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Pathogen adaptation to different hosts can lead to specialization and, when coupled with reproductive isolation, genome-wide differentiation and ecological speciation. We tested the hypothesis of host specialization among California populations of Eutypa lata (causal fungus of Eutypa dieback of grapevine and apricot), which is reported from >90 species. Genetic analyses of nine microsatellite loci in 182 isolates from three hosts (grapevine, apricot, and willow) at three locations were complemented by cross-inoculations on cultivated hosts grapevine and apricot to reveal patterns of host specialization. The cultivated hosts are likely more important sources of inoculum than the wild host willow, based on our findings of higher pathogen prevalence and allelic richness in grapevine and apricot. High levels of gene flow among all three hosts and locations, and no grouping by clustering analyses, suggest neither host nor geographic differentiation. Cross-inoculations revealed diversified phenotypes harboring various performance levels in grapevine and apricot, with no apparent correlation with their host of origin. Such phenotypic diversity may enable this pathogen to persist and reproduce as a generalist. Regular genetic reshuffling through sexual recombination, frequent immigration among hosts, and the lack of habitat choice in this passively dispersed fungus may prevent fixation of alleles controlling host specialization. [ABSTRACT FROM AUTHOR]

Eutypa dieback of grapevine is caused by Eutypa lata in production areas with Mediterranean climates in California, Australasia, Europe, and South Africa. Eutypa dieback has also been described in the colder, eastern North American vineyards where cultivars adapted from native Vitis spp. (e.g., Vitis x labruscana 'Concord') are primarily grown. However, the causal agents associated with the diseases in this region have not been conclusively identified. Examination of 48 vine-yards showing symptoms of dieback in the northeastern United States (Connecticut, Massachusetts, Michigan, New York, Ohio, and Rhode Island) and Ontario, Canada revealed that vineyards were mainly in-fected by Eutypa spp. other than E. lata. Multigene phylogenies (inter- nal transcribed spacer ribosomal DNA, ß-tubulin, and RNA polymer-ase II) of isolates recovered from these vineyards indicated that Eutypa dieback is caused primarily by an undescribed Eutypa sp. and E. lae-vata. Eutypa sp. was recovered from 56% of the vineyards examined, whereas E. laevata and E. lata were less far common (17 and 6%, respectively). Fruiting body morphology and spore dimensions supported phylogenetic separation of the three taxa. Pathogenicity tests conducted on Vitis vinifera 'Chardonnay' in the greenhouse and in the field verified that all three species were able to cause wood canker and to infect pruning wounds, respectively. [ABSTRACT FROM AUTHOR]

In eastern North America, Phomopsis cane and leaf spot, caused by Phomopsis viticola, is a foliar disease of grape but, in the Mediterra-nean climate of western North America, P. viticola is primarily associ-ated with wood cankers, along with other Diaporthe spp. To determine the identity of wood-infecting Diaporthe spp. in eastern North Amer-ica, 65 isolates were cultured from 190 wood-canker samples from 23 vineyards with a history of Phomopsis cane and leaf spot. Identifica-tion of 29 representative isolates was based initially on morphology, followed by phylogenetic analyses of DNA sequences of the ribosomal DNA internal transcribed spacer region, elongation factor subunit 1-a, and actin in comparison with those of type specimens. Three species were identified: P. viticola, P. fukushii, and Diaporthe eres. Inoc-ulations onto woody stems of potted Vitis labruscana 'Concord' and V. vinifera 'Chardonnay' showed that D. eres and P. fukushii were pathogenic (mean lesion lengths of 7.4 and 7.1 mm, respectively, compared with 3.5 mm for noninoculated controls) but significantly less so than wood-canker and leaf-spot isolates of P. viticola (13.5 mm). All three species infected pruning wounds of Concord and Char-donnay in the field. Our finding of pathogenic, wood-infecting Di-aporthe spp. in all 23 vineyards suggests a frequent co-occurrence of the foliar symptoms of Phomopsis cane and leaf spot and wood can-kers, although the latter are not always due to P. viticola. [ABSTRACT FROM AUTHOR]