By: Becky Garrison
During the Oregon Wine Symposium, held virtually from February 15-17, 2022, Akif Eskalen, Professor of Cooperative Extension, UC Davis, offered his insights into the identification, biology, epidemiology and current management strategies of grapevine trunk diseases.
In Eskalen’s estimation, GTDs are considered one of the most significant challenges for viticulture across the globe. These harmful diseases can be found on the spurs, cordon, trunk and rootstock of the grapevine and gain entry primarily via pruning wounds. GTDs are caused by a broad range of permanent, wood-colonizing fungal pathogens. They are also present as part of normal grapevine microbiota, with environmental factors triggering them to switch from normal to pathogenic.
When examining the fungal pathogens responsible for creating GTDs, Eskalen found that most of these pathogens produce overwintering fruiting structures containing infectious spores. These overwintering structures are found in old pruning wounds, harvesting debris, the bark surface of infected vines and other woody perennial crops such as nut and fruit trees.
Researchers have identified more than 130 different fungal species associated with GTDs. Commonly found in both young and mature vines was Black Foot disease, and Petri disease was found predominant in young vines. Researchers found Esca (black measles), Botryosphaeria dieback, Eutypa and Phomopsis dieback in mature vines. These are called canker diseases because they cause characteristic perennial cankers in the vines. The word canker comes from the Greek word “cancer,” which describes dead tissue in living organisms. Perennial cankers cause spur, cordon, and trunk dieback, ultimately resulting in the death of the entire vine. Other significant symptoms of the presence of GTDS in vines include poor vigor, stunted shoots, leaf chlorosis and stripe, berry specks and shoot and tendril dieback.
In analyzing the lifestyle of this disease, Eskalen said that if the infection cannot be controlled in the cordon, it’s going to move into the main trunk. Once it settles there, the trouble starts. As the main trunk is the essential part of the plant, the pathogen will move faster because more woody tissues and nutrition are in that area.
Should the pathogen reach the graft union, then it is too late to do anything about managing GTDs. In some cases, black foot and charcoal rot diseases are caused by a soil-borne pathogen complex. Once they colonize the roots and lower level of the trunk, nothing much can be done to save the vines.
Through Eskalen’s research at the UC Cooperative Extension Eskalen Lab, he’s discovered that pruning wounds are the primary entry point for these fungal pathogens.
“Some of them could be entering the plant without the pruning wounds, but the major entry point is the pruning wounds because…there is no defense mechanism,” Eskalen said. As soon as the fungal spores land on the tip of the wound, sap provides enough nutrients to the fungi for them to colonize.
Another infection method is latent infection, occurring when the fungal pathogens release from their source and land on the tissue without causing further problems until the right conditions come, such as when the vine is stressed by factors like nutrition, climate change and irrigation.
According to Eskalen Lab’s research, conducted with California-based farmers, most spores appear to be released during precipitation events from December to February. Since this time overlaps with pruning season, there’s a window for the fruiting bodies (i.e., overwintering spore structures) to release fungal spores onto exposed pruning wounds and cause infection. Eskalen estimates that pruning wounds could be susceptible for several months and that pruning wound protection is essential in vineyards during this time.
Mitigating Grapevine Trunk Disease
GTDs are vascular diseases, which means they can colonize the wood part of the plant. As the symptoms above are caused by mycotoxin, or toxins produced by the fungus in the vascular tissue, applying fungicide, or anything from outside, to control it will not work because these pathogens are present inside the wood.
Pruning wound protection strategies alongside cultural practices are the best strategies to mitigate GTDs. Cultural practices focus on sanitation, including using clean material when establishing a new vineyard, removing pruned and infected material, and pruning dead shoots, spurs, and cordons below the symptomatic tissue.
In Eskalen’s estimation, the most effective way to protect pruning wounds from airborne fungal spores of GTDs is to apply registered fungicides or biological pruning wound protectants annually. These treatments should be sprayed the same day if applying synthetic chemicals. To avoid inclement weather from washing the solution away, apply these protectants after pruning and during a dry weather window. If beneficial fungal species, known as biocontrol agents, are used, Eskalen said it could be better to have sap accumulation on the wound so they can colonize and compete with the pathogens.
In their research, Eskalen Lab found that delayed pruning after the high disease pressure period was another option for mitigating GTD infection. Eskalen said some of the powdery mildew fungicides might control some of the GTD pathogens, but the lab doesn’t spray powdery mildew fungicides during the dormant season.
Commercial chemical protectants shown to be effective in controlling GTDs include a combination of Rally and Topsin-M. Currently, Eskalen Labs is researching biological wound protectants that are more sustainable.
Assessing the Economic Impact of GTDs
According to the data collected by Eskalen dating back twenty years, if a grower did not do any preventative practices for GTDs when initially planting the vines, the vineyard will start to see GTDs within the first five to 15 years. By then, the vineyard has matured, and removal becomes far more difficult than if the recommended treatments had been applied from the beginning.
Those looking for a more in-depth analysis of the work of Eskalen Lab and the latest research on treatments for GTDs, go to https://ucanr.edu/sites/eskalenlab.
Eskalen Lab’s website offers these suggestions for managing GTDs in the nursery and vineyard.
Preventative Management in Nursery
• Treat pruning wounds on mother plants to prevent new infections.
• Sanitation in mother fields and during the entire nursery process.
• Disinfect grafting machines regularly.
• Reduction of the cutting hydration period.
• Apply control products (chemicals or biologicals) as a dip after grafting, before storage and/or before dispatch.
• Hot water treatment of dormant nursery plants prior to dispatch.
Preventative Management in Vineyards
• Use the cleanest plant material available when establish new vineyards.
• Protection of pruning wounds with effective registered chemicals and/or biological control agents is the most effective way to prevent new infections from air-borne spores of GTD fungal pathogens. More than one application may be necessary to protect the pruning wound during its susceptible time period.
• Minimize stress conditions on young vines after planting.
• In applicable, In VSP systems, double pruning has shown to facilitate late pruning of large acreage vineyards and thus, reduce infection.
• Prune dead shoots, spurs and cordons below the symptomatic tissue (at least a few inches past the last symptomatic wood).
• Make a clean and smooth pruning cut to speed up the callusing process at the pruning wound.
• Sanitation is very important in the vineyard. Remove pruned and infected plant materials away to prevent the development and increase of GTD fungi overwintering structures in the vineyard.
• Remedial surgery, where visible infected parts of the vine (spurs, cordons and/or trunk) are removed, can be an effective strategy to remove the pathogen from the vine (primarily when cuts are done 7” to 10” below the visual canker tissue) and thus, prolong the lifespan of vineyards.
Preventative Management in Vineyards
• Protect pruning wounds.
• Use disease free, clean plant materials when establish new vineyards.
• Apply good cultural practices to minimize stress on young and mature vines.
• Delay dormant pruning to avoid potential pathogen dissemination during winter precipitation and to reduce the susceptibility.
• If applicable, consider doing double pruning to reduce fungal spore infection during winter moths.
• Prune dead shoots, spurs and cordons below the symptomatic tissue (at least a few inches below).
• Make a clean and smooth pruning cut to speed up the callusing process at the pruning wound.
• Remove pruned plant materials away from the vineyard to prevent fungi to form pycnidia and perithecia.
