Cool, damp weather has been welcome through much of Kansas this fall, but the conditions have sparked leaf rust in wheat, Kansas State University scientists said.

Producers are expressing concern that their fields are turning yellow and wondering if it's wheat streak mosaic, said K-State Research and Extension crop specialist Jim Shroyer. In this case, however, there are chlorotic (dying) areas with large orange pustules on the leaves. That is what's giving the field the yellow color and how it can be identified as leaf rust.

"I don't recall it ever being this widespread before," Shroyer said. "We always see some of this in south central Kansas, but it's quite unusual to see it so far west," he said, citing calls he's received from Wichita, Smith, Logan and Comanche counties, among others.

"Leaf rust has been observed throughout the state from south central to northwest Kansas and in north central Oklahoma as well," said K- State Extension plant pathologist Jim Stack. "Last week, I saw severe leaf rust in fields of volunteer wheat in Cheyenne and Sherman counties as well as in newly-emerged wheat."

"The number one question is, will it hurt the wheat? Generally, the answer is no, but I've had reports that some small wheat is dying," Shroyer said.

Although the severities in some fields are high, the probability that it will impact winter survival or final yield remains very small, Stack said.

Shroyer said the sooner wheat goes dormant, the better. Conversely, the longer wheat keeps growing this fall, if temperatures don't get cold enough, the rust could continue to develop.

Growers sometimes ask if leaf rust can overwinter and damage the wheat in the spring, he said.

"Generally, the answer is no, it won't overwinter if we have a normal cold winter. However, if we have an unusually warm winter, then it's possible leaf rust could overwinter and get an early start next spring," the agronomist said.

He explained that the relatively early wheat planting, coupled with fairly good moisture and foggy, cool, but not cold conditions kept wheat leaves damp for long periods of time. These are conditions that foster leaf rust growth.

No research has been done on leaf rust treatment in the fall, Shroyer said, because the threat usually disappears with cold weather.

"It appears that most of the leaf rust is on the old leaves and the newer leaves are okay - that's a good sign. If it stays warm, the newer leaves may become infected as well. I just can't imagine that a fungicide would be profitable or practical," he said, adding that the situation is different this year than in most years. In most years the wheat goes dormant "and that's the end of the story."

Stack said that Bob Hunger, a long-time wheat researcher from Oklahoma State University, has said he has never seen a case of fall leaf rust affecting final yield and that fungicide applications in the fall are economically unwarranted. Unless the winter is unusually mild, most of the rust spores will be killed by low winter temperatures.

K-State Research and Extension is a short name for the Kansas State University Agricultural Experiment Station and Cooperative Extension Service, a program designed to generate and distribute useful knowledge for the well-being of Kansans. Supported by county, state, federal and private funds, the program has county Extension offices, experiment fields, area Extension offices and regional research centers statewide. Its headquarters is on the K-State campus in Manhattan.