PHYSIOLOGICAL LEAF-SPOT OF WINTER-WHEAT

A leaf spot of unknown etiology damages winter wheat in the northwestern United States. The symptoms are similar to Septoria leaf blotch and tan spot, but the causal agents of these diseases are not present. Leaf spot symptoms are described. Dominant microbial colonists of the spots were Cladosporium herbarum, C. macrocarpum, C. cladosporioides, and Alternaria species. Leaf spot symptoms could not be induced on fresh leaf tissues with any of the inoculation or incubation procedures examined. Fungicides toxic in vitro to the dominant fungal colonists failed to suppress disease or increase grain yields in field experiments. Regression analyses of leaf spot severity and yield indicated that leaf spot reduced yield potential by 10% during each of 2 yr. No evidence is presented to indicate that this leaf spot is of microbial origin. Retention of the name physiologic leaf spot is suggested.