Leaf spot diseases beginning to develop in winter wheat

Farm Forum

Article by Emmanuel Byamukama with contributions from Bob Fanning and Connie Strunk.

The moisture we have had in the past several days has been conducive for foliar diseases to develop in wheat. A few fields visited and samples submitted to the SDSU Plant Diagnostic Clinic indicate tan spot and bacterial leaf streak are beginning to develop on lower leaves of the wheat plants (Fig. 1 A). Young tan spot lesions start as tan brown flecks that expand into lens-shaped tan blotches with a yellow margin (Fig 1 B). Tan spot lesions can coalesce and become darker brown at their centers.

Tan spot is caused by a fungal pathogen Pyrenophora tritici-repentis which survives in wheat stubble. Infected plants in spring and summer provide source of inoculum for the following season (Fig. 2). The fungus requires 6-48 hours of a wet period for spores to be produced. The spores are then dispersed by wind and rain splash. Maturing lesions also serve as a source of secondary inoculum.

Tan spot can cause significant yield losses if infection is severe at flag leaf. Tan spot can be effectively managed by burying crop residue, through crop rotation and foliar fungicides. For wheat sown into wheat stubble, an early fungicide application may be recommended if tan spot is severe. Otherwise, apply a fungicide at flag leaf to protect the flag leaf if tan spot is moderate to severe.

Another disease that has been confirmed on wheat is bacterial leaf streak (BLS). BLS, like the name suggests, is caused by a bacteria called Xanthomonas campestris pv. translucens. When the bacteria infects the leaves, it causes leaf streak. When it infects the wheat head, it causes black chaff of wheat.

The bacteria survives on and in the seed and in the crop residues. Infected leaves have water soaked and necrotic streaks. The bacteria enter the wheat plants through natural openings and through wounds. Further spreading of the bacterium is through rain splash, overhead irrigation, plant-to-plant contact and insects such as aphids.

Once BLS is observed in the field, there is little that can be done to control it. However, if BLS is known to occur in a field, use certified seed and plant resistant cultivars to manage the disease.

Other diseases of economic importance that may develop on wheat in the course of the season include stripe rust, leaf rust, powdery mildew and Fusarium head blight (FHB). Both stripe rust and leaf rust have been detected in Nebraska, but the risk there remains very low. Rust inoculum must come from southern states into South Dakota. Powdery mildew develops when there is high humidity and poor air circulation in lower parts of the canopy. FHB develops if there is high humidity at flowering and corn, wheat, sorghum or millet residue is present. The Fusarium fungus infects wheat heads through the flowers.

Fungal leaf diseases can be effectively controlled by a timely fungicide application. The most economic benefit of fungicide application in wheat is when it is done to protect the flag leaf. Growers should scout wheat at the flag leaf stage and be ready to apply a fungicide treatment to protect the flag leaf if the incidence of fungal diseases is moderate to high and weather conditions are conducive for severe disease to develop. For FHB, fungicide application should be targeted at flowering if weather conditions call for increased FHB risk. Growers should check the National FHB prediction tool for their respective areas when deciding on a fungicide application for FHB. Refer to the North Central Regional Committee on management of small grain diseases for fungicide recommendations for various small grain diseases.