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Differential Host Range Reaction of Citrus and Citrus Relatives to Citrus Canker and Citrus Bacterial Spot Determined by Leaf Mesophyll Susceptibility. T. R. Gottwald, Research Plant Pathologist, USDA, Agricultural Research Service, Orlando, FL 32803. J. H. Graham, E. L. Civerolo, H. C. Barrett, and C. J. Hearn. Professor, IFAS, CREC, University of Florida, Lake Alfred 33850; USDA, National Program Staff, Beltsville, MD 20708; and Research Plant Geneticists, USDA, Agricultural Research Service, Orlando, FL 32803. Plant Dis. 77:1004-1009. Accepted for publication 21 June 1993. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1993. DOI: 10.1094/PD-77-1004.

The leaf mesophyll susceptibility of 54 citrus species, cultivars, and relatives to Xanthomonas campestris pv. citrumelo, the cause of citrus bacterial spot, was evaluated in Hastings, Florida, during 1989 and 1990. A similar host range of 53 citrus species, cultivars, and relatives was tested in Beltsville, Maryland, during 1991 to compare their differential susceptibility to X. c. citri, which causes citrus canker, and to X. c. citrumelo by inoculations on foliage of the same trees in replicated field plots. Field-grown trees were pruned to stimulate synchronous leaf flush for inoculation by a modified pinprick method. Lesion size at 60 days (Hastings plots) or 45 days (Beltsville plots) postinoculation was used to quantify leaf mesophyll susceptibility. For X. c. citrumelo inoculations, lesion expansion was greatest on cultivars of trifoliate orange and trifoliate orange hybrids. Smaller lesions formed on Citrus spp. such as grapefruit, sweet orange, sour orange, mandarin, lemon, and their hybrids, with the exception of Key lime, which developed lesions similar to those formed on trifoliate hybrids. Susceptibility of most citrus types to X. c. citri was more general. Lesion 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 orange. Because pinprick inoculations cause wounds and open the leaf mesophyll to direct colonization by bacteria, this method bypasses stomatal infection and does not consider other factors that may affect field resistance.

Keyword(s): epidemiology, field inoculation.