Relationships Between Onion Leaf Age and Susceptibility toAlternaria porri

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Abstract
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 Allium 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 5.5, 5, 4.5, 3.5 and 2.5 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 Allium porri as they aged, and emerging leaves became more susceptible as the bulbs approached maturity.

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