Posted 29 July 2013. PMN Crop News.
Cereal Rusts and Bacterial Leaf Streak Appearing
Source: South Dakota State University Press Release. www.sdstate.edu
Brookings, South Dakota (July 9, 2013)--SDSU Extension specialists scouted several South Dakota winter wheat fields the first week in July and found leaf rust and stripe rust in several counties but only found stem rust in Brookings County.
Leaf rust was found in Davison, Lyman, Hughes, and Brooking Counties and stripe rust was found in Brookings, Lyman, Brule and Charles Mix Counties.
"Stripe and leaf rusts were found to be at trace levels in all fields where rust was found. Stem rust in Brookings County was found only on the Robidoux cultivar which is susceptible to common stem rust races in the Great Plains region, in the screening nursery at moderate severity," said Emmanuel Byamukama, SDSU Extension Plant Pathology Specialist.
Byamukama explained that stripe rust, which is also called yellow rust, causes yellow, blister-like lesions that are arranged in stripes. Whereas, leaf rust which is also known as orange rust, causes small, orange-brown blister like lesions on leaves. Stem rust, also called black rust, has reddish-brown color pustules and these cause tearing as they burst through the outer layers of plant tissues. The stem rust can occur on leaf sheaths, leaves, and stems.
"The presence of these rusts in a few wheat fields in South Dakota indicates that there is rust inoculum in the state. However, there is low chance of rain in the next seven days, therefore the probability of these rusts causing significant yield loss remains low, especially for wheat fields that are in the grain fill stage," Byamukama said.
He adds that growers should nevertheless scout their fields and be ready to apply a fungicide to protect the flag leaf against rust and other fungal infections if wet weather conditions are predicted.
Review fungicide recommendations for controlling cereal rusts.
Byamukama explained the conditions for each of the rusts.
"Stripe rust requires low temperatures, less than 60o F, and at least eight hours of free moisture like heavy dew on plants to infect plants. Leaf rust requires between 65 to 77o F and at least six hours of free moisture, while stem rust requires warmer temperatures 65-85 o F and at least 6 hours of free moisture," he said.
Fusarium head blight and other diseases
While scouting fields, fusarium head blight (FHB) was observed in winter wheat fields in Charles Mix, Davison and Brookings Counties at low incidence. Byamukama said the dry weather conditions forecasted indicate reduced risk for FHB development in the next few days. To view the Scab Prediction website visit, www.wheatscab.psu.edu.
Bacterial leaf streak was another disease noted in some of the surveyed wheat fields. He said that bacterial leaf streak was between low to moderate severity for most fields except fields that had hail damage.
"Fields which experienced hail damage seemed to have elevated severity of bacterial leaf streak. If bacteria infects the wheat heads it causes black chaff, where the wheat head becomes brown-black with necrotic streaks and blotches mainly on the glumes," Byamukama said. "This disease cannot be managed by applying fungicides. Use of clean seed and resistant/tolerant cultivars are the recommended management practices."
Leaf spot diseases, such as tan spot, septoria/stagnaspora blotch, were mainly found in the mid-to-lower canopy in most wheat fields.
"The pathogens that cause these leaf spot diseases survive on residues and require wet conditions for infection to occur. Dry periods like we are having currently prevent new infections and lesion expansion to occur. The fungal leaf spot diseases can be managed effectively through wheat residue management and through use of fungicides," he said.
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