Identifying the Routes of Infection of Eutypa Dieback among Vineyards, Orchards, and Riparian Areas in California

Our objective was to determine if Prunus orchards (stonefruits, almond) and riparian areas are sources of Eutypa inoculum (sexual spores, ascospores) for vineyards. To address this goal, we identified the direction of spore dispersal among vineyards,orchards, and riparian areas. We gathered 216 isolates from four locations in California (San Benito Co., Solano Co., Napa Co., Merced Co.), and five hosts (grapevine, apricot, cherry, almond, willow). In all locations, 30–50% of apricot trees and grapevines sampled were infected with Eutypa. This is in contrast with the low recovery of Eutypa from cherry orchards in Napa (3%) and Solano (0%), as well as from willow in all locations (4–24%). We obtained a genetic fingerprint for each isolate with a combination of nine microsatellite loci in order to examine the distribution of genetic diversity among host plants and locations. Bayesian analysis did not reveal any genetic grouping of isolates based on location or host of origins, thus suggesting that the collection of 216 isolates forms a large, genetically-homogenous population in California. Almost all isolates had a unique genetic fingerprint, supporting the important role of ascospores in infection of all hosts examined. There were no differences in genetic diversity among locations. In contrast, Eutypa populations from willow were significantly less diverse than those from grape (mean number of alleles per locus = 3.1 vs 2.6; P = 0.007). Both the low incidence of Eutypa in riparian areas and the lower genetic diversity in samples from willow suggest that Eutypa populations from riparian areas may not constitute important sources of inoculum for infections of cultivated crops. However, hierarchical analysis of molecular variance indicated that there were no significant genetic differences among samples collected either from different locations or from different host plants. Indeed, 99% of the total genetic variation found among all isolates was actually present within a single field. Furthermore, genetic differentiation among all samples was very low and non-significant (Fst = 0.008; P = 0.12), suggesting high levels of spore dispersal among the different host plants. Our findings suggest that Eutypa spores from Prunus orchards serve as inoculum reservoirs for vineyard infections (and vice versa), whereas infections in riparian areas likely originate from vineyards and orchards. In spite of the lack of genetic evidence of host specialization, we continued with the 2nd stage of the work, which is to test the hypothesis of host specialization in Eutypa with complimentary, cross-pathogenicity tests. Results of such tests will be acquired in the winter 2012-2013.