Wheat Disease Update, May 20, 2011

From Ned Tisserat, Extension Specialist and Professor
CSU Bioagricultural Sciences and Pest Management

The wet cool weather over most of the eastern part of Colorado slowed crop development. I have had reports of some barley yellow dwarf and some dryland root rot, but not any samples or confirmations of rust diseases. About the closest report of rust is from central Kansas. So right now it is looking good for us. However, the weather the past week was very conducive for stripe rust and it would be good to keep an eye out for any hot spots in the field. Stripe rust usually appears in small patches or foci in a field.

One question I commonly get is whether it is worth it to put on a fungicide spray at flowering even though no stripe or leaf rust is present in the field or in the general vicinity. The thought is that this fungicide application would be good ‘insurance’ in case stripe rust or leaf rust were to develop after flowering (sort of the last legal point for fungicide application). We had just that situation in Ft. Collins last year. I applied fungicides at flowering – very little stripe rust was present. However, the disease blew up about 10 days after flowering and completely covered the flag leaf of non-fungicide treated plants. The plants treated with fungicides at flowering were almost completely free of rust pustules – the chemical control worked beautifully. Yet there was no difference in yield or test weight among the untreated and treated plants (see the attached table [download id=”325″]– one clarification about terminology, incidence means the percentage of plants in the plots with rust whereas severity is the average area damaged on the flag leaf – severity in this case is the more important measurement). Also note that the early fungicide sprays at jointing were completely unnecessary. We had similar results in Ft. Collins in 2009. Based on these data, I am skeptical that insurance sprays are worth it. On the other hand, if you have moderate rust in your field at flag leaf or flowering, fungicide sprays can provide a substantial yield boost. But we would have to have a major change this year to see a significant rust epidemic in Colorado. Yes, I know, I’m tempting fate with statements like that.

I have included below reports from neighboring states:

Oklahoma: (Dr. Bob Hunger) Over the last week, I have been at field days in north central OK as well as in fields/trials around Stillwater. Leaf rust has reached levels of 65-80% around Stillwater where there has been more rainfall, but levels on susceptible varieties are not consistent from field to field. A few pustules of stem rust also were noted on stems of McNair 701 in a trap plot at Stillwater and have been sent to the CDL. Yesterday I attended a field day north of Ponca City (@ Kildare – about 50 miles north of Stillwater) and then drove straight west to Hwy 132 and south to Lahoma (about 15 miles west of Enid, OK). Leaf rust was at low incidence at all the stops I made. I did find a few plants with whiteheads that showed evidence of dryland (Fusarium) root rot, and an occasional symptom of septoria/tan spot. Most of the wheat was at soft to medium dough. Some fields looked decent while others were thin and obviously drought/heat stressed. Barley yellow dwarf was still the most prevalent disease, but I believe the discoloration associated with BYD must have been enhanced by the drought and/or heat and/or freeze and/or high winds we have experienced this spring.

Reports/excerpts of reports from other states:
Kansas (Dr. Erick De Wolf, Wheat Plant Pathologist, Kansas State University), 13-May: Disease scouting this week reveals that leaf rust is present at very low levels in parts of central Kansas. The most recent find came from McPherson county where leaf rust was found at low levels on the second leaf down within the canopy. The incidence of leaf rust was still less than 5%. Stripe rust was also reported By Joe Martin, KSU wheat breeder, in Ellis county near Hays Kansas (Central, KS) this week. Clayton Seaman, who works with Joe, reports finding only a single lesion in the mid canopy at this location. Weather conditions remain dry in most areas of the state with severe drought in western Kansas. Wheat in north central Kansas has been heading and flowering this week and some fields in the south are well into the grain filling stages of growth. The rust diseases may increase near the end of the season, but are not expected to be a major factor in most fields this year.

Ned Tisserat
Extension Specialist and Professor
Bioagricultural Sciences and Pest Management
Plant Sciences 1177
Ft. Collins CO 80523
970-491-3862 (FAX)

This entry was posted in Colorado Wheat Blog, News & Events, Wheat Pest and Disease Update. Bookmark the permalink.