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Tan Spot - Be on the Lookout

By Sherri Roberts, Crops Extension Specialist, Weyburn

June 2022

Three things are required for any disease to occur: a disease-causing organism, a susceptible host and the proper environment. Continued rains in the eastern side of the province are possibly setting things up for a perfect storm when it comes to fungal diseases and producers should be scouting more.

One such fungal disease starting to show up is tan spot or yellow leaf spot. The fungal pathogen Pyrenophora tritica-repentis is the disease-causing organism. This disease affects both spring and winter wheat, durum and certain tame grasses including smooth brome. Currently, eight different races have been identified in the Prairies and they are host specific. Symptoms start out as small, dark brown to black spots that will first appear on the lower leaves. These spots expand out and develop longitudinally along the plant’s veins.

Tan Spot on Wheat
Typical symptoms of tan spot on wheat

To distinguish these lesions from similar ones created by stagonospora/septoria leaf blotch, examine the lesions with a hand lens. Tan spot lesions will NOT have pycnidia (small, black fruiting bodies).

This fungus can overwinter on crop residue, both in standing stubble and surface trash. Due to prairie winter conditions, the reproductive organisms that allow this disease to overwinter are delayed in developing and pseudothecia (sexual fruiting bodies) form on the residue. As a result, rotations that contain wheat back-to-back are extremely vulnerable to this disease. Studies done in Saskatchewan have shown two-year-old wheat stubble is even a greater source of infected residue. The spores that spread this disease are produced in the spring and are carried on the wind or rain to young plants. Symptoms will appear five-seven days after infection.

During the growing season, conidia (asexually produced spores) are produced in old lesions on wet leaves. These conidia will become air borne as the infected leaves dry. At least six hours of leaf wetness is required before these conidia can infect and colonize new leaf tissues. Temperatures below 20 C as or above 28 C will slow development. Hot temperatures in the summer will often prevent spread of this disease to the upper leaves. Generally, this disease affects the lower leaves early in the season.

Tan Spot Blotches
If moisture and temperature conditions are favorable, lesions will coalesce together and form irregular blotches.

If weather conditions are cool and wet during formation of the flag leaf, a fungicide application may be warranted to prevent the disease from destroying photosynthetic leaf area and infecting the seed. If seed heads are infected and conditions are conducive to disease development, the mature kernels may appear to have a reddish discolouration on the germ end of the kernels. This is known as red smudge, which can be confused with fusarium head blight infected kernels. Planting seeds with large numbers of red smudge kernels could result in reduced stands.

The most effective method of controlling tan spot is an appropriate crop rotation. There should be at least a three-year spread between wheat crops. Fungicide applications need to be accessed as to the economic benefits.

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