USDA table grape breeders are approaching a solution to powdery mildew, a fungal pest that has nagged grape growers for thousands of years.
The disease, Uncinula necator, an annual threat to grapes, appears first as chlorotic spots on leaves and later as white, powdery material on infected foliage and fruit, causing the fruit to be unmarketable.
The temperature-driven pathogen traditionally has been fought with sulfur and more recently with a host of fungicides, along with sophisticated and expensive weather monitoring systems to signal its onset and guide treatments.
David Ramming, UDSA Agricultural Research Service research horticulturist at Parlier, Calif., and leader of the breeding campaign, said they have harnessed naturally occurring powdery mildew resistance from a number of wild grape species.
However, releases of new varieties to nurseries and growers are still years away, pending extensive selections and evaluations.
Ramming reported on progress of the research, supported by the California Table Grape Commission, at the recent San Joaquin Valley Table Grape Seminar in Visalia.
Noting that powdery mildew is the single most important fungal disease of grapes in the world, he said the tonnage of sulfur (17.5 million pounds on table grapes in 2003) and fungicides applied for it is a continuing environmental concern. Table grapes typically need an average of seven to eight treatments for the disease per season, making a significant expense for growers.
In addition, the disease has been able to generate new forms resistant to a succession of fungicides formulated to control it.
Although disease-resistant programs for grapes have been in development for the past century, only limited progress has been made against powdery mildew and the general quality of fruit from resistant cultivars has been poor.
However, collaborative research between Ramming's group at Parlier and their USDA counterparts at Geneva, N.Y., has identified resistance in species including Muscadinia rotundifolia, Vitis romanetii, and Vitis aesitvalis.
Using these species, the researchers have developed molecular markers to aid in the selection of resistant types. The markers allow for rapid selection, replacing the years of crosses and greenhouse and field studies required for conventional selection.
“Our ultimate goal,” Ramming said, “is to develop both table grape and raisin varieties that are resistant to powdery mildew.”
The promising resistance mechanism, he added, apparently prevents penetration of the plant by the fungus. Experience with other crops suggests that this type of resistance will repel various races of it. “So this should be a very durable source of resistance.”
The breeders are “pyramiding” multiple genes from the resistant sources to produce individual plants with maximum resistance.
“We have a large number of selections already planted in plots at the Parlier USDA station,” he said. Meanwhile, they continue to work with additional resistant species as they do backcrossing with selections made earlier.
They made 11 seedless by seedless crosses for powdery mildew. Ramming has been using the “embryo rescue” technique he and his staff developed in the early 1980s. The laboratory procedure involves surgically removing embryos for hybridization with two seedless parents for nearly 100 percent seedless progeny. It is much more efficient and reduces the ten years needed for conventional breeding to five.
The crosses are being planted in a greenhouse for further selection and about half of them will advance to planting in the field.
Work in 2007 showed that a very high proportion of the desired table grape fruit traits, such as large berries, flavor, and seedlessness, along with powdery mildew resistance, could likely be preserved in various crosses.
“Although we have increased fruit quality while keeping resistance, no individual has all the commercial table grape qualities necessary to compete with current industry standards,” Ramming said.
Another seminar presenter, Joseph L. Smilanick, USDA research plant pathologist at Parlier, reported on his continuing investigations of preharvest applications of fungicides to control gray mold, or Botrytis cinerea, the most important postharvest disease of table grapes.
The Redglobe table grape is particularly susceptible to the disease because of the large grape's numerous fractures in the cuticle on the surface of its berries.
During 2007 in a laboratory experiment on 38 B. cinerea isolates, Smilanick said, “Only five isolates with a relatively low level of resistance to Elevate were found, while 25 or more of the 38 isolates collected were resistant to the other fungicides.”
The other fungicides, ranked in descending order of potency to reduce disease by 50 percent, are Scala, Vanguard, Rovral, Pristine, and Topsin.
“This is not to say that Elevate is necessarily the best material,” Smilanick said. “Other studies show that Elevate had good activity in laboratory studies but was not very rain-fast in the field.”
He added that his tests were only preliminary and a larger number of isolates would be needed to characterize and confirm the amount of fungicide resistance in populations of the disease. Plans are to move the experiments into a commercial vineyard for more information.
Since resistance to fungicides has been detected, he said surveys in Kern, Tulare, Fresno, and Madera counties for about 200 isolates of the disease will give a better indication of the amount of B. cinerea in San Joaquin Valley vineyards.
Another finding of his work was that if labeled rates of fungicides are applied, residues typically will not be more than one-third of the limits allowed.
Jennifer Hashim-Buckey, Kern County farm advisor, worked in commercial vineyards in the overall project with Smilanick. She told the seminar two of three fungicide application programs with Pristine, Vanguard, and Scala reduced the prevalence of preharvest bunch rot by about 50 percent. “Significant reductions in postharvest non-Botrytis slip skin,” she added, “were observed in the regime that incorporated two applications of Scala and one of Pristine. One evident benefit of the fungicide programs we evaluated includes a reduction in the number of clusters with summer bunch rot.”
Those were reduced from 49 percent among untreated clusters just before harvest to as low as 22 percent among those treated with the most effective fungicide schedule.
“This is a substantial increase in yield, berry quality, and the speed of harvest would be improved because harvesters would have less trimming to do,” she said.
Hashim-Buckey also said control of postharvest non-Botrytis slip skin on Redglobe grapes was significant, although the magnitude of reduction in the prevalence of infected berries was not large, from 0.39 percent among untreated berries to 0.23 percent among berries treated with the best fungicide regime.