With stripe rust moving as far north as Salina, Kan., as of April 5, and leaf rust found as far north as Riley County in eastern Kansas and Ness County in western Kansas, it's a race to the finish line for the Kansas wheat crop, according to Erick DeWolf, plant pathologist for K-State Research and Extension.
Early spring rains have farmers and crop consultants on alert for the spread of these diseases, and DeWolf said farmers need to be checking fields for possible infestations.
For instance, the crop is heading out in southern Kansas. If farmers see active rust lesions in wheat fields now, be prepared to spray the crop with a fungicide.
If you don't see rust lesions at heading, a fungicide application may not be warranted. "But you do need to continue scouting those fields," recommended DeWolf, who added that growers need to remain vigilant and take steps to protect the crop until after the crop has flowered.
With rust moving into northern Kansas, Nebraska wheat growers should be scouting now for rust diseases, too, a University of Nebraska-Lincoln Extension plant pathologist said.
"The detection of leaf and stripe rusts in Kansas indicates that we are likely to see these diseases in Nebraska in the next two to three weeks," said Stephen Wegulo, UNL Extension plant pathologist in the university's Institute of Agriculture and Natural Resources.
Nebraska wheat is about two weeks ahead of normal crop development this year because of the unseasonably warm temperatures this winter and spring, he said.
Diligent field scouting will help farmers overcome the uncertainty of this year's race of stripe rust. "It appears that some varieties that looked vulnerable during our last stripe rust outbreak are not developing stripe rust this year," DeWolf said. "However, many varieties, including Tam 111, Everest, Armour and Cedar - all of which showed resistance in 2010 - are appearing susceptible to the variant of stripe rust that is showing up this year."
Growers have until a given fungicide's Pre-Harvest Interval, or the growth stage restriction on the label, to apply a fungicide. DeWolf advised farmers that for best results, apply fungicide between the growing stages of flag leaf fully extended (Feekes 9), and fully headed (Feekes 10.5). Some fungicides can be applied during the flowering stages of growth but generally have a 30-day pre-harvest interval. Once the flag leaf has emerged, farmers will have about two weeks to scout for disease and make a final decision about fungicide application.
Farmers have a choice of two basic categories of foliar fungicides:
- Strobilurins prevent diseases only and should be applied before symptoms appear. They have somewhat longer residual activity than triazoles.
- Triazoles are a better choice when diseases are already present. They can inhibit infections that have already started, and some of these products are more upwardly mobile within the leaves, than are strobilurins. However, movement within the plant is limited.
- Combination products contain both triazole and strobilurin modes of action.
In several years of yield trials, the application of fungicides to susceptible varieties between flag leaf and flowering resulted in a yield boost of 4 percent to 14 percent. This research was conducted at K-State.
Growers should use the best available information about varieties to determine that variety's resistance to diseases. Then, make an application decision based on the activity of disease in the fields. If diseases are present prior to heading, and the field has good yield potential, a fungicide application is more likely to pay off. Seed production fields are also a priority, DeWolf said.
"The decision to apply a fungicide product is reinforced by a forecast for moderate temperatures and frequent rain, because these conditions favor continued disease development," DeWolf added.
"A forecast for extreme temperatures, either cold or heat, or prolonged dry conditions might enable farmers to delay a decision to use a fungicide."
But with above normal temperatures in the forecast and if rainfall occurs, that combination could also lead to rapid development of foliar diseases in wheat including powdery mildew and fungal leaf spots.
"These have already been observed in southeast Nebraska," Wegulo said. There also have been reports of general yellowing of wheat fields.
This condition is common at the current growth stage and can be caused by several factors, including inadequate fertilization and virus diseases. If the yellowing is of a general nature with no obvious virus symptoms (stunting and leaf mosaics, mottling or streaking), it is most likely caused by inadequate fertilization.
"The wheat crop usually grows out of this condition and greens up as the growing season progresses," he said.
Wheat growers should scout fields routinely so timely management decisions can be made.
For more information about scouting wheat fields, including treatment information, visit www.ksre.ksu.edu or cropwatch.unl.edu.