Now is the time to be scouting wheat fields vigilantly for signs of leaf rust, stripe rust and other foliar diseases such as powdery mildew and tan spot, Kansas State University specialists said in a Webinar presented Monday morning.
A new strain of stripe rust that attacks even those varieties with known genetic resistance was spotted in south Texas earlier this spring. Foliar diseases usually over-winter in the southern part of the U.S. and arrive in Kansas in early to mid-May on the prevailing southwesterly winds.
Rust has been spotted in multiple locations in central Kansas from south to north, said K-State plant pathologist Erick DeWolf, including on genetically resistant varieties. He said powdery mildew has also been seen and tan spot is showing up, especially in non-till, continuous wheat fields.
In Monday’s Webinar, K-State agronomist Jim Shroyer presented information on wheat growth stages and the best window of time for applying fungicides to get maximum yield protection.
Most fungicides have a pre-harvest interval of at least 30 days or a label restriction that they are not to be used after flowering, he said.
Wheat across the state is still early enough in development to be significantly affected by rust infection, he said, especially in North Central and Northwest Kansas.
“The most important thing is to be able to identify the flag leaf and protect it,” he said. “More than 75% of the nutrients to the head of the wheat plant are produced in the flag leaf.”
Growers can spot the flag leaf because it is the last leaf to emerge above the second growth node.
He said the optimum time for herbicide application is in the days between the emergence of the head and flowering, usually a seven to 10 day window.
The earlier foliar diseases are spotted the greater their potential to significantly reduce yields, he said.
“If you have wheat that has a fully emerged head and no sign of rust, then you really won’t see much advantage from applying fungicide,” he said. “But if you have rust just as the flag leaf is fully emerging, then you can get a significant yield bump from an application of fungicide.
There are three fungicide options, the strobilurins, such as Headline and Quadris; the Triazones such as Caramba, Folicur, Proline, Prosaro and Tilt and the Mixed Mode products such as Quilt, Stratego and Twinline.
For the stripe rust that is showing up on resistant varieties, the best option is to apply mixed mode fungicides or a combination of strobilurins, which have curative abilities, and mixed modes.
Products range in price from $4 an acre to $20 an acre depending on the product, the rate of application and the cost of application, he said. A yield response of about 10 percent or five to six bushels per acre is average in most of Kansas, the experts said.
The most favorable response can be in varieties with susceptibility to multiple diseases, while the least bang for the buck comes in varieties that have resistance to multiple diseases.
Experts said it is definitely time to apply a fungicide if you spot disease in a field with yield potential of more than 40 bushels per acre or in seed production fields; where risk of disease is high or has already been seen on the flag leaf .