Brent Griffin, Extension Agent in Prairie County, found at least four hot spots of stripe rust on his family’s farm in northern Lonoke County on December 19, 2012. This beats the record set last January by about one month. The hot spots are now about 3 feet in diameter and are on an unknown variety that was planted on October 8. The finding was confirmed by Gene Milus, U of A wheat pathologist in Fayetteville, on January 9 after receiving samples collected by Jason Kelley, Extension Wheat and Feed Grains Agronomist. Given that all contemporary varieties are more or less susceptible to stripe rust during early growth stages, there likely are more hot spots developing throughout Arkansas and surrounding states.
The most common type of stripe rust resistance in contemporary varieties is classified as race specific, adult-plant resistance. This means that plants up to the jointing stage are susceptible and become more resistant as they mature and that the stripe rust fungus can evolve new races to overcome the resistance. The number of different resistance genes in contemporary varieties is unknown, but the stripe rust fungus has been evolving to overcome several of them. Even if a particular variety was resistant last year, there is no guarantee that it will be resistant this year. Furthermore, even if a variety turns out to be resistant, it will still be susceptible during the next two months. New races arise through mutations, and the probability of any particular mutation increases as the population size (number of spores) of the fungus increases. Therefore, a fungicide application is likely to be cost effective if hot spots are found in a field or if one has a field of the same variety that is known to have hot spots in a nearby field, and controlling stripe rust early will reduce the probability for new races.
There is still plenty of time to make an early fungicide application. Tank-mixing a fungicide with the herbicide used to control broadleaf weeds will save the cost of a separate application. Applying this tank mix during February or early March would be an optimal time to control both weeds and stripe rust. Research during 2012 showed that Tilt, Folicur, or Headline fungicides quickly inhibited the ability of spores to infect plants even though new spores were produced for several days after the fungicide application. Other fungicides registered on wheat likely would have similar effects. Generic formulations of Tilt and Folicur are inexpensive. This early application should control stripe rust long enough to determine if adult-plant resistance will be effective for the rest of the season or if another application at boot stage will be needed.