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Background: A peculiar leaf spot complex that results in necrosis of leaf blades and yield losses has been reported in wheat fields of the Pacific Northwest and Great Plains. Incidence and severity have been observed to be more problematic during high moisture seasons or in high rainfall environments. Previous research has shown this leaf spot complex is not pathogenic in origin.   Hence, the  phenomenon has been referred to as a "physiologic leaf spot" of wheat.   Leaf spot occurrence and severity has a strong genetic basis as is it affects only specific wheat varieties.  Recent, investigations indicate this problem is associated with inadequate plant chloride nutrition and is easily corrected by adding chloride-containing fertilizers.

 
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Physiologic leaf spot in the field

Redwin winter wheat

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Symptoms:

Symptom appearance differs depending on lesion's age and wheat variety. Frequently, the first symptoms are small, dark-brown or yellow spots on lower leaves. A chlorotic halo may surround a darker colored center in some varieties. Initially, the lesions are circular to oblong in shape (1 to 10 mm diam) with distinct margins. Lesion size and numbers increase as the plant matures. Lesions become necrotic or bleached white to gray in color within a few weeks. In severely affected plants, spots coalesced and resulted in premature leaf senescence.

Usually, symptoms will first become evident at flag leaf emergence to boot stages. The symptoms first appear on the lower leaves. Lesions progress to newer tissue or leaves as they become fully-emerged. Lesions may or may not progress to the flag leaves. Leaf spot severity (percentage of leaf tissue affected by chlorotic or necrotic lesions) will typically increase through the early-grain fill period. At this time symptoms are typically more severe on the flag-1 (first leaf below the flag) and flag-2 (second leaf below the flag) leaves than on the flag leaf .

Lesions are evident on the leaf sheath in CDC Kestrel as well as the leaf blades.

In the past, physiologic leaf spot of wheat has been mistaken as Septoria leaf blotch (incited by Septoria tritici) and tan spot (incited by Pyrenophora tritici-repentis) due to the similarity in symptom expression. Application of fungicides, e.g. benomyl and propiconazole, has no affect on symptoms as this phenomenon is not microbial in origin.