Grapevine Canker Diseases in California
Grapevine cordon and trunk canker diseases constitute the primary causes of plant mortality and economic losses in the California grape industry. For many years it was thought that the decline and dieback observed in California grape growing regions commonly referred as Eutypa Dieback was due only to Eutypa lata. However, in previous studies, we isolated several other Diatrypaceae species as well as various Botryosphaeria spp from cankers found in the wood of declining grapevine. Fields surveys conducted in California since 2003 have showed the major significance of Botryosphaeria spp in canker disease of grapevine.
Surveys extended during 2005 have allowed our laboratory to obtain comprehensive identification and distribution of nine distinct Botryosphaeria spp associated with grapevine cankers in California. Also, Botryosphaeria species appeared as the main group of fungi isolated from wedge shape cankers in many California grapevine growing areas. Botryosphaeria spp were further distinguished based on specific morphological and genetic characteristics. Species concepts and identification were confirmed from phylogenetic analyses of the ITS, beta-tubulin and elongation-factor genes. Both morphological and molecular analyses have allowed our laboratory to build up tools for diagnosis and separation of the different Botryosphaeria species isolated from grapevine cankers in California. Study of the effect of temperature on colony growth revealed different optimal temperatures among Botryosphaeria species suggesting the specific adaptation and distribution of these different Botryosphaeria species to the various wine regions of California.
Study of cankers occurring in native plant species in the vicinity of vineyards were examined as to offer additional models of canker development to enlighten some of the major aspects of canker formation in grapevine. Following this type of investigation we were able to identify new alternate host plants and putative sources of inoculum for these major grapevine pathogens. Also, our observations have supported the idea that wood cankers in grapevine as well as in native trees are results of the activity of specific fungal association rather than a single fungal species as previously believed. AFLP (Amplified fragment-length polymorphism) is being applied on our assorted collection of E. lata isolates from California gathered in the past 5 years. Preliminary results did not detect occurrence of subpopulations of E. lata with regard to the geographical and ecological (host specificity) distribution of our isolates. Similarly, sequencing of the RPB2 gene for E. lata confirmed our previously proposed delineation of the species within the wide range of ecologically and geographically diverse isolate collection.
Results of pathogenicity tests showed that all Botryosphaeria species found in California were pathogenic to grapevines. However, pathogen virulence appeared to differ depending on the species. Boron-based chemicals have been shown to be effective to control E. lata when applied on pruning wounds. However, our results indicated that pruning wounds were susceptible to several fungal pathogens responsible for branch dieback of grapevine, and we also recovered several of these fungi in concert form wood cankers. Therefore, we are now testing the ability of boron-based material in comparison to other fungicides to control several wood decay fungi of grapes including E. lata, Botryosphaeria rhodina, B. dothidea, B. obtusa, B. sarmentorum, Phaeomoniella chlamydospora, Phaeoacremonium aleophilum and Pleurostomophora richardsiae