DIFFERENTIAL HOST-RANGE REACTION OF CITRUS AND CITRUS RELATIVES TO CITRUS CANKER AND CITRUS BACTERIAL SPOT DETERMINED BY LEAF MESOPHYLL SUSCEPTIBILITY

The leaf mesophyll susceptibility of 54 citrus species, cultivars, and relatives to Xanthomonas campestris pv. citrumelo, the cause of citru s bacterial spot, was evaluated in Hastings, Florida, during 1989 and 1990. A similar host range of 53 citrus species, cultivars, and relati ves was tested in Beltsville, Maryland, during 1991 to compare their d ifferential susceptibility to X. c. citri, which causes citrus canker, and to X. c. citrumelo by inoculations on foliage of the same trees i n replicated field plots. Field-grown trees were pruned to stimulate s ynchronous leaf flush for inoculation by a modified pinprick method. L esion size at 60 days (Hastings plots) or 45 days (Beltsville plots) p ostinoculation was used to quantify leaf mesophyll susceptibility. For X. c. citrumelo inoculations, lesion expansion was greatest on cultiv ars of trifoliate orange and trifoliate orange hybrids. Smaller lesion s formed on Citrus spp. such as grapefruit, sweet orange, sour orange, mandarin, lemon, and their hybrids, with the exception of Key lime, w hich developed lesions similar to those formed on trifoliate hybrids. Susceptibility of most citrus types to X. c. citri was more general. L esion sizes resulting from pinprick inoculations with X. c. citri were not significantly different among Citrus spp. and hybrids, indicating a general susceptibility of leaf mesophyll. Smaller lesions generally formed on citrus relatives, including some cultivars of trifoliate or ange. Because pinprick inoculations cause wounds and open the leaf mes ophyll to direct colonization by bacteria, this method bypasses stomat al infection and does not consider other factors that may affect field resistance.