Cool weather slows spread of disease

COLUMBUS – Cooler than normal temperatures in the week of May 27 to June 2 may have limited the occurrence of Fusarium head scab in Ohio winter wheat fields, said Pat Lipps, plant pathologist, Ohio State University.

An OSU forecasting model rates the risk for infections as generally low in areas north of Interstate 70 in Ohio, and a moderate risk exists in areas to the south. The forecast is based on weather data collected during the pre-flowering and post-flowering stages of wheat development.

“Head scab is favored by wet conditions during the seven days preceding flowering, and several days of rain during flowering if temperatures are above 60 degrees Fahrenheit,” Lipps said. “Most locations have had sufficient rain at flowering, but temperatures have been below or only slightly above 60.”

Ohio’s regional average temperatures were in the upper 50s for the week ending June 3, which is 7-8 degrees below normal, according to the Ohio Crop Progress Report. All areas reported above-average precipitation for the growing season except the northeast, which had a deficit. Ohio’s average precipitation is about 1.6 inches above normal for the growing season.

Wheat scouting.

Forecasts help, but nothing replaces good old-fashioned scouting in order to determine if wheat is infected, Lipps said. All Ohio wheat is either past or approaching 10 days after flowering – Feeke’s Growth Stage 10.5 – which is when symptoms appear.

“If head scab is going to be important this year, we’ll know about it soon,” Lipps said. “Symptoms should become evident by late this week (June 3-9) or next week (June 10-16).”

Head scab symptoms include dead or bleached-out florets on heads scattered throughout a field. Generally, not all florets will show symptoms while the rest of the head is green. By harvest, seeds in infected heads will be shriveled and have a white to pinkish color.

Harvest management.

Knowing the extent of a head scab problem is important for harvest management. Fields with high levels of infection require special handling to prevent mixing with healthy wheat. High levels of scabby kernels may indicate high levels of vomitoxin, which is undesirable for baking. In addition, vomitoxin can cause health problems in livestock.

Growers can see the head scab disease forecast at the Ohio Field Crop Disease Web site, at www.oardc.ohio-state.edu/ohiofieldcropdisease/. Click on the area titled, 2001 Ohio Wheat Head Scab Risk. A map at the left shows results of the first model based on weather conditions prior to flowering, when it forecasts a high threat for west-central and central Ohio. The map on the right incorporates weather data from the first model plus an additional 10 days of post-flowering data.

The site.

Lipps cautions growers that the Web site is for informational purposes only. They should not use it to make farm management decisions. The site also includes color pictures of scab-infected kernels. Click on “Wheat,” which links to the “Ohio Wheat Diseases” page, and then click on “head scab” to get to the pictures.

Cool weather also may have slowed down the spread of Stagonospora leaf and glume blotch, but can favor powdery mildew, Lipps said. In any event, Feeke’s Growth State 10.5 is past the time when fungicides can be safely applied.

Stagonospora is developing on susceptible varieties. Yield loss may be minimized if grain filling occurs before the disease gets severe on the flag leaf and head. Stagonospora will develop quickly if temperatures rise above 80 degrees. Leaf blotch symptoms are tan oval spots with yellow halos on leaves. Identify glume blotch by dark brown to purple blotches on the glumes of the head.

As for powdery mildew, expect yield losses in fields with plants that have mildew on the flag leaves. Temperatures above 80 degrees would stop mildew development.