Penetration through leaf stomata and strains of Xanthomonas campestris in citrus cultivars varying in susceptibility to bacterial diseases

Leaf stomata and the pressures required to effect water congestion of tissue and bacterial penetration and growth in leaves were compared for selected cultivars of citrus species and relatives that vary in susceptibility to Asiatic citrus canker and citrus bacterial spot caused by Xanthomonas campestris pv. citri and X. c. pv. citrumelo, respectively. The differences among cultivars in structure and density of stomata on leaves expanded by two thirds (most susceptible stage to infection) and leaves fully expanded (least susceptible) were not related to previously reported susceptibility to citrus canker. Leaves, two-thirds expanded, of citrus cultivars were inoculated with X. c. citri or X. c. citrumelo after pretreatment at three impact pressures to yield incipient water congestion of tissue, full congestion, and congestion with damage to the epidermis. The number of lesions of citrus canker and citrus bacterial spot increased with degree of water congestion, but there was no interaction among cultivars with impact pressure. The number of bacteria that penetrated and the growth of either X. c. citri or X. c. citrumelo in leaves did not vary significantly among cultivars from 5 to 48 h. Populations continued to increase up to 168 h in citrus cultivars susceptible to citrus canker and in trifoliate orange and its hybrids susceptible to citrus bacterial spot. After 48-72 h, populations of X. c. citri were significantly lower in Cleopatra mandarin and in trifoliate orange, which are moderately resistant to citrus canker, and growth of X. c. citrumelo ceased in citrus species that are highly resistant to citrus bacterial spot. The number of bacteria recovered from within the infiltrated area at 5 h corresponded with the number of lesions of citrus canker and citrus bacterial spot at 168 h, suggesting that individual lesions developed from infections of stomata. In susceptible cultivars, lesion development was often correlated with bacterial populations at 168 h, but these factors were not correlated in cultivars resistant to citrus bacterial spot. Thus, resistance of citrus leaf tissue was expressed not as reduction in the number of bacteria that penetrated through stomata, but as a reduction in bacterial growth after 72 h.