Grapevine Canker Diseases
Until recently, grapevine dieback in California grapevines was attributed mainly to the fungus Eutypa lata in the family Diatrypaceae. This fungus has been known as the causal agent for Eutypa dieback. However, we have shown that dieback of grapevines in California is also caused by other fungi in the family Diatrypaceae (Eutypa leptoplaca, Eutypella and Diatrypella) as well as by several Botryosphaeria spp. To date, we have identified at least nine species of Diatrypaceous fungi associated with grapevine cankers as well as nine different species of Botryosphaeria, which appeared to constitute the main pathogens isolated from grapevine cankers statewide (Úrbez-Torres, et al., 2006; Úrbez-Torres, et al., 2007). Isolation from cankers and spore trapping studies in the table grape areas of the Coachella Valley have revealed the high incidence of Eutypella vitis (Diatrypaceae). This is a new pathogen in Coachella Valley table grape area. In addition, isolations from cankers have shown Phomopsis viticola to be the most common pathogen isolated from grapevine cankers in table and raisin cultivars in Fresno and Tulare Cos (Úrbez-Torres, et al., 2006). Results of spore trapping studies have shown that Botryosphaeria spores were mainly trapped following rainfall events in Napa Valley, Arbuckle, Lodi and San Luis Obispo, and following overhead irrigation in the Coachella Valley. Botryosphaeria spores were trapped from September 2006 to Jan., 2007. Interestingly, Botryosphaeria spores were also trapped without rainfall or irrigation in Arbuckle, suggesting that other environmental factors may contribute to spore release. Temperature studies for mycelial growth and pycnidial formation have shown different optimum temperature regimes among Botryosphaeria spp found in California, which agree with the actual differences in terms of geographical distribution of Botryosphaeria spp in California. We have developed new assays to evaluate the pathogenicity or virulence of the various fungi associated with grapevine dieback. Indeed, pathogenicity testing using green shoots in the laboratory as well as in the field appeared as a reliable method to rapidly assess fungal pathogenicity. Surveys for the host range as well as perithecia of Botryosphaeria spp in California have revealed additional host plants and sources of inoculum for these pathogens. These findings are providing a better understanding of the disease cycle of Botryosphaeria canker disease. Also, our work has stressed the importance of riparian and forest systems adjacent to vineyards in understanding the epidemiology, disease cycle and development of canker diseases in grapevines. Double pruning was shown to be an effective cultural practice which completely eliminates canker formation by Eutypa spp (Weber, E. et al., 2007) and we are now testing this practice against the more rapidly moving Botryosphaeria spp. Finally, our research has offered alternative control methods for E. lata using boron based chemicals. The use of 3-5% boron mixed with a commercial tree wound paste gave excellent disease control (91%). Interestingly, 1%boric acid also gave excellent disease control (82%).