Wheat Disease Update – April 2, 2018
Dr. Kirk Broders, Plant Pathologist
Department of Bioagricultural Sciences and Pest Management
Colorado State University
The 2018 wheat season is well underway. The good news is that this is the first disease update, as there are no reports of stripe rust overwintering in Colorado so far in 2018, and so no need for earlier communications. The bad news is the drought conditions present across the state that helped prevent stripe rust from overwintering. As you can see at droughtmonitor.unl.edu, 90% of Colorado is under drought conditions, with the southeast part of the state in severe/extreme drought.
However, over the course of the last two weeks, areas north of I-70 have seen enough precipitation to help the wheat break dormancy and begin growing. I have received several messages regarding my recommendation on whether to include a fungicide along with the early herbicide application growers are planning on putting on in the next couple weeks. Given that the risk of stripe rust is very low at this point in the season, I do not think it is necessary, or worth spending the money, to make an early fungicide application. The only exception would be for wheat grown under center pivot irrigation, or wheat planted into wheat fields where tan spot, stagonospora blotch or powdery mildew were observed last year (see disease updates below for pics and info), as the pathogens can survive in the wheat residue. All three of these pathogens are Ascomycete fungi, which means fungicides in the strobilurin or triazole (See fungicide table below) class will be effective at controlling these pathogens.
The other major disease to keep an eye out for early this season is the mite-vectored viruses. Last year both the incidence (% of fields infected) and severity (% of yield lost) were at very high levels across regions of Colorado, Kansas and Nebraska. The significant mite and pathogen population in the region, the presence of volunteer wheat, and warm temperature into late November, likely allowed wheat curl mites to be more active in the fall for a longer period of time, resulting in a greater number of infections in some areas. Once wheat is infected with Wheat Streak Mosaic Virus, Triticum Mosaic virus, or the aphid-transmitted Barley Yellow Dwarf Virus there is no chemical treatment that can eliminate the pathogen. If you believe you are observing virus symptoms in any of your fields, but are unsure of the diagnosis, I would encourage you to send samples to the CSU Plant Disease Diagnostic Clinic.
Finally, while stripe rust is not here yet, it is only a matter or time (and weather). The fungus, Puccinia striiformis, overwinters in southern Texas and then starts to become active when there is precipitation and cool temperatures. On March 21, Dr. Clark Neely (Small Grains/Oilseed Extension Specialist; Texas A&M AgriLife Extension) indicated that, “A report from Uvalde, TX late last week indicated stripe rust had increased significantly on susceptible checks. A fungicide trial in College Station, TX showed stripe rust building in the lower canopy. Flag leaves were still pretty clean, but F1 and F2 leaves were 5% or greater in much of the trial. The field surrounding the trial did not show any obvious signs of stripe rust at this time, but showed heavier levels of leaf rust. Talked to a grower in the Waco area and he reported leaf rust building in his wheat fields as well. Expecting dry conditions through the weekend but then turning wet for much of next week, which could facilitate further development.”
On March 23, Dr. Dr. Amir Ibrahim (Professor & Small Grains Breeder/Geneticist; Texas A&M AgriLife Research) indicated a somewhat similar scenario for wheat rust in south and central Texas by stating that, “Uvalde seems to be the first location where we detect wheat stripe rust every year. Stripe rust is also active at Castroville, College Station and McGregor. Warming temperatures expected at these locations may slow it significantly during coming weeks.”
Based on these two reports, Dr. Bob Hunger (Professor & Extension Wheat Pathologist; Oklahoma State University) stated “Based on these two reports (from Texas) and the recent weather conditions in Oklahoma, I would expect stripe and leaf rust to begin to appear across Oklahoma where moisture was received over the last week. It likely has been too dry in western/northwestern OK and the panhandle to facilitate wheat rusts.”
In summary, stripe rust is still restricted to the southern Texas, but this can change quickly if there is ample precipitation combined with cooler temperatures. However, for now Colorado wheat growers should not see stripe rust for several more weeks. I will keep you posted if there is any changes in the stripe rust situation in Texas and Oklahoma and how this may impact fungicide application timing decisions.