Relationships Between Onion Leaf Age and Susceptibility to Alternaria porri. Marvin E. Miller, Assistant Professor, Texas Agricultural Experiment Station, Weslaco 78596. Plant Dis. 67:284-286. Accepted for publication 27 July 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-284.

Measurements of onion (Allium cepa) leaf tissue damaged by Alternaria porri infections were taken at weekly intervals from the time of bulb initiation until bulb maturity to determine relationships between age of onion plants and susceptibility to A. porri. Levels of leaf damage were significantly lower (P = 0. 05) on younger leaves than on older leaves each week, except 1 wk before bulb maturity on New Mexico Yellow Grano (NMYG) and at maturity on both NMYG and Texas Grano 502 onions. Leaves that emerged 9, 8, 7, 6, and 5 wk before bulb maturity required 5½, 5, 4½, 3½, and 2½ wk, respectively, to reach 50% leaf damage, whereas leaves emerging 2, 3, and 4 wk before bulb maturity exceeded 50% leaf damage within 2 wk. Individual onion leaves were more susceptible to A. porri as they aged, and emerging leaves became more susceptible as the bulbs approached maturity.