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Wheat leaf rust developments

By Bryan R. Butler, Ag Today

Monday, May 14, 2007

Arv Grybauskas, extension field crops plant pathologist at the University of Maryland, says producers involved in wheat production need to be aware that there is a potential for leaf rust development this season that might require fungicide treatment on susceptible cultivars. Interestingly, there appears to be little threat of stripe rust this season. Powdery mildew this year is pretty strong, so susceptible cultivars should be scouted and considered for treatment this season.

We have found a couple of pustules of leaf rust (not stripe rust) on a highly susceptible cultivar on the Eastern Shore. Considering we typically don't see leaf rust until well after heading, and much of the wheat in the state has not yet produced a flag leaf, there is a potential for disease to develop to damaging levels.

Leaf rust generally does not over-winter or, in this case, over-summer very well in our area. Because of that, our outbreaks usually depend on development of the disease in more southern regions of the U.S. and then getting moved into our region by wind.

The most likely routes that spores reach us are from wheat-producing states south of us, and potentially from wheat producing states from the west and southwest of us. The only current reports of leaf rust are from Oklahoma, Kansas, North Carolina and Virginia. It's the North Carolina reports that are of the most interest to us. They have some cases that appear to be significant enough to be threatening to their production fields. Virginia so far only has a few low-incidence reports.

At this point, it does not appear to be an epidemic. But there is plenty of time for rust to develop to damaging levels, regardless of where the spores can come from if the weather turns warm and wet. If there was a real hot spot somewhere, Arv would be more concerned. This is not a jump-to-go-spray situation, but let's get people looking so we can better assess the potential threat. If you have seen any rust pustules, which are reddish, or stripe rust, which is yellow, let us know. Depending on what you find makes the next step more obvious.

First, it is important to know what cultivar of wheat is being grown. There are plenty that have good resistance to leaf rust. The resistant and even the moderately resistant cultivars are unlikely to need any fungicide but we need to scout them in case there is a new race appearing, or if by chance stripe rust makes an appearance. The only recent ratings of resistance to the likely race of leaf rust that is prevalent is from the Virginia program.

Susceptible cultivars could benefit from a fungicide application if rust begins to develop on them. You don't want to spray right away if there is no obvious threat in the field or neighboring field. Fungicides will only last about three weeks, and for a single application to give the most bang for the buck, you need to protect the flag leaf for as long as possible. The application window is flag leaf through heading, if rust is a threat.

If less than 3 percent of the plants are infected, any registered product will work well. If there is more than 3 percent infection, then you need to move quickly and use a triazole or mixture that includes a triazole such as Tilt, Quilt, or Stratego. There is a newly registered triazole from Bayer, Proline (prothioconazole), that we don't have much experience with. This product was supposed to be registered as a mix with the active ingredient of Folicur (tebuconazole) for scab control but the Environmental Protection Agency held up the full label on tebuconazole.

I believe the original rationale for the mixture was that prothioconazole was a little weak against rust by itself. If this product is used, make sure the label rate for rust is used.