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Survival of Xanthomonas axonopodis pv. citri in Leaf Lesions Under Tropical Environmental Conditions and Simulated Splash Dispersal of Inoculum

First, third, and fifth authors: Research Plant Pathologists and Statistician, CIRAD Pôle de Protection des Plantes, 7 chemin de l'IRAT, 97410 Saint Pierre, Réunion Island, France; and second and fourth authors: IRD, UR075, Résistance des Plantes, BP 5045, 34032 Montpellier Cedex, France

Asiatic citrus canker (ACC) is a severe disease of several citrus species and hybrids in many tropical and subtropical areas. Populations of Xanthomonas axonopodis pv. citri in leaf and twig lesions are the most important inoculum source for secondary infections. In areas with a marked winter season (e.g., Argentina and Japan), low temperatures induce a decrease of 10² to 10⁴ in population sizes in lesions, thus creating a discontinuity in the X. axonopodis pv. citri life cycle. The purpose of this study was to evaluate the dynamics of X. axonopodis pv. citri populations in leaf lesions exposed to the mild winter temperatures prevailing in a tropical environment. Internal X. axonopodis pv. citri population levels in Mexican lime leaf lesions reached 10⁶ to 10⁷ CFU lesion^-1 whatever the lesion size. These densities, however, were not strongly negatively affected by winter temperatures prevailing under experimental conditions. The estimated decrease in internal X. axonopodis pv. citri population sizes was approximately 10-fold. When exposed to 35 mm h^-1 of simulated rainfall, internal population sizes decreased over time by ≈1 log unit for lesions 1 and 2 months old, but did not for older lesions. A microscopic examination indicated that lignin-like compounds are present in lesions more than 6 months old. The slow decrease over time of X. axonopodis pv. citri population sizes in leaf lesions may be the balanced result of defense reactions by the host at late stages of disease development, and the concomitant multiplication of the pathogen at the margin of old lesions. We conclude that the epidemiological significance of overwintered leaf lesions in the tropics is higher than that reported in other areas.

Additional keywords: histology, rainfall simulation device.