Losses to canker diseases in California vineyards range into hundreds of millions of dollars each year, and plant pathologists are taking a closer look at how the fungal pathogens, including some “new” ones, move about.
Doug Gubler, Extension plant pathologist at the University of California, Davis, says no less than 17 species of fungi invade pruning wounds to cause “dead arm” disorders in grapevines globally.
Symptoms of the group, most readily seen in vines 10 or more years old, include general vine decline and dead arms, cordons and trunks once the pathogens obstruct the vines' vascular systems. Yield losses occur, and eventually the vine dies.
When diseased portions are cut cross-wise, the same darkened and wedge-shaped portions, long-known among California grape growers as signs of Eutypa dieback, are revealed.
According to Gubler, for years plant pathologists believed the cankers were all caused by Eutypa lata, because of their similar appearance in the field and a lack of complete laboratory tests.
But in recent years, more intensive research involving sophisticated DNA identification methods confirmed the presence of multiple species of fungi from the Diatrypaceae family, which includes Eutypa, Botryosphaeria, and others.
Surveys by Gubler's assistant, graduate student Joe R. Urbez-Torres, during the past few years turned up nine species of Botryosphaeria, or “Bot canker,” in California vineyards, making them the most prevalent fungi associated with grapevine cankers in the state.
The Bot canker species are pruning-wound pathogens that get into vines during periods of winter rainfall.
“But,” Gubler said, “there's still a lot we don't understand about the effects of temperature and other things that allow spore release and subsequent infection of pruning wounds.”
Although additional species are now involved, he said methods of control, such as pruning late in the dormant season and double-pruning, will probably remain the same as those used traditionally for Eutypa.
Botryosphaeria is most common in California from Merced County south and even occurs on table grapes in the Coachella Valley, despite dry conditions there, emerging in vineyards irrigated by sprinklers.
Eutypa lata is most common in northern and coastal California vineyards and orchards. It is widespread largely because one of its primary hosts is willow, an abundant species throughout the state.
Botryosphaeria species, Gubler added, have generally been overlooked in trees and vines around the world. Until a few years ago, they were largely considered saprophytes, or secondary colonizers, that are weak pathogens that move into vines after some injury. More recently, tests have shown they do, in fact, carry the infection on their own.
Plant pathologists around the world have become aggressive in learning more about Botryosphaeria species during the past five years. Gubler recalled that during a global conference at Davis in September of 2006 about half of the 110 viticulture talks dealt with Bot canker.
In their survey spanning three years, Urbez-Torres and Gubler investigated 166 wine, table, and raisin vineyards in 21 California counties and removed 1,700 canker samples, 90 percent of which were found to be infected with one or another species of Botryosphaeria.
They have developed a new test to identify the nine Botryosphaeria species found in California as well as some of the others occurring elsewhere.
Their research uncovered high incidence of Eutypella vitis in the Coachella Valley for the first time and found Phomopsis viticola to be the most common canker pathogen in table and raisin varieties in Fresno and Tulare counties.
Among other findings, they learned that B. rhodina thrives in temperatures of about 91 degrees F. and is one of the most common species in the Central Valley. B. obtusa, B. parva, and B. viticola are more common in the milder climate of the Napa Valley.
B. rhodina and B. parva were found in the testing to be two of the most severe in virulence and pathogenicity of the California group. “Unfortunately,” Gubler said, “these are two that we have in the Central Valley.” Although the others move less rapidly, they nevertheless do result in infection.
For the last couple of years, the researchers have been collecting spores trapped in vineyards throughout the state by farm advisors and then growing these out in the lab for identification.
The traps are simple, consisting of glass slides coated with Vaseline and left on cordons. After traps arrive at the lab, the spores are washed off the slides, filtered, and grown out in cultures.
Gubler and Urbez-Torres found a direct connection between the amount of spores released and rainfall or sprinkler irrigation, and the amount of infection was also greater where pruning wounds were slower to heal.
In the Coachella Valley, where disease occurred but collected spore counts did not seem to be sufficient for infection, they learned that B. rhodina and one other species of Botryosphaeria survive on pruning debris and sprinkler irrigation causes them to release spores to infect vines during pruning.
Gubler said the Bot species they are working with produce picnidia within a few weeks after infection is initiated. They release spores that await the next opportunity to be carried by splashing water.
Sources of vineyard-threatening Botryosphaeria inoculum for a vineyard include pistachio, cherry, almond, and other tree crops and other surrounding crops. “Once a tree starts producing the fruiting bodies, it is pretty well too late to do anything but cut down the tree and burn it,” Gubler said.
Reviews of scientific literature from around the world, he said, show an amazing amount of different Botryosphaeria symptoms according to location and cultivar, including shortened internodes, bud necrosis, vascular necrosis, cane dieback, trunk dieback, cankers, cane bleaching, graft failure, and others.
Gubler also measured the rate of development on both green and dormant tissue of the group and found B. rhodina, B. parva, and B. lutea can move to buds so rapidly they kill tissue before buds can push in the spring. That is why the disease rarely shows foliar symptoms.
He said in only one instance, an infected vine at the Kearney Agricultural Center at Parlier, were foliar symptoms attributed to Botryosphaeria observed.
Canker diseases inflict losses on raisin and table varieties, but estimates of Eutypa (and now including Botryosphaeria) damage to wine varieties alone were set at more than $260 million annually in 2001, based on studies by Jerome B. Siebert, an agricultural economist at the University of California, Berkeley.