Abstracts of Selected Papers Relating to Physiologic Leaf Spot

Smiley, R. W., L.-M. Gillespie-Sasse, W. Uddin, H.P. Collins, and M.A. Stoltz. 1993. Physiologic leaf spot of winter wheat. Plant Disease 77:521-527.

A leaf spot of unknown etiology damages winter wheat in the northwestern U.S.A. Symptoms are similar to Septoria leaf blotch and tan spot but 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 spp. Leaf spot symptoms could not be induced on fresh leaf tissues with any inoculation and 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 two years. No evidence is presented to indicate that this leaf spot is of microbial origin. Retention of the name physiologic leaf spot is suggested.

Smiley, R. W., W, Uddin, P.K. Zwer, D.J. Wysocki, D.A. Ball, T.G. Chastain, and P.E. Rasmussen. 1993. Influence of crop management practices on physiologic leaf spot of winter wheat. Plant Disease 77:803-810.

Response of physiologic leaf spot to management of winter wheat was examined in the semiarid Pacific Northwest. Winter wheat cultivars exhibited large differences in susceptibility. The dominant cultivar (Stephens) produced in the region was the most susceptible of cultivars evaluated. Disease became less severe in response to delayed planting and increasing rates of nitrogen fertilizer. Leaf spot severity was reduced and grain yield increased by foliar application of urea + calcium chloride, but not by application of urea + micronutrients. Disease was more severe in annual wheat than in rotations of wheat with fallow or peas. Leaf spot severity was not affected by application of diclofop-methyl herbicide, timing or source of soil-applied nitrogen, or burning or not burning the stubble of previous wheat crops. Amounts of residue on the soil surface had inconsistent effects on leaf spot severity. It is concluded that the severity of physiologic leaf spot can be reduced by management of wheat cultivar selection, crop rotation, planting date, and plant nutrition.
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