Is it tan spot or physiological leaf spot in your winter wheat?

What might look like tan spot or septoria on winter wheat could be physiological leaf spot and it’s important to know the difference.
If it’s physiological leaf spot there’s no point spending money on a fungicide because it won’t cure the problem, which usually has little impact on yield anyway.
Suspected cases of physiological leaf spot have been seen in the eastern region of Manitoba.
Physiological leaf spot symptoms starts as small yellow necrotic spots (one to three mm) on the upper leaves. As they develop, a dark brown center forms in this necrotic spot, which is similar to tan spot. However, physiological leaf spots are usually more "blocky" or "angular" and are often defined by the leaf veins.
Tan spot lesions are more oval or elliptical and often have a yellow halo surrounding the necrotic (dead) spot. This is more evident as the lesions expand.
Physiological leaf spot is associated with low chloride levels in the plant and soil.
Tissue analysis to determine tissue chloride levels can be done quickly by most soil test laboratories. For a list of soil test labs, visit MAFRI’s website at
Contact the lab to determine the proper way to obtain and ship a sample.
The sufficient level for flag leaf chloride in wheat is between 0.2 to 0.5 per cent.
Plant diseases can be cultured and identified at the Crop Diagnostic Centre in Winnipeg. However, this isn’t the best route for confirming physiological leaf spot as it does so through the process of elimination.
Regardless of how physiological leaf spot is confirmed, confirmation should be performed on a field-by-field basis.
And remember that both may actually occur on the same leaves.
• Physiological leaf spot is strongly related to genetic predisposition and only appears in certain environmental conditions.
• Physiological leaf spot is favoured by cool, wet spring and summer conditions that promote vigorous plant growth. Rainy days in combination with sunny, warm days can also favour it.
• CDC Falcon, which is a popular winter wheat variety in Manitoba, is very susceptible. However in Saskatchewan studies, non-spotting varieties responded similarly to low levels of chloride.
• Physiological leaf spot is not reduced by fungicide applications.
• There are no corrective chloride treatments.
• Visible observations of leaf tissue affected would suggest that yield reduction could occur, but yield responses to applied potassium chloride have been modest at best. In Saskatchewan trials yield response ranged from zero to 15 per cent, with an average of three per cent. In one Manitoba trial in 1999, the flag leaf had 36 per cent leaf spotting versus 22 per cent where potassium chloride was applied yet no yield difference was measured.
• If physiological leaf spot is observed, soil test and consider applying  potash on future winter wheat crops.

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