Wheat Leaf Rust and Stripe Rust Update: March 2, 2012

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist March 2, 2012 13:03 Updated

Stripe rust present on a wheat flag leaf.

Over the past week to 10 days additional stripe rust (yellow rust) reports have occurred from the Mississippi Delta and neighboring states.  At present, stripe rust has been positively confirmed/identified in Bolivar, Coahoma, Sunflower, and Washington counties.

In addition to stripe rust, leaf rust has been identified in several fields in eastern, southern, and western MS.  Last Monday (2/20/12), two fields in Noxubee County (east Mississippi) were identified as having active, sporulating leaf rust by a consultant.  On Wednesday afternoon (2/22), I was in the area and observed sproulating leaf rust pustules at low levels in two fields.  In addition, on Tuesday of this week (2/28) a consultant reported leaf rust from Smith County in south MS and Coahoma County in western MS.  Typically, leaf rust can be identified at this time in our wheat production area as the fungus favors cooler temperatures.  Presently, the leaf rust is scattered in fields and in some cases was observed only on volunteer wheat plants, especially in eastern MS.  Currently, I am not near as concerned (at least as of today March 2, 2012) about leaf rust as I am stripe rust.  Recognize that the major difference between the two diseases has to do with the pattern of pustules on a wheat leaf.  Stripe rust will be more organized, pustules appear smaller in size, and the overall color of the spores within the pustule is much lighter and generally more yellow.  Leaf rust pustules appear a little larger, are less organized (not present in a “stripe” pattern), and when comparing the color of the sporulation are more orange (darker) than stripe rust spores.  See attached photos.

Last Monday (2/20) I observed a field in Bolivar County with several hot spots along the edge of the field and one large hot spot in the middle of the field.  In 10 days’ time the hot spots had grown from a 5 sq ft area to approximately 400 sq ft in size.  As of yesterday (3/1/12) stripe rust was present throughout the crop canopy and had created the typical “flash” of yellowing that can be observed when sunlight hits the hot spot from the right direction.  The wheat field was between Feekes 7 and Feekes 8 and a fungicide was applied.  However, do not mistake low spots in a field and the general yellow color that some low spots can exhibit if standing water has been an issue for any period of time.  Scout wheat fields and observe the yellow areas to determine the cause even if you think it is just a low area.  In January, when stripe rust was initially observed in MS the fungus was present on some of the lowest leaves on the plant.

Leaf rust of wheat. Notice the irregular pustule patter as well as color of the spores present within the pustule.

Arkansas and Louisiana are also reporting stripe rust from advanced wheat plants.  See the AR update at: http://www.arkansas-crops.com/2012/03/01/wheat-stripe-rust-field-observations-and-update/

With the current environmental conditions continuing to be unseasonably warm and humid with ample rainfall throughout the majority of the state it appears like we could be set up for a stripe rust epidemic.

Typically, as wheat plants age they progress from a more susceptible growth stage to growth stages where the plant becomes more resistant to infection, this is more typically referred to as “adult-plant resistance”.  However, one of the greatest concerns at this point has to do with the race of the stripe rust fungus that may be present this season.  Even though samples have been sent off for race identification it may be months before we know the specific race involved in the stripe rust we are observing today.  The race of the stripe rust fungus we’ve encountered most recently appears to be able to handle warmer temperatures and infect plants and cause disease more rapidly.  In addition, based on research conducted in Arkansas in the recent past, adult-plant resistance has been observed to be race specific.  Essentially, the level of adult-plant resistance observed in a particular wheat variety may depend on the race of the stripe rust fungus.

Making a decision to apply a fungicide is difficult.  Presently, we can’t determine what the environment for the next 2.5 to 3 months will be.  Keep in mind, that if you choose to make a fungicide application at an early growth stage that depending on what the environment does over the next 2.5 to 3 months it is possible a second fungicide application will be necessary.  Making a decision to apply a fungicide should be based on SEVERAL key factors.  However, the level of importance of each factor will differ on a case-by-case basis and within each field so there is no presumed “order” to the factors below.

Notice yellow area near center of photo immediately adjacent to water furrow. Upon careful inspection, areas in fields that appear to be from standing water can be "hot spots" of stripe rust. Pay careful attention when scouting yellow areas.

1. Likely the single most important factor: a field’s yield potential.  The yield potential of a particular field will likely dictate whether or not a fungicide application is an economical decision.  For example, if the wheat field has remained wet for the majority of the season and a lot of thin areas are present, the wheat may have a low yield potential.

2. Wheat variety planted.  Applying a fungicide to a variety with tolerance/resistance to stripe rust may NOT provide an economic response.  As I have stated in the past, find out what variety was planted and consult either the company information or information from the AR, LA, or MS variety trials with regards to disease reaction of the specific variety.

3. Presence of a disease and severity of the disease present.  At this point, I’m most concerned about stripe rust.

4. A fungicide will NOT make up for a management practice that have not been followed correctly.  If you have skipped a nitrogen application (or all nitrogen applications) then applying the fungicide is NOT going to make up for that situation.  A fungicide should be applied for one reason, and one reason only, to prevent yield loss as the result of a foliar fungal disease.

5. Depending upon growth stage timing of fungicide application, prevailing environment following the application, susceptibility of the wheat variety, and presence of stripe rust, a single fungicide application may not effectively reduce the risk of yield loss caused by stripe rust.  Keep this in mind when choosing a fungicide since there are products available that will be much cheaper in the long run if more than one application of a fungicide is required.

Should you decide to make a fungicide application based on the above factors, a table of some of the labeled fungicides and their expected efficacy against the stripe rust fungus has been included.  Please keep in mind, that some of the generic products mentioned in the footnotes may not be labeled for use in MS.

stripe rust fungicide table

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist March 2, 2012 13:03 Updated
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