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USDA trials target table grape gray mold controls

A little Botrytis cinerea, or “noble rot,” is a good thing on wine grapes for concentrating sugars in specialty wines, but the same gray mold is quite unwelcome when it destroys California table grapes in cold storage.

Although canopy management during the growing season and fumigation with sulfur dioxide (SO2) gas after harvest have long been used with success to control the Botrytis, SO2 can have a side-effect of bleaching berries of red-pigmented varieties such as Redglobe.

That's why Joseph L. Smilanick, a plant pathologist at USDA's San Joaquin Valley Agricultural Sciences Center in Parlier, and his colleagues have been searching for alternatives to SO2 fumigation.

He reported on his early research during the recent San Joaquin Valley Table Grape Seminar in Visalia.

He is evaluating two approaches, first, biofumigation with gases produced by another fungus, Muscodor albus, and second, short-term fumigation with ozone. When combined, these may have potential as an SO2 alternative.

Gases given off by M. albus are known to control fungal pathogens in peaches, apples and vegetable seedlings. With that in mind, Smilanick exposed cultures of it on rye grain to table grapes inoculated with B. cinerea spores and kept in cold storage. Only the gases, not the M. albus itself, came in contact with the fruit.

Control percentages

He said he observed control of the mold ranging from 50 percent when the culture was present under bags of grape clusters to more than 90 percent when the culture was enclosed with the fruit in clamshell containers.

Smilanick said he will continue the studies in 2006 with various rates of M. albus in expanded, semi-commercial trials. AgraQuest of Davis is developing prototype generator sheets containing the fungus to be placed inside grape boxes with liners.

After precooling and a period of cold storage, the incidence of decay and other aspects of quality will be determined and compared to grapes treated with SO2.

In his experiments with ozone, Smilanick said he hopes to develop new storage technologies for table grapes with the gas, which is approved by the U.S. Food and Drug Administration and has been examined for reducing decay organisms in many fruits and vegetables.

He reported that a single fumigation with 5,000 ppm of ozone for one hour reduced gray mold incidence in grapes in storage by 50 percent to 70 percent.

The 2005 investigation used high concentrations of ozone in a vacuum chamber originally designed for treating spices. The 2006 work will be done in a larger chamber with capacity for four pallets of grapes.

“If successful,” Smilanick said, “this approach would most likely be used in place of initial SO2 fumigation during precooling of grapes.”

One possibility he will be evaluating is short-term fumigation with high doses of ozone, followed by biofumigation with in-package generators containing M. albus applied at harvest in the same fashion as those currently used for SO2.

Smilanick is making no conclusions at this point. However, he said, “So far, levels of gray mold by both alternatives have been lower than those reported for SO2 when it has been used properly.

“Ozone gas fumigation at the right dosage may be able to compete with SO2, particularly during forced air, initial fumigation, and many cold storage facilities in use today may already be sufficiently sealed and corrosion resistant to tolerate ozone.”

He will also be carefully monitoring treated grapes for phytotoxicity.

Girdling timing

Bill Peacock, Tulare County farm advisor, disclosed his 2005-season observations on the optimum timing of girdling of Princess and Summer Royal table grapes, plus a re-examination of the practice on Thompson Seedless.

Girdling concentrates carbohydrates and plant hormones in the vine above the girdle point. Its timing is critical to accomplish desired improvements in berry size, maturity, set, and yield.

For Princess, a recent USDA release that is sensitive to early flower cluster necrosis, Peacock said the best girdling time depends on the vineyard's history with the physiological disorder linked to low light and low temperatures.

“Vineyards with a history,” he said, “should be girdled during full bloom to maximize yield. Berry size, however, will be similar to the ungirdled control. Vineyards without a history of necrosis should be girdled about a week after full bloom when shatter first begins. This maximized berry size and correspondingly increased yield.”

Peacock said girdling post-bloom had no effect on flower cluster necrosis. Excessively vigorous Princess vineyards suffer the most from the necrosis since carbohydrates are drawn to support shoot growth at the expense of flowers.

Princess berry weight was greatest when the girdle was made at the beginning of shatter or a week later at berry set. The response dropped off quickly when vines were girdled later than berry set.

Experiments on Summer Royal indicated the best time for girdling was from full bloom to early shatter, or one week past full bloom. This timing maximized both yield (by 24 percent) and berry size.

Unlike Princess, the Summer Royal vines were free of flower cluster necrosis, so the bloom time girdles did not increase the number of clusters at harvest.

Some surprises

Peacock said the trial on Thompson Seedless gave some surprising results. The common practice for girdling Thompson Seedless to increase berry size is at berry set, approximately two weeks after full bloom, and many vineyards are girdled a week or so after berry set.

“Our data suggest,” he said, “that vines should be girdled no later than berry set to maximize berry size. A week after berry set is too late. Growers should experiment to determine specific response in their vineyards.”

Thompson Seedless growers, he added, also need to re-evaluate timing of girdling intended to increase berry size. “Girdling earlier than berry set does not appear to affect berries set per cluster, and this is a primary concern for girdling early.”

Since Thompson Seedless was not afflicted with early flower cluster necrosis, the bloom time girdles did not increase the number of clusters at harvest.

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