Colonization and Distribution ofXanthomonas phaseoliandXanthomonas phaseolivar.fuscansin Field-Grown Navy Beans

Abstract

Rifampin-resistant mutants of X. phaseoli (Xp) and X. phaseoli var. fuscans (Xpf) were used to study the multiplication and distribution of inoculum of bean blight bacteria in field-grown navy (pea) beans (P. vulgaris). Increase of Xp and Xpf in bean leaves resembled standard bacterial growth curves in vitro. Mean doubling times of Xpf and Xp were 19.4 and 18.8 h, respectively, during the exponential growth phase. Numbers of Xpf and Xp peaked during the stationary phase and remained stable until leaf abscission; a slow death phase accompanied leaf decomposition on the soil surface. Symptom development on leaves required an inoculum density of at least 5 .times. 106 bacteria/20 cm2 of leaf tissue, usually corresponding to the density in the early stationary phase. The multiplication of Xp and Xpf in the leaf canopy from seedling to reproductive phases of plant development described by a series of growth curves displaced over time, with each curve representing bacterial multiplication on individual leaves of the main stem. Correlation of the sequence of symptom expression with the increase of the bacteria explained the late disease development that is characteristic of common and fuscous blights. Both Xpf and Xp spread throughout the canopy by rain, bud colonization and systemic movement. At least 10% of the Xpf and Xp leaf population was removed as secondary inoculum during rainfall. Isolates Xpf R10 and Xp Ra were systemic in bean stems and roots, with doubling times of 22.8 and 23.8 h, respectively in stems.