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S M ART VITICULTURE by Dr. Richard Smart Grapevine trunk diseases: A larger problem than ever posed by phylloxera? "T runk Disease, a New Global Vineyard Threat" was the theme of my spring 2012 column. The PWV editor inserted a box called "California Input" written by Paul Verdegaal, University of California farm advisor. This comment on my column contains some familiar sentiments made by people around the world explaining poor performance of newly planted vineyards. The "young vineyard decline" of some new vineyards in California was blamed on issues such as "improper planting, replanted vineyards, pushing vines to produce early, deficit irrigation" and extraordinarily "spur pruning" (hardly a new practice in California!), as perhaps contributing to the problem. These assertions may be true, but I for one am very skeptical and far from being convinced. The question remains: Would these stresses be important if the young vines planted were healthy and not affected by trunk disease? Diseased plants usually are more susceptible to stress, trunk disease included. I understand there remains a reluctance of some California nursery operators (and others worldwide) to admit that some of the vines they are selling can be infected with trunk disease, which can jeopardize the establishment and growth of young vineyards. Infected vines may not be fit for planting purposes! My column followed up a text written by Dr. Doug Gubler and others from the University of California in the January/ February 2005 PWV, where they discussed grapevine trunk diseases in California. Most of the Gubler et al. content was about the then well-recognized Eutypa disease, but the end of the report included some new research about "Bot canker," caused by a group of Botryosphaeriaceae fungi. A lot has occurred in the eight years since this report was published in PWV. Present situation in California Gubler's Ph.D. student (by then co-author, José Úrbez-Torres from Spain) and others published in 2006 an extensive survey of California vineyards aiming to identify the species of Botryosphaeriaceae associated with canker symptoms in California Trunk disease symptoms in a one-year-old grafted vine exported by a California nursery. vineyards.1 In the past, grapevine canker and consequent dieback symptoms had always been associated with Eutypa. The 2006 survey was extensive, including 1,735 samples from 166 vineyards located in 21 counties. Botryosphaeriaceae species were found to be surprisingly widespread, being present in 148 of the 166 vineyards sampled, around 90%. In fact they were more abundant than the Eutypa dieback causal fungus Eutypa lata in the grapevine cankers tested. There were also seven Botryosphaeriaceae species discovered. The authors concluded that Botryosphaeriaceae may be a more important cause of grapevine dieback than was previously recognized. Úrbez-Torres and Gubler published a 2009 study of the pathogenicity of the nine Botryosphaeria species in California.2 They showed that all nine species were able to infect both young and old vines, causing cankers, discoloration and streaking of the wood. However, there was a difference in virulence between the species from very virulent to weakly virulent, which indicates that the species of Botryosphaeria present in a vineyard may be a very important consideration. The most virulent species are Botryosphaeria rhodina (aka Lasiodiplodia theobromae) and Botryosphaeria lutea (aka Neofusicoccum luteum). A 2011 study by Úrbez-Torres looked at susceptibility of grapevine pruning wounds to two of the more virulent strains of Botryosphaeria. 3 Experiments were done in the field using Chardonnay and Cabernet Sauvignon in the Napa Valley. Vines were pruned at different times and the pruning wounds inoculated or not, up to 84 days after pruning. Results showed that pruning wound susceptibility was highest in November and lowest in March. Infection was highest immediately after pruning and declined with the age of the wound, but less so than for Eutypa. Based on this research and similar studies in California about Eutypa dieback, ideally growers should avoid wet weather when pruning to reduce risk of infection, but this is not always possible. Other studies have confirmed that the trunk disease situation in California may be more serious than was previously understood. For example P.E. Rolshausen and others published a 2010 study of pruning wound susceptibility to grapevine trunk diseases.4 The Botryosphaeria species tested had the highest infection rates of recent pruning wounds around 80%, and greater than Eutypa at around 30%. These results highlight the potential threat of these pathogens if they become established in production vineyards. Very importantly, the authors also encourage the protection of pruning wounds for mother-vine source blocks used for plant material production in nurseries. These have been found worldwide to be the major source of contamination of young plants produced by nurseries.12 Investigations continue about the role of protective sprays for pruning wounds. The practice of "double pruning" developed at the Gubler laboratory enables growers with large acreage to reduce infection by Botryosphaeria and Eutypa by a follow-up pruning in the spring with less rainfall. Elsewhere in the world More recent studies have shown the presence of significant trunk diseases in Arkansas and Missouri, due to 15 different fungi, by Úrbez-Torres and others.5 These were all new reports, and highlight that grapevine trunk diseases can affect grapevine health of inter-specific hybrids and vinifera. pr actica l win ery & vin eya r d JANUARY 20 14 9