Outcrossing and diversity of vegetative compatibility types in populations of Eutypa lata from grapevine

The sources of inoculum of Eutypa dieback of grapevines, caused by Eutypa lata, are not fully understood. Ascospores are thought to be the main source, but in populations with low disease incidence conidia may be important. Earlier studies of this disease in Italy and Germany did not report sexual structures (perithecia) of E. lata on grapevines, leaving open the possibility that populations of this fungus are clonal in some areas. However, we found perithecia in 24% and 43% of symptomatic vines in vineyards in Italy and Germany, respectively. Spatial patterns of symptomatic vines in the German vineyard were significantly aggregated but the aggregation was not strong, as might be expected if the fungus dispersed by conidia or mycelium on pruning tools. In contrast, vines with stromata and perithecia were not significantly aggregated in space. Vegetative compatibility (vc) types segregated among ascospore progeny in all perithecia sampled from both vineyards, indicating that this fungus consistently out-crosses in these populations. Finally, we found a high diversity of vc types in both vineyards, where isolates from different vines (with one exception) had unique vc types. These results reject the hypothesis that clonal reproduction of E. lata is epidemiologically significant, and strongly suggests that ascospores of E. lata are an important source of inoculum. Furthermore, E. lata's mating system is characterized by outcrossing in nature.