Reports of wheat stripe rust have really jumped this week with confirmed cases of stripe rust now being reported in 9 counties in east-central Arkansas. The levels of stripe rust being found range from individual infected leaves to large “hot spots”. The reports of large hot spots is particularly alarming since much of the wheat in the area where stripe rust is being reported is just now beginning to joint. The hot spots that I have seen have stripe rust all the way to the top of the plant and many of the lower leaves have already died from stripe rust. Unfortunately in many fields the wheat variety is unknown so scouting all fields is needed at this point and not just fields you suspect may be planted to a variety that was susceptible to stripe rust last year. A list of varieties that stripe rust is showing up on will be available in the next few days.
The following is an update on the situation from Dr. Gene Milus, Professor of Plant Pathology at the U of A:
The early and relatively widespread occurrence of stripe rust in Arkansas is cause for concern. Although the full impact of stripe rust will not be known until May, this could be a “perfect storm” of a stripe rust epidemic. We are dealing with several unknowns, but it would be good to get everyone in the wheat community on the same page with what is known about stripe rust.
Based on research at the University of Arkansas that was funded by the Arkansas Wheat Promotion Board and others, we know that we have a new strain of the stripe rust fungus since 2000 and that this strain causes more disease more quickly and is better adapted to warmer temperatures than the old strain. We also know that the most common type of stripe rust resistance among contemporary soft red winter wheat varieties is adult-plant resistance. This means that seedlings and young plants are susceptible, but the plants become more resistant as they mature. This type of resistance also may be more effective at warmer temperatures. This type of resistance has worked well here in the past when stripe rust first shows up in late March. The adult-plant resistance in some varieties has been highly effective (no symptoms on upper leaves) while the adult-plant resistance in other varieties has only been moderately effective (some stripe rust on upper leaves).
To complicate the situation, several examples of this adult-plant resistance have been shown to be race specific. This means that the resistance is effective against some races of stripe rust but not others. At this time, it is impossible to know whether or not the stripe rust in our wheat fields will overcome the adult-plant resistance in particular varieties. We also do not know how favorable the rest of the growing season will be for stripe rust. However, stripe rust has another 2.5 months to cause damage, and the weather does not have to be very favorable over this long period to have a serious epidemic. Therefore, there likely will be several outcomes depending on whether particular varieties have adult-plant resistance to the stripe rust that is in our fields and the level of this resistance. If particular varieties have no adult-plant resistance, three fungicide applications likely will be needed to prevent significant yield and test weight losses. Varieties with moderate levels of resistance likely will need one or two fungicide applications, and varieties with a high level of resistance likely could get by without any fungicide.
Given the knowns and unknowns, if there are hot spots of stripe rust in a field, it may be prudent to tank mix a fungicide with the broadleaf herbicide application (if still needed to apply). The objective of this early application is to stop rust development for approximately 3 weeks and allow time for adult-plant resistance to express. After 3 weeks, the disease situation can be reassessed. This early fungicide application also would stop leaf blotch and leaf rust that are ahead of normal in several fields. Any of the fungicides registered on wheat would work, but it may be more cost-effective to use an inexpensive fungicide now and use a more expensive fungicide later if needed.