Epidemiology of Botryosphaeria spp. and the control of trunk diseases
Botryosphaeria species have emerged as important diseases in vineyards throughout the world. Because there are no cures for these diseases, one possible control strategy is to surgically remove all of the infected tissue, retrain the vines, and then use a variety of cultural and/or chemical controls to manage the disease. In collaboration with Sutter Homes vineyard, we established two trials in Zinfandel vines, one in 2005 and the other 2006, in a vineyard that is challenged by Botryosphaeria obtusa (=Diplodia seriata). The first years of the trials were described previously Epstein et al. & Huffsmith et al. (2008); the vines have regrown vigorously. This year, vines surgically treated and retrained four years ago yielded an average of 54 pounds of fruit per vine. However, the vines have also become reinfected at a much faster rate than we anticipated, and we now postulate that a key to Botryosphaeria species control is to start with clean plants from the nursery. Our trial has demonstrated the following. Painting the pruning wounds with the durable paint Duration does not cause phytotoxicity and has significantly reduced disease, in some but not in all measures of incidence. Destructive sampling has indicated that 29%of the pruning cuts were not completely painted to the margins of the wound; the data are consistent with the notion that tiny crevices may be prime infection sites. In the unpainted vines, we estimate a minimum of approximately 25 infections and 8 infections per vine in the four and five-year-old trials. B. obtuse mycelium can grow and induce the plant to release the compounds that cause discoloration from infections that originated on other pruning wounds. That is, shoots in which there has never been a pruning cut can be discolored at the time of pruning. In both trials, in January 2010, over one-third to half of the new pruning cuts were apparently already infected. Currently we are culturing pathogens from the trial so that we can assess the reliability of our visual assessments, and can better evaluate the rapidity of pathogen dissemination in the vineyard. Our current hypothesis is that the surgically retrained vines were reinfected from conidia in pycnidia that remained on the rootstock after surgery; this is consistent with our previous spore collection data. In our final objective, we tested phosphite as a treatment for grapevine trunk diseases; although the literature only indicates that it can be effective against some of Oomycete pathogens, some growers are interested in its use. In a greenhouse trial, we inoculated five pathogens and a mock-inoculated control into 3309 rootstock, and then estimated the amount of fungal growth by measuring the amount of discoloration. Based on the amount of growth in the untreated tissue, we list the pathogens that we tested from most to least pathogenic: Phaoacremonium aleophilum; Phaeomoniella chlamydospora; Botryosphaeria obtusa, Botryosphaeria rhodina, and Eutypa lata. Phosphite (1%Nutri-Phite® P+K) was applied as a drench to soil three times at six week intervals. There was no evidence that phosphite reduced fungal growth in the rootstock.