur during periods before, during, and after flowering for head scab to become a problem. In addition, treating the field with a fungicide prior to head emergence will not eliminate the risk of the disease. Fungicide applications, if warranted, should be timed for flowering. But with that said the decision will all depend on the environment encountered leading up to flowering.

For continued updates on the national and specifically the southern scab situation there is an internet model that will provide information with regards to the potential risk from the disease. Risk is assessed based on the presence of a conducive environment. However, the risk model/tool should be used carefully as it suggests that a conducive environment is likely but this does not mean that:

a) wheat is in a susceptible growth stage
b) a fungicide is necessary at that time
c) wheat is even present in that area
d) that this disease will even occur following the predicted risk model.

Like any predictive model numerous options need to be weighed including whether or not a scab-susceptible variety is even present. The tool can be accessed at http://www.wheatscab.psu.edu along with additional information on the disease itself. At present the model suggests there is some medium risk in some southern parts of the state; however, the information that is used to produce this risk is based on weather events that have occurred over the past 7 days. Remember, like any predictive model this does not likely translate into a treatable issue at present.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist March 24, 2011 20:54
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