Physiologic Leaf Spot

Physiologic leaf spot is an important malady in the Pacific Northwest because the leaf spot symptoms are very similar to those caused by Septoria diseases and tan spot. The biology and epidemiology of the disease was studied when it became clear that many thousands of acres of wheat were being sprayed to control "Septoria" or "tan spot". Research showed that physiologic leaf spot does not respond to applications of fungicides. More recent studies by others, prompted by a finding discussed in our paper on disease management, have shown that symptom expression is reduced and grain yield is improved by applying chloride fertilizer to soil before planting, or by applying chloride to foliage before flag leaf emergence. A group of nutrient-deficiency diseases including "physiologic leaf spot" as it is expressed in the Pacific Northwest are being renamed "Chloride- and zinc-deficient leaf spots" in the 3rd edition of the APS Compendium of Wheat Diseases and Insects. Representative papers include the following.