Current UC Viticulture Research

American Vineyard Foundation
California Competitive Grant Program for Research in Viticulture and Enology

We examined evidence of spore dispersal (gene flow) among vineyards (Vitis vinifera), orchards (stone fruits apricot and cherry), and riparian areas (wild host willow). We gathered 151 strains from three locations in California (St. Helena, Napa County; Hollister, San Benito County; Winters, Solano County). Spatial distances among the three locations ranged from 60 to 350 km. 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. Population genetic analyses included calculating of summary statistics to compare genetic diversity within and among populations, hierarchical analysis of molecular variance (AMOVA), Bayesian clustering analysis, and testing for isolation-by-distance patterns.

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 in riparian areas. For example, despite recovery attempts from 96 willow trees with wood cankers in Solano, we recovered only five strains (a very low incidence of 4%). Instead, other species of fungi from the same family as Eutypa, the Diatrypaceae, were more frequently recovered from willow (e.g., Cryptosphaeria sp, Cryptovalsa sp, Eutypa laevata), and incidence of these closely-related fungi was higher in willow than in the cultivated crops. Genetic diversity indices were similar among all strains, although those of Napa were slightly more diversified than those of San Benito. Our findings of high haplotypic richness and absence of linkage disequilibrium among microsatellite loci suggest 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 (AMOVA) indicated that genetic differences in Eutypa populations among locations were greater than those among hosts. Bayesian analysis identified two genetic clusters that were not clearly explained by host plant or location.. The lack of a substantive biological basis to support the output of this Bayesian analysis led us to consider this genetic structure as marginal. Thus, the two genetic clusters were not considered biologically relevant and the data were subsequently treated with classical population genetics analyses, where groups of strains obtained from a given host at a given location were considered a ‘sample’. When examined at a local spatial scale (< 65 km), we identified an isolation-by-distance pattern in the genetic structure of the pathogen, i.e. an increase in genetic differentiation with increasing spatial distances between samples. This finding has important consequences for the epidemiology of Eutypa dieback in California. First, most ascospores are likely transported by wind locally and ascospore dispersal over distances up to 65 km may be very limited. 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 willow strains (presence of alleles not shared with strains collected from other hosts) suggests that there is limited ascospore exchange between willows in riparian areas and the cultivated hosts in both vineyards and stone-fruit orchards.

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. This research verifies past findings and provides new insights in the epidemiology of Eutypa dieback in California. We confirmed that ascospores (i.e., sexual reproduction) are important in the spread of Eutypa dieback, albeit at a limited spatial scale. A new finding from our study is that Eutypa spores are likely exchanged between grapes and stone fruits. As apricot, cherry, and grape can serve as inoculum sources for each other, and at local spatial scales (<65 km), removal of infected hosts upon which the pathogen is forming sexual fruiting bodies (i.e., rogueing out dying vines and trees harboring the typical stroma of Eutypa dieback) may limit local spore production and hence limit the number of pruning wound infections. Lastly, considering the very low incidence of Eutypa in riparian areas supporting willow and the genetic divergence of willow strains from those of cultivated crops, riparian areas may not be a significant source of inoculum for grape or susceptible stone fruits.

Figure 1. Table of hierarchical analysis of molecular variance (AMOVA), to test for the effects of host plant or location on the genetic diversity of Eutypa populations.

Figure 2. Figure of Bayesian clustering analysis, to test for the presence of patterns of genetic diversity across the entire data set.

Kendra Baumgartner
W. Douglas Gubler
Renaud Travadon
Philippe Rolshausen

Baumgartner K, Bergemann SE, Fujiyoshi P, Rolshausen PE, Gubler WD. 2009. Microsatellite markers for the grapevine pathogen, Eutypa lata (pdf). Molecular Ecology Resources 9:222-224.