Wheat Scouting and a little more about rusts
Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison
On April 2nd I scouted my first wheat plots of the 2013 season. Theses plots were located at the Arlington Agricultural Research Station located in Columbia Co. The snow was finally mostly off this field and plants were readily visible. Spotty areas of ice damage and winter desiccation were evident as the field was scanned (Fig. 1). On closer look, many of the wheat plants appear to be in mediocre or poor shape (Fig. 2). However, many of these plants still had green crowns and white roots. For more information about assessing stands of wheat like this one, consult Dr. Conley’s blog at http://thesoyreport.blogspot.com.
Additionally I scouted these stands of wheat for any signs of pathogens and plant disease. No diseases where readily evident in these plots at this time. As the weather gets warmer and spring rains begin to set in, growers and scouts should scout wheat more frequently. These weather conditions are conducive for many diseases of wheat.
Last week I also had the opportunity to attend a 1-day workshop at the USDA-ARS Cereal Disease Laboratory located on the University of Minnesota campus in St. Paul. At the workshop attendees were updated on the latest information about rusts of wheat. The main focus was on stem rust of wheat, but we also talked about the status of leaf rust and stripe rust.
You will remember from the last issue of the Wisconsin Crop Manager, I talked a bit about rusts in the context of treating them using fungicides. All rusts (stripe rust, leaf rust, and stem rust) are favored by wet weather and periods of extended leaf wetness. However, temperature optimums vary among the different rusts. Stripe rust is favored during periods when temperatures are between 55 and 65F, while leaf rust tends to occur at a higher temperature range of 60 to 72F. Stem rust tends to occur at temperatures between 60 and 104F. In most years, spores of the rust pathogens will not readily overwinter in Wisconsin. Cold temperatures (<32 F) will reduce the ability of rust pathogens to overwinter. In the case of stem rust spores especially, spores are brought into Wisconsin on wind currents from southern states.
In years where there is extended snow cover, two of the wheat rusts may have the ability of overwintering in more northern latitudes in the U.S. Some work has been done to show that even in the Minneapolis area, the leaf rust pathogen can overwinter on wheat debris under an insulating blanket of snow. If you consider that the stripe rust pathogen tends to like cooler temperatures, the likelihood of this pathogen overwintering is even higher. The stem rust pathogen is least likely to overwinter in the northern U.S. because it prefers much warmer conditions. These trends are evident in the 2012 observation maps that were prepared by the USDA-ARS Cereal Disease lab. Patterns of leaf rust (Fig. 3) and stripe rust (Fig. 4) reporting were much more uniform across the U.S. in 2012, reflecting the fact that these two rusts might have the increased ability to overwinter in more northern latitudes, and that more races of these rusts might be present. Maps of stem rust observations show that this disease is less uniform in occurrence indicating that there are not only fewer races, but the pathogen has to be blown in from locales such as far southern Texas each season.
Be aware, that the occurrence of leaf rust and stripe rust is more likely in Wisconsin than is stem rust, but all three rusts can occur here on winter wheat. Crop Management professionals should scout stands of wheat frequently for rust prior to heading. As with any disease, the success of a rust management decision will be higher, the earlier you catch the epidemic. Be sure that you walk the field and check multiple locations, perhaps walking in a zigzag or W-shaped pattern checking plants frequently along the route. For more information on how to identify rusts and other foliar diseases of wheat visit: http://fyi.uwex.edu/fieldcroppathology/files/2010/11/Wheat_Disease_ID.pdf