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Watch Wheat Disease


The winter wheat crop is looking the best it has in years, but don’t take your eyes off it for too long.

USDA has pegged the winter wheat crop at 60% in good-to-excellent condition — the highest-quality rating in seven years. But good wheat growing conditions in the spring can also be conducive to disease. So if you spy a yellow or discolored patch in the field, stop the truck and investigate, said Oklahoma State University Extension wheat pathologist Bob Hunger.

“When you see those possible hot spots, stop and see if it’s foliar disease, aphids or virus symptoms,” he said. “It could also be fertility issues — cold, wet soils can cause wheat to yellow up, too.”

With 6% of the country’s winter wheat starting to head, the ideal window for fungicide applications is opening up for many. Now is a good time to brush up on the common early season wheat diseases and their spring symptoms.

STRIPE RUST: Stripe rust has been a common villain in the Southern Great Plains in recent years. The disease overwinters in Mexico and parts of Texas and can travel rapidly north via wind and storms.

Stripe rust pustules tend to be yellow or orange in color and confined to interveinal spaces on the leaf, creating the disease’s signature striping pattern.

Fortunately, stripe rust has been laying low this year, Hunger said. Reports from Texas and Oklahoma have been sparse, and spring temperatures may soon move beyond the disease’s preferred range. “Stripe rust usually appears earlier than leaf rust, because it does better with cooler temps in 50s and low 60s,” Hunger explained.

LEAF RUST: Leaf rust remains a possible threat for most winter wheat growers, Hunger said. It prefers temperatures in the 70s and plenty of moisture. The pustules are rusty brown and will appear scattered across a leaf, Hunger said.

“I would watch for leaf rust because it can still come on later in the spring,” he added. “We’ve got a pretty good-looking crop so we don’t want to take a big ding from leaf rust coming in late.”

THE SPOTTY BUNCH: The leaf spot diseases that can attack wheat — tan spot, Septoria tritici blotch and Septoria/Stagonospora nodorum blotch — also favor the moderate temperatures and wet weather that spring commonly brings.

Each of these diseases produces spotty tan lesions on young wheat leaves, although they vary in size and development. See more details on each here: https://www.ag.ndsu.edu/…

THE VIRAL VILLAINS: The worst-case scenario for spring wheat disease is almost certainly a wheat virus, which cannot be treated in-season.

Wheat streak mosaic, triticum mosaic and High Plains mosaic are all spread by the wheat curl mite — a tiny insect that survives on volunteer wheat and can infect plants in the fall or early spring.

Wheat streak mosaic (WSM) caused widespread infection and yield loss in the Plains in 2017, and the disease also surfaced in 2018. Given plentiful moisture and volunteer wheat in the summer and fall of last year, farmers would be wise to be on alert for the disease in the spring.

However, Hunger is hopeful that late planting of the winter wheat crop last year may have helped stave off WSM infections this past fall. Nonetheless, growers should scout for the disease’s tell-tale pale yellow streaks that can spread across a wheat leaf, in a mosaic pattern. Don’t confuse it with some nutrient deficiencies, which cause more uniform, top-to-bottom interveinal streaks, he added.

See more on wheat streak mosaic here: http://extensionpublications.unl.edu/…

Barley yellow dwarf (BYD), spread by a variety of aphid species, also causes early season yellowing of wheat leaves. The disease can also cause purple discoloration of the leaf, and plants will be stunted, Hunger said. See more here: https://cropwatch.unl.edu/…

Finally, University of Illinois plant pathologist Nathan Kleczewski recommended that wheat growers be on the lookout for soilborne wheat viruses that favor cool, wet conditions: wheat spindle streak virus and wheat soilborne mosaic virus.

Both are caused by soilborne microbes and cause chlorotic, stunted plants, often in low-lying compacted sections of the field. Fortunately, good genetic resistance is available for these two diseases in most wheat varieties, Hunger added.

Source: Emily Unglesbee, DTN

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