Identifying the Routes of Infection of Eutypa Dieback Among Vineyards, Orchards, and Riparian Areas in California
Our goal is to determine if Prunus orchards and riparian areas are sources of Eutypa lata inoculum for vineyards. We examined evidence of spore dispersal (gene flow) among vineyards (wine-grape cultivars), orchards (stone fruits apricot and cherry), and riparian areas (wild host willow). We gathered 151 strains from four locations in California (Merced, Napa, San Benito, and Solano Counties), representing larger collections from grapevine, apricot, cherry, and willow, and a small subset of strains from both almond and pear. Incidence of Eutypa dieback was consistently high in all vineyards and apricot orchards (40 to 50%). In contrast, incidence was low in cherry orchards and from willow. We obtained a genetic fingerprint for each strain (haplotype) with a set of nine microsatellite loci, and examined the distribution of genetic diversity among hosts and locations. Genetic diversity indices were similar among strains, although those of Napa were slightly more diversified than those of San Benito. Our finding of high haplotypic richness suggests that sexual reproduction is the mode of reproduction that shapes the genetic structure of the pathogen on all hosts examined, and supports the important role of wind-dispersed sexual spores (ascospores), and not asexual spores (conidia) as infectious propagules. Hierarchical analysis of molecular variance indicated that genetic differences in Eutypa populations among locations were greater than those among hosts. When examined at a local spatial scale (<65 km), we identified an isolation-by-distance pattern in the genetic structure of the pathogen. This finding has important consequences for the epidemiology of Eutypa dieback in California. First, ascospore dispersal may be limited to distances less than 65 km. Second, ascospores may be exchanged among grape, cherry, and apricot. In contrast, the low incidence of Eutypa on willow in riparian areas and the presence of unshared alleles in strains from willow may indicate limited ascospore exchange between riparians areas and the cultivated hosts in vineyards and stone fruit orchards. Therefore, Eutypa populations from willow are not likely to constitute important sources of inoculum for primary infections of cultivated crops. Additional sampling and pathogenicity tests (Objective 2, proposed for funding cycle 2011-2012) are required to confirm these findings.