Cereal rusts and bacterial leaf streak appearing in wheat

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click image to zoomFig. 1. Stem rust on Rubidoux cultivar in the winter wheat nursery at Brookings on June 30, 2013. Scouting for winter wheat diseases was done in Davison, Douglas, Brule, Charles Mix, Lyman, Aurora, Hughes, Hand, Kingsbury and Brookings counties in South Dakota over the last week. Leaf rust was found in Davison, Lyman, Hughes, and Brooking counties. Stripe rust was found in Brookings, Lyman, Brule and Charles Mix counties while stem rust was found only in Brookings County.

click image to zoomFig. 2. Stripe rust just beginning to develop on winter wheat in Southeast South Dakota on June 26, 2013. Stripe and leaf rusts were found to be at trace levels in all fields that were found with these rusts. Stem rust in Brookings County was found only on the Robidoux cultivar (susceptible to common stem rust races in the Great Plains region) in the screening nursery at moderate severity (Fig. 1). Stripe rust (also called yellow rust) causes yellow, blister-like lesions that are arranged in stripes (hence the name) (Fig. 2). Leaf rust (also known as orange rust) causes small, orange-brown blister like lesions on leaves (Fig. 3). 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 (Fig.1). The stem rust can occur on leaf sheaths, leaves, and stems.

click image to zoomFig. 3. Leaf rust seen on flag leaf in Davison County on June 26, 2013. 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 7 days, therefore the probability of these rusts causing significant yield loss remains low, especially for wheat fields that are in the grain fill stage. 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 (see link for the fungicides recommended for controlling cereal rusts). Stripe rust requires low temperatures (<60° F) and at least 8 hours of free moisture (heavy dew on plants) to infect plants. Leaf rust requires between 65 to 77° F and at least 6 hours of free moisture, while stem rust requires warmer temperatures 65-85° F and at least 6 hours of free moisture.

Fig. 4. Fusarium head blight on winter wheat in Douglas County on June 26, 2013. Fusarium head blight (FHB) was observed in winter wheat fields in Charles Mix, Davison and Brookings at low incidence (Fig. 4). The dry weather conditions in the weather forecast indicate reduced risk for FHB development in the next few days (see the Scab Prediction website).

Bacterial leaf streak was another disease noted in some of the surveyed wheat fields. Bacterial leaf streak was between low to moderate severity for most fields except fields that had hail damage. These fields seemed to have elevated severity of bacterial leaf streak. If the bacterial infects the wheat heads, it causes black chaff, where the wheat head becomes brown-black with necrotic streaks and blotches mainly on the glumes. This disease cannot be managed by applying fungicides! Use of clean seed and resistant/tolerant cultivars are the recommended management practices.

Leaf spot diseases (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. Fig. 5. Severe bacterial leaf streak in a winter wheat screening nursery at Brookings on June 30, 2013.

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