Histopathology and Bacterial Populations of Atypical Symptoms-Inducing Xanthomonas axonopodis pv. citri Strains in Leaves of Grapefruit and Mexican Lime

Abstract

Histopathology of atypical symptoms-inducing Xanthomonas axonopodis pv. citri (Xac) strains XW47 (type Xac-A') and XW121 (type Xac-A') in citrus leaves was examined with light and electron microscopy, and compared with that of a typical Asiatic citrus canker strain XW19 (type Xac-A). Strain XW47 induces typical canker lesions on leaves of Mexican lime (Citrus aurantifolia) but causes flat necrotic lesions with water soaked margin on leaves of grapefruit (C. paradisi): whereas strain XW121 induces restricted and raised corky lesions without watersoaked margin on both citrus leaves. Light microscopic examinations showed that XW47 induced hyperplasia and hypertrophy of mesophyll cells and ruptures of the epidermis as those observed in Mexican lime leaves induced by XW19. However, neither hypertrophic cells nor eruption of the epidermis were observed in the brownish necrotic and hyperplastic mesophyll tissues induced by XW47 on grapefruit leaves. A distinct layer of lignified tissues around the brownish disintegrated erumpent tissues in which masses of fibrillar matrix agglutinated with bacterial cells was observed in the sections of a restricted and raised corky lesion induced by strain XW121. Bacterial populations of strain XW121 in mesophyll tissues were approximately 10^3 folds lower than those of XW19 and XW47 under a lower inoculum concentration, but increased to those of XW19 and XW47 under a higher inoculum concentration. Although XW19 and XW47 induced distinct symptoms on grapefruit leaves, no significant difference of bacterial growth was observed.