Wheat Disease Update – September 13, 2016

Wheat Disease Update – September 13, 2016

Dr. Kirk Broders, Plant Pathologist

Department of Bioagricultural Sciences and Pest Management

Colorado State University

Planting winter wheat is getting underway in Colorado and there are a several disease topics growers need to consider at this time of year. Making good disease management decisions this fall will have positive impacts on reducing disease the following spring and summer.

Planting Date: Planting wheat early significantly increases the likelihood that disease such as mite-transmitted viruses, the aphid/barley yellow dwarf complex, and root and foot rots will be more prevalent and more severe. For much of eastern Colorado the optimum planting dates are between September 10 and October 15.

Use fungicide-treated seed. Fungicide seed treatments reduce losses caused by seed-transmitted and soilborne fungal pathogens of wheat. Seed-borne pathogens controlled by fungicide seed treatments include the bunts and smuts, including common bunt (also called stinking smut), loose smut, and flag smut. The names are quite similar and can be confusing. Common bunt and flag smut spores carryover ON seed or in the soil, whereas loose smut carries over IN the seed. The good news is that seed treatments are highly effective in controlling  bunts and smuts. Soilborne diseases controlled by fungicide seed treatments include Rhizoctonia and Pythium rot roots, and Fusarium root and crown rots.  Some seed treatment products contain a fungicide and an insecticide and offer additional protection against fall diseases and insect vectors, such as aphids, which can transmit  Barley Yellow Dwarf Virus and Cereal Yellow Dwarf Virus. However, the insecticide does not protect against mites and therefore will not protect against wheat curl mite-vectored viruses such as wheat streak mosaic, wheat mosaic and triticum mosaic.

Manage mite-transmitted virus diseases. These include Wheat Streak Mosaic Virus (WSMV), Wheat Mosaic Virus (previously called high plains disease or high plains virus), and Triticum Mosaic (TrMV). All of these viruses are transmitted by the wheat curl mite (WCM). Problems from this disease complex occur when the wheat curl mite that transmits the viruses is able to over-summer in significant numbers. However, the mite cannot survive off green plants for more than a day or two at the most; therefore, the mite needs a ‘green bridge’ to be able to survive until the new crop emerges in the fall. 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. In Colorado the greatest ‘green bridge’ risk results when pre-harvest hail rapidly results in pre-harvest volunteer wheat.  The mites quickly carry the viruses to this volunteer and attempt to ride out the summer.  The mite and viruses can also survive on some other grasses present through the summer, but the next most significant risk for mite-virus over-summering is corn.  Mite populations can establish on the corn and carry the viruses through corn and move to the new wheat crop in the fall.  Damage around corn fields will be variable but will depend on how green the corn stays (irrigated corn is at greater risk than dryland).

Significant August rains can improve the condition of dryland corn and extend its ‘greenness’ further into the fall when it can overlap with wheat emergence.  The severity of this spread will depend on the extent of this corn-wheat overlap and on the fall weather conditions (e.g. greater risk with warm extended fall).  Even though the risk of disease from adjacent corn is not as extreme as from pre-harvest volunteer, the gradient or border effects from mite and virus spread can be significant.  Virus risk around these areas can be managed by avoiding early planting in these areas to minimize this overlap.  Few commercial varieties have strong resistance to this virus complex, but a few newer varieties do carry virus resistant genes (e.g. CSU variety Snowmass; KSU varieties RonL, Oakley CL, Clara CL; UNL variety Mace).

Research completed near Akron, CO has demonstrated that the most reliable strategy for controlling virus infection is to plant between September 15 – October 15. Keep in mind this is a general rule and may vary from year to year. This year the corn is still green in many areas of eastern Colorado and may still represent a significant reservoir for the WCM. Waiting until the end of September will allow for the corn to adequately dry down and reduce the wheat curl mite population, while still allowing the wheat ample time to germinate and establish prior to the onset of colder temperatures. Evidence from our research indicates that even the most resistant varieties Snowmass and Mace show a yield penalty when planted early (before September 1) in areas of high wheat curl mite and virus pressure. However, they perform better than susceptible varieties Hatcher and Pronghorn at the earliest planting date, when virus pressure is at the greatest. In contrast, all varieties performed significantly better when planted after September 25th, and Hatcher performed as well or better than the resistant cultivars when planted in late September, indicating there was limited virus pressure after September 25.

Scout, but do not spray, for stripe and leaf rust in the fall. In the last few years stripe rust and leaf rust have survived on volunteer wheat and some weeds such as jointed goat grass through the summer and until emergence of fall-planted wheat. The spores of the rust fungi then disperse and infect the new emerged wheat plants. It is important to scout fields for rust in the fall, but do not apply fungicide in the fall. Instead, these fields should be noted and scouted early in the spring for the presence of active sporulation of stripe rust. If stripe rust appears on wheat in the fall, and there is consistent snow cover through the winter, the stripe rust pathogen has the ability to overwinter in Colorado, as was observed in a few locations in 2015-16. If there is active sporulation in the spring an early fungicide application is recommended to prevent further spread of the pathogen.



Kirk  D. Broders, PhD
Assistant Professor
Department of Bioagricultural Sciences and Pest Management
Colorado State University

Office: 970-491-0850

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