Induced Systemic Protection in Cucumber: Effects of Inoculum Density on Symptom Development Caused byColletotrichum lagenariumin Previously Infected and Uninfected Plants

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Abstract
Systemic protection induced by infection of leaf 1 with Colletotrichum lagenarium was not overcome by high levels of challenge inoculation of leaf 2 with C. lagenarium (107 conidia per milliliter). The cucumber cultivar Marketer was more susceptible than [Wisconsin] SMR-58 to C. lagenarium; similarly, the level of resistance induced by systemic protection was less in Marketer than in SMR-58. Lesions were more numerous, larger, and became necrotic earlier in Marketer than in SMR-58 and on unprotected leaf 2 compared to protected leaf 2. In general, the lower the concentration of challenge inoculum, the lower the total number and the more slowly chlorotic lesions became necrotic. However, the amount of added resistance induced by systemic protection, as expressed as the difference in inoculum density to obtain the same degree of symptoms in protected plants as in unprotected plants, was slightly greater in Marketer (100- to > 1,000-fold) than in SMR-58 (10- to 1,000-fold). The rate of necrotic lesion expansion in leaf 2 on unprotected plants was similar for Marketer and SMR-58 and was independent of the concentration of challenge inoculum. Necrotic lesion size was dependent on the inoculum concentration. The apparent rate of necrotic lesion expansion was lower in protected leaf 2 while lesions were less than 1 mm in diameter. When lesion size exceeded this, however, expansion rates were similar. The interpretation and significance of the latter are discussed.