With predictions for an El Nino winter and spring, California pest control and farm advisers are cautioning grape growers to be particularly careful about botrytis pressure.

Wet years seem to produce more botrytis bunch rot on susceptible varieties such as Zinfandel, Petite Sirah and Chardonnay, says Bill Thomas, a pest control adviser for Mid Valley Agricultural Services in Linden, Calif.

“We typically recommend an application of Vangard at bloom or at bunch closure to protect against botrytis bunch rot,” Thomas says. “And in heavy botrytis years, most of our growers would make two applications of Vangard if they felt grape prices were high enough.”

Therein lies the challenge. In the past few years, it has been upward and onward for grape prices, but no more. Prices have been depressed for most for two years and for some for three years as the wine grape industry goes through an oversupply period. Thomas says growers were in the position to spend money freely to prevent diseases when times were good.

“That changed this year,” he says, noting that prices have fallen from $600 to $150 a ton. “The falling prices made our growers take a hard look at what they were spending and whether they were getting value from it.”

Roger Duncan, a the University of California Cooperative Extension farm advisor in Modesto, says the economy forced some grape growers to reduce their fertilizer, insecticide and fungicide applications to minimize costs. Some producers have decided to pull out their grape vines and plant other crops such as almonds, although that is not as prevalent in his area as in others because producers did not over-plant vineyards during the last boom.

2002 light year

It helped that botrytis bunch rot wasn't much of a problem in 2002, so many growers only made one application of Vangard prior to bunch closure and thinned grape bunches to reduce infestations, Thomas says.

But things are expected to change this winter and spring with wet weather predicted with the El Nino weather pattern. And adding to growers' concerns is the increasing focus on grape quality, Thomas says.

Grape growers produced generally a large crop this past season and buyers looked for quality rather than volume. Quality often meant the difference between finding a home for the grapes and not.

“If you did not have quality this year, you would not have been able to meet your contracted standards, and they could have rejected the grapes,” says Thomas, who works with growers in San Joaquin and Sacramento counties.

Duncan says growers in his area stay on guard for botrytis and summer bunch rot.

“In Stanislaus County, commonly half our rot is botrytis and the other half is what we call summer bunch rot or sour rot,” he notes. “Vangard is a very good fungicide against botrytis, however results are variable against sour rot.”

Botrytis bunch rot has a reputation for causing most bunch rot, says Duncan, who has conducted botrytis and other bunch rot research. It is the primary rot organism in the cooler areas along the coast, however, it many times is not the culprit in the Central Valley.

Controlling botrytis, however, can help stave off summer rot, Duncan explains. If Botrytis starts, it may open the door for a secondary fungus.

‘Little time bombs’

“You get all these little senescent flower parts that are infected with botrytis at bloom, and they remain through the season within the grape cluster like little time bombs,” Duncan says. “If we get a substantial, mid-summer rain, these infected flower parts can lead to botrytis bunch rot. Even if it doesn't turn into a full-fledged botrytis infection, you can have an opening or crack in the grape that will allow weaker pathogens to enter. Canopy management is very important in managing botrytis and rot.”

Botrytis can even affect grapes in storage. Table grapes can have problems with storage rot, even if the grapes were not infected in the field, Duncan says. He adds that using Vangard during the growing season can help reduce the development of botrytis in cold storage.

Mid-season rains and even ideal growing conditions which lead to compact clusters of grapes can make grapes susceptible to bunch rot, Duncan notes.

“Zinfandel is notorious for bunch rot,” he adds. “Chardonnay can be bad. Barbera can be susceptible — so keep vigilant about disease control and treat botrytis prior to its onset.

“If you wait until botrytis hits, it's too late,” Duncan says.