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64 P R A C T I C A L W I N E R Y & V I N E YA R D October 2015 T runk diseases threaten vine- yards around the world, and the International Organization of Vine and Wine (OIV) has now taken an active interest. This column describes a new systematic approach to overcome trunk diseases in mature vine- yards. The basic concept of timely trunk renewal (TTR) is certainly not new, but the application to trunk disease — espe- cially to entire vineyard blocks — is prac- ticed by very few growers. Trunk renewal is not a new process for California. University of California Extension workers Jim Kissler and Bob Sisson used trunk and cordon renewal in the 1980s and showed that that retraining from the cordon or the trunk works very well for infected vines. However, the process and its application to the present battle with trunk diseases generally has been overlooked. The pres- ent emphasis in California is to encourage early preventative practice such as double or late pruning and wound protection with chemicals. In the past five years I have interacted with many scientists in the international trunk disease research community. Several of them made useful comments on a draft protocol for trunk renewal, which have been incorporated below. Grapevine trunk diseases There are four major grapevine trunk dis- eases, all caused by different, taxonomi- cally unrelated fungi. These diseases are called Esca, Eutypa dieback, Botryosphaeria dieback and Phomopsis dieback. Esca is a major problem in Europe, but it is less common in California. Eutypa dieback occurs around the world and in California. It is the trunk disease that has been the most thoroughly exam- ined on a scientific basis. Botryosphaeria and Phomopsis dieback are also global but not so well understood by researchers nor recognized by many growers. The extent of Botryosphaeria infec- tion in California relative to Eutypa was described by J.R. Úrbez-Torres (2006). There is presently no single, universally agreed control strategy for these diseases once they are established in the vineyard. My belief is that trunk renewal will fill that role, just as the use of grafted grape- vines led to control of phylloxera. Grower attitudes vary as to how much damage is a threshold level for removal and replanting, but I hear a figure of 20% yield loss frequently. California economic studies indicate that early intervention of wound protection is necessary to restrict disease spread and loss of income, much earlier than 20%. Trunk renewal in perspective Present recommendations for trunk disease control in California are for preventative action starting in young vineyards and include delayed prun- ing or double pruning and treatment of pruning wounds with fungicides such as Topsin M (thiophanate-methyl; United Phosphorus Inc., King of Prussia, Penn.) and Rally (myclobutanil; Dow Agrosciences LLC, Indianapolis, Ind.) before a rain event or with non-chemi- cal materials, such as boric acid (Tech- Gro B-Lock; Nutrient Technologies, Inc., Dinuba, Calif.) or VitiSeal (VitiSeal International LLC, San Diego, Calif.). Boric acid only controls Eutypa, but with addition of Topsin M all diseases are controlled. TTR can be seen as supplementary to these procedures, namely in mature vine- yards, and allows for the treatment of early symptom vines and those most at risk because of their location near infected vines. S M A R T V I T I C U L T U R E by Dr. Richard Smart Figure 2. A vine in England with two suckers in line with the vine row and on both sides of the trunk. These are ideally placed to replace the existing diseased trunk with two healthy trunks, each one supporting a cordon. The trunk could be removed either the following winter or the next winter, with minimal or no crop loss. The principal means of spread of trunk diseases is by winter pruning-wound infection with wind-dispersed or splash- dispersed spores (depending on the pathogen), which are released in rainy weather. Trunk diseases are insidious, especially Botryosphaeria dieback, which does not show symptoms until long after infection. Surveys from grapevine nurser- ies around the world have detected the causal pathogens of Esca and Black foot, and other fungi contaminating grafted vines, which can lead to infection in new vineyards. Thus, the problem will likely worsen. The damage Under extreme conditions, vine death due to Esca and Botryosphaeria can occur before the vine matures and begins to form fruit, but commonly they and the other trunk diseases first show symptoms (dead spurs, stunted shoots or foliar symptoms) between five and seven years of age. As more and more vines develop symptoms, yields decline. RICHARD SMART Timely Trunk Renewal to Overcome Trunk Disease