Wheat Disease Outlook

Stripe Rust on the Move; Leaf Spot Diseases Surfacing

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Stripe rust is among the foliar wheat diseases taking advantage of cool temperatures and plenty of moisture to infect winter wheat in the Southern Great Plains this spring. (DTN photo by Pamela Smith)

ROCKVILLE, Md. (DTN) -- Cool temperatures and plentiful moisture in the Southern Great Plains have allowed rust and leaf spot diseases to thrive and spread easily this spring.

Stripe rust has become prominent in southwestern Oklahoma and is poised to catch a ride on storm systems into Kansas, said Oklahoma State University Extension Wheat Pathologist Bob Hunger. Leaf spot diseases such as tan spot and Septoria tritici blotch aren't as good at hitchhiking, but they are also taking advantage of damp fields and cool weather to speckle the canopies of many Oklahoma wheat fields, Hunger added.

STRIPE RUST ON THE MOVE

The yellow and orange pustules of stripe rust, which typically arrange themselves in tell-tale stripes on wheat leaves, prefer moderate temperatures between 50 and 60 degrees Fahrenheit. Once spring days start warming into the 80s, with nighttime temperatures in the upper-60s, stripe rust development slows, Hunger said.

That leaves plenty of time this spring for Plains growers to see the disease spread in their fields, he noted. "Stripe rust will start slowing down as we warm up, but we need temperatures to be consistently up in the 80s in the day and 60s at night," he said.

Kansas State University Extension wheat and alfalfa pathologist Erick De Wolf agreed. "With the wheat crop in south-central and southeast Kansas approaching or already at the flag leaf emergence stages of growth, growers are encouraged to be on the lookout for diseases," he warned in a university newsletter recently.

Because wheat can rapidly put on new leaves prior to heading, he urged growers to look deep into the canopy for older leaves when scouting.

"Focus on leaves that were present over the last two weeks," he wrote. "These leaves have a higher probability of infection than the new leaves at the top of the canopy."

See more on stripe rust from Hunger here: http://entoplp.okstate.edu/… and DeWolf here: https://webapp.agron.ksu.edu/….

TROUBLING SPOTS

Leaf spot diseases such as Septoria tritici blotch and tan spot are opportunistic lurkers, which often linger on wheat residue, always ready to infect fields if conditions are right, Hunger said.

Normally, they don't rise to the level of economic damage, or even infect leaves beyond the lower canopy, but this year has been different, Hunger added.

"Typically, they don't get up on flag leaf, but this year they have to some extent," he said. "When it gets there, they can have a yield impact."

Both diseases will initially surface on the lowest leaves, close to the ground, Hunger explained.

Tan spot infections start as small oval or diamond-shaped spots, which expand into larger lesion, often with a yellow halo and a small brown spot in the center, as this North Dakota State University guide explains: https://www.ag.ndsu.edu/….

Septoria tritici blotch lesions start as small yellow flecks that expand in to larger, irregular-shaped brown lesions that sometimes have a bleached center with dark specks -- the spore-producing bodies, or "pycnidia." See more from Ohio State here: https://ohioline.osu.edu/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

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(PS/AG)

Emily Unglesbee