Stripe Rust Update – April 11, 2016
Update from Kirk Broders, Assistant Professor in the Department of Bioagricultural Sciences and Pest Management, Colorado State University. Wheat in the Fort Collins area varies, but most is still in the tillering stage (Feekes 5). Wheat around the state is also at several different growth stages ranging from tillering to stem elongation (Feekes 6-7). Regarding disease stripe rust is still present in locations where it was initially reported, but the dry weather has kept it from spreading outside of those initial early detection locations on the western slope and the Prospect Valley area near Keenesburg. I have also received a report of stripe rust near Stratton. I believe the level of disease was low, but it does appear to be present in that area of the state as well. I don’t believe there has been much movement or increase in stripe rust because the weather has not been conducive (no free moisture on leaves). Similar reports from Oklahoma confirm this slow down in stripe rust movement. However, we are getting a much needed rain here in Fort Collins as I write this message. There is rain in the forecast for much of eastern Colorado this evening and over the course of the next week, and stripe rust could start to become very active again. In addition, the rains bring suitable conditions for leaf rust as well, which has not been reported from many locations in Colorado, but is present in most of the western Kansas counties that border Colorado. However, this rain is certainly needed in many areas of the the state, particularly in the southeast, where early signs of drought stress are starting to be reported. The other disease that will start showing up as the wheat really starts to grow are the virus diseases, specifically wheat streak mosaic virus (WSMV). Initial foliar symptoms of wheat streak mosaic virus typically show up in the spring. The pattern of the disease in the field is tied to the distribution of its vector, the wheat curl mite. Typically the mite survives on volunteer wheat, corn, or other weedy grasses until wheat is planted in the fall and then the mite moves back to wheat as it begins to emerge. Therefore, mosaic patterns will usually show up on the edges of the field first or areas of the field that had significant volunteer wheat the prior summer. Affected wheat plants are typically stunted with mottled, streaked leaves. The streaks consist of yellow, discontinuous dashes running parallel to the veins. We have research plots in Akron with high populations of the wheat curl mite and WSMV and symptoms have really started to develop in the last week. I would expect symptoms in infected fields throughout the rest of the state to start developing over the coming weeks. You can find some good photos of WSMV virus symptoms and a description of wheat viruses here (http://store.msuextension.org/publications/AgandNaturalResources/MT200911AG.pdf). There area also two other virus vectored by the wheat curl mite, Tritium Mosaic virus and Wheat Mosiac Virus. We may also start seeing early symptoms of Barley Yellow Dwarf Virus (BYDV). This virus is transmitted by aphids, and so virus symptoms associated with high populations of aphids are likely the cause of BYDV. If you are observing virus symptoms in your fields you can send samples to either the CSU diagnostic lab or directly to Dr. Broders at the address in the heading of this message. We are conducting a survey of virus pathogens in Colorado and as a part of the project will be able cover the cost of virus diagnosis. Reports/excerpts of reports from other states: Kansas: Dr. Erick DeWolf; Extn Plant Pathologist; Kansas State University; Manhattan, KS; Apr 8, 2016: “The wheat crop in Kansas is now at the flag leaf emergence stage of growth in much of southern and central Kansas. The crop is at mid to late joining in the west central and northwestern regions of the state. Stripe rust continues to be my primary focus this week with new reports from additional counties and further disease development in central Kansas. The disease is still limited to the lower leaves for the most part with occasional mid canopy leaves with trace levels. The incidence stripe rust on the lower leaves of susceptible varieties ranges from 1-30%. Leaf rust was observed at multiple locations in central Kanas this week also. Leaf rust is also on the lower leaves with only trace levels found in most plots. I did observe a few fields and plots in Reno and McPherson counties with incidence of leaf rust approaching 90% on the lower leaves. The severity of the infection was still low (<10%) in most cases. The dry conditions may be slowing the spread temporarily but growers should be watching this situation carefully. Be prepared to apply a fungicide if disease continues to progress.” Oklahoma: Dr. Bob Hunger; Extension Wheat Pathologist; Oklahoma State University: Wheat varies around Stillwater, but mostly is at flag leaf emerged with the heads approaching the boot. Brian Olson (my Technician) reported mostly the same at Lahoma (15 miles west of Enid in north-central OK). Yesterday I was in wheat fields near Talala, OK in NE OK, and wheat there ranged from flag leaf emerging to awns starting to appear. Also, more wheat is showing or starting to show drought stress across much of the state. Regarding disease, stripe rust remains the most prevalent disease with actively sporulating pustules still quite evident. I don’t believe there has been much movement/increase in stripe rust because the weather has not been conducive (no free moisture on leaves). Gary Strickland (Extn Educator; Jackson County in SW OK) indicated the same thing to me earlier this past week, i.e., that foliar disease (primarily stripe rust) seems to be in a “holding pattern.” At Lahoma, Brian indicated seeing stripe rust but only rarely on the flag leaves; mostly on leaves below the flag and at varying degrees of severity. I also have seen leaf rust pustules on lower leaves around Stillwater, but at low frequency. Powdery mildew also has become more apparent around Stillwater but only in a few locations. Also around Stillwater and in northeast OK yesterday, I saw colonies of greenbug but not in great numbers. However, because of these aphids and earlier high numbers of aphids (especially bird cherry-oat aphids), I am starting to see more and more “hot spots” of barley yellow dwarf. Texas: Dr. Amir Ibrahim, Professor & Small Grains Breeder/Geneticist, Texas A&M AgriLife Research, Apr 8, 2016 [This is from a report on the Rust Evaluation Nursery located near Castroville, TX]: “Wheat leaf rust (Puccinia triticina) has spread uniformly in our trials, populations, and disease screening rows. Wheat stripe rust (Puccinia striiformis f.sp. tritici) has uniformly spread in the lower canopy across the field at Castroville in February but warming temperatures have slowed it or stopped it, and it is inactive at this point at Castroville. Stripe rust has caused significant damage to the lower canopy in susceptible types. Symptoms can still be seen on the flag leaves of very susceptible wheats such as ‘Redhawk’. The wheat lower canopy does not contribute significantly to grain yield, but such significant damage to the lower canopy will be reflected in yield somehow even though no symptoms can be detected on the upper canopy in general and flag leaves specifically.