Genetic differentiation and host specificity among populations of Alternaria spp. causing brown spot of grapefruit and tangerine x grapefruit hybridsin Florida

Alternaria spp. were sampled from brown spot lesions in several geographica lly separated citrus groves and different grapefruit and tangerine x grapef ruit hybrid cultivars in Florida and screened for variation at 16 putative random amplified polymorphic DNA loci. Populations of the pathogen on two h ybrids, Minneola and Orlando, in five locations throughout Florida were mod erately differentiated (Nei's coefficient of gene differentiation [G(ST)] = 0.12) among locations. The hypothesis that host-specialized forms of Alter naria spp. cause brown spot on different Citrus spp. and cultivars was test ed by estimating genetic differentiation among isolates sampled from differ ent hosts and by pathogenicity assays. Isolates sampled from grapefruit and the hybrid cv. Nova were genetically distinct from isolates sampled from o ther hybrid cultivars including Robinson, Sunburst, Minneola, Orlando, and Murcott. No differentiation could be detected among isolates sampled from t his latter group of hybrids. Quantitative pathogenicity assays on leaves us ing spray inoculation revealed that 'Nova' isolates were not significantly more pathogenic on 'Nova' compared with isolates from 'Minneola' and 'Orlan do'. Similarly, grapefruit isolates were not significantly more pathogenic on grapefruit compared with isolates from 'Minneola'. Isolates from all hos ts had similar disease rankings on each inoculated cultivar, with 'Minneola ' the most susceptible, followed in decreasing order of susceptibility by ' Orlando', 'Sunburst', 'Nova', and 'Duncan' grapefruit. Rough lemon was gene rally immune to all isolates tested; however, occasional brown spot lesions were observed on leaves of this host with isolates from grapefruit. No evi dence was found to support the hypothesis that unique genotypes of the path ogen, which are more virulent on 'Sunburst' or grapefruit, have been introd uced to Florida. Populations of Alternaria spp. causing brown spot of citru s on grapefruit and 'Nova' in Florida are genetically distinct from isolate s on other cultivars, and we speculate that these populations are in the ea rly stages of adaptation to and possible speciation on these hosts.