VIEW ARTICLE

Leptosphaerulina briosiana on Alfalfa: Relation of Lesion Size to Leaf Age and Light Intensity. K. T. Leath, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, University Park, PA 16802; R. R. Hill, Jr., Research Agronomist, Agricultural Research Service, U.S. Department of Agriculture, University Park, PA 16802. Phytopathology 64:243-245. Accepted for publication 3 October 1973. DOI: 10.1094/Phyto-64-243.

A relationship between the size of lesions caused by Leptosphaerulina briosiana and the age of alfalfa leaves was observed in greenhouse and growth chamber tests. Lesions on the youngest (top) leaves were the largest and decreased in size on the next three successively older leaves of three susceptible clones; small lesions formed on all leaves of two resistant clones. The gradient in lesion size was greater under high (21,528 lumens/m²) than under low (9,688 lumens/m²) postinoculation illumination. The large, susceptible-type lesions formed only under the high light condition. A lesion-size gradient occurred in high light even when all leaves received essentially the same amount of light energy. Plants that did not have a gradient in lesion size also did not produce pale green, succulent leaves at the top of the stem. Postinoculation illumination of 21,528 lumens/m² or greater and vigorously growing plants are prerequisite to selection for resistance to L. briosiana.