Botryosphaeria is often overlooked as a pathogen for canker diseases of grapevines, but Dr. Doug Gubler of the University of California, Davis wants growers to have a closer look at the fungus.
Learning what to look for is a first step, however.
Botryosphaeria cankers are often confused with the eutypa cankers that have long been considered the primary canker of California grapevines, Gubler told growers attending the British Columbia, Canada, Wine Grape Council’s annual viticulture and enology conference in Penticton, British Columbia this past summer.
While botryosphaeria was known in southern California, it wasn’t suspected in northern parts of the state. Moreover, where it did occur, it was believed to be taking advantage of existing eutypa infections.
No foliar symptoms
Throw in confusion regarding symptoms, and botryosphaeria—which doesn’t give rise to foliar symptoms—frequently went undiagnosed. Both disorders cause a web-shaped canker.
"When you’ve got them out in the field, you can’t tell the difference between botryosphaeria canker and eutypa canker," Gubler said.
Now, with nine species identified in California, Gubler considers botryosphaeria a major concern. "Botryosphaeria seems to be the most prevalent species causing canker in California," he said.
The nine species identified in California include B. rhodina and B. lutea, warm-climate species identified only in the San Joaquin and the Coachella valleys, as well as Mexico. Seven cool-climate species have been identified, including B. australis, B. dothidea, B. obtuse, B. parva, B. stevensii, and the more-rare B. iberica, found only in the central coast, and B. viticola.
"These fungi ecologically have their niche in California. They grow and cause disease in areas that kind of match their optimum temperature growth requirement," Gubler said.
The most destructive species are B. parva, B. lutea and B. rhodina, which can move through 15 centimeters of vine in just two months, leaving a significant amount of dead wood. During trials, cuttings were killed a mere five weeks after inoculation.
Most species can kill vines rapidly and need to be pruned out immediately after they’re found, Gubler said.
Cankers usually start appearing within three years of the organism being present in the vineyard. While it was originally thought that the fungus was moving into vineyards from adjacent orchards, forests, and trees in riparian areas, Gubler said the fungus might have already been present in vineyards and simply be moving between host species—including vines—before causing an infection.
A number of factors create optimum conditions for botryosphaeria infections to take hold.
Temperatures have to be in excess of 5°C (41°F), while spore release usually occurs following rainfall. The spores land on a wounded vine and move through the plant tissues toward the roots.
"These organisms are very closely tied to rainfall, in terms of when their spores are released, and we know that for infection to occur, there has to be some kind of wetness on the cut surface where the spore lands on," Gubler said. "So, in this area, if you’re out trimming and you’ve got temperatures of five degrees or lower and it starts to rain, then you probably don’t have a lot to worry about in terms of cankers during that particular rainstorm."
Sprinkler irrigation can be as effective in dry areas as rainfall in facilitating an infection.
"We know that with sprinkler irrigation, we can also get spore release," Gubler said.
To control the fungus, Gubler recommended double pruning, which has been shown effective against most diseases that take advantage of pruning wounds. Double pruning involves making two passes through a vineyard, once to prune vines to a uniform height, and a second time to prune the vines when the risk of infection is lowest.
"Double pruning, we’ve been able to show, will control every single disease that occurs on grapevines through pruning wounds," Gubler said.
There’s also been some success with spray applications of the fungicide myclobutanil (Rally) and phosphorein, a combination Gubler’s research indicates kills just about every fungus that afflicts grapevines through pruning wounds.
"We think it’s going to be more economical than going out and painting pruning wounds by hand," he said.
Regardless of what pruning method is used, or when it’s done, Gubler urged growers to remove cuttings and dead wood downwind from vineyards to prevent spores from drifting back into the vines and risking future infections.
Botryosphaeria is a new pathogen in British Columbia’s Okanagan Valley. Dan O’Gorman of the Pacific Agri-Food Research Centre in Summerland reported that relatively crude spore traps have identified two vineyards with vine decline. The dominant botryosphaeria species identified in the surveys is B. iberica.
B. iberica is a cool-weather species being picked up in March through May in the Okanagan. B. parva has also been identified, suggesting that growers are facing at least one of the most aggressive canker pathogens seen in California.
While it is too early to point out trends in British Columbia, O’Gorman highlighted a correlation between rainfall and temperature and spore presence.
Ring nematodes may play a role in facilitating infections by damaging the roots of vines.