Rains spur wheat scab scouting

Aerial applicator, Jeff Chorman, of Allen Chorman & Son, applies fungicide to a Queen Anne’s County wheat field on May 4. (Photo by Sean Clougherty)

After a mostly dry April, multiple inches of rain coming April 28-30 brought much needed moisture to small grains fields, but for wheat at or near the flowering stage, it may also increase the risk of fursarium head blight, also called scab.
While forecasting tools for the Mid-Atlantic region show scab risk to be only slightly elevated, plant pathologists urge continued scouting in fields to help make management decisions on fungicide application.
Alyssa Koehler, University of Delaware Extension plant pathologist said growers can expect flowering in wheat to start three to five days after full head emergence, but a series of cool nights can push that window to seven to 10 days.
Fungicide is best applied in the time period between 50 percent of the wheat heads flowering to four to six days after flowering.
“You will be looking for bright yellow anthers in the center of the wheat head to signal the start of flowering,” Koehler wrote in the April 28 edition of the University’s Weekly Crop Update. “Anthers can remain attached after flowering but become pale white.”
Koehler added data from recent years has shown that it is better to spray a little bit after first flowering than too soon, particularly to suppress vomitoxin.
“If you spray too early, heads that have not emerged (secondary tillers) will not be protected by the fungicide application,” Koehler said adding the rain and lower temperatures should allow for good grain fill.
Another management aid is the U.S. Wheat & Barley Scab Initiative’s Fusarium Risk Tool, available at wheatscab.psu.edu.
The tool forecasts risk two, four and six days out from the current date and provides commentary from disease specialists from throughout the region.
Variety plays a role in the decision-making process, too.
According to the Fusarium Risk Tool on May 5, winter wheat varieties “very susceptible” to scab were deemed high risk in nearly all of Delaware and Pennsylvania and most of central and eastern Maryland. Areas of the states not in high risk were in the medium risk category.
Stepping down to varieties labeled “susceptible” to scab showed a drastic difference in the tool, moving Delaware and most of Maryland to a low-risk rating.
The trend continued for the tool’s “moderately susceptible” and “moderately resistant” settings.
In USDA’s May 1 Crop Progress and Condition Report, Maryland winter wheat was rated 84 percent Good, and Maryland barley was 87 percent Good.
In Delaware, barley was rated 57 percent fair and 38 percent Good and wheat was rated 80 percent Good.
Scab risk in barley was on its way up as more of the crop reaches head emergence, according to Alyssa Collins, Penn State plant pathologist, but noted some good news in fungicide application.
“Work done by researchers in North Carolina found that the best spray timing for protecting winter barley from scab is application six days after 100% heading,” Collins wrote in a May 2 post to the Fusarium Risk Tool. “With this new guidance we should get better (vomitoxin) reduction in barley than under our previous recommendation which targeted 50-percent heading.”
Caramba, Miravis Ace, Prosaro, Prosaro Pro, and Sphaerex all provide very good scab suppression, Collins said.
The Miravis Ace label allows for earlier application than Caramba or Prosaro, but best results are still achieved when application is timed after full heading in barley.
Collins urged growers to not use any of the strobilurins (Quadris, Headline), or strobilurin/triazole combinations (Twinline, Quilt, Stratego) at flowering or later as there is evidence that they may cause an increase in mycotoxin production.