We are receiving many reports of MSU winter wheat variety ‘Bobcat’ showing foliar leaf spots across the state. The leaf spots are tan to brown in color with yellow halos around them and resemble those associated with tan spot disease or chloride deficiency. Lesion from chloride deficiency tend to have more discrete margins between affected and nonaffected tissue while the margins of fungal lesions tend to be more diffuse. We have observed leaf spot symptoms in Bobcat in previous years, as well, but this year’s drought conditions and temperature extremes have likely exacerbated this phenotype.
At the Schutter Diagnostic Lab we have assessed several ‘Bobcat’ samples for the presence of the tan spot pathogen (Pyrenophora tritici-repentis) or other pathogenic causes. At this point, we have no convincing evidence that the observed spotting is caused by a fungal pathogen. The leaf spots are more likely of physiological nature (physiological leaf spot, PLS), which is often caused by chloride deficiency. If disease is ruled out, then chloride deficiency is suspect. In most winter wheat fields this year, it’s likely too late to have a large benefit with a chloride treatment, but if you decide to try, 10-20 lb potash (0 0 60) per acre should be sufficient (5-10 lb Cl/acre) or a similar amount of chloride applied as a liquid (for example calcium chloride or ammonium chloride).
If you observe leaf spot symptoms we encourage you to drop a sample (whole plant) at your local Extension office or submit directly to the Schutter Diagnostic Lab and await diagnosis before applying fungicides (sample submission instructions and forms can be found on the Diagnostic Lab website). If you want to rule out or rule in chloride deficiency, the suggested critical level in plant material (entire plant) is between 0.1 and 0.4%, with large yield increases and leaf spot decreases when chloride was applied at tissue Cl concentrations below 0.1%. Please note that the Schutter Diagnostic Lab does offer tissue analysis or soil testing services.
See https://store.msuextension.org/publications/AgandNaturalResources/NM7.pdf for more information.
Don't hesitate to reach out with any questions or concerns.
Extension Field Crop Pathologist
Extension Soil Fertility Specialist