Fresno County Calif growers Alan Kasparian left and Ed Nikssarian visit during the San Joaqun Valley Grape Symposium held in Easton in January

Fresno County, Calif. growers Alan Kasparian (left) and Ed Nikssarian visit during the San Joaqun Valley Grape Symposium held in Easton in January.

Drought, UC specialists update grape growers on issues

The crux of the California drought problem is “the deficit.” On average, a year of rainfall is missing over the three-and one-half-year drought statewide, and about a two-year deficit in the San Joaquin Valley. The relatively new herbicide Mission, combined with Roundup, is effective against some stubborn weeds, says the University of California's Kurt Hembree.

As yet another El Niño-spawned rain storm pelted the area where they were meeting, California grape growers received good and bad news as they gathered for a San Joaquin Valley Grape Symposium held in Easton.

First the good news - this El Niño is the real deal, “currently a strong El Niño,” said Jerald Meadows, warning coordination meteorologist with the National Weather Service in Hanford.

Then, the bad, in response to a question, “We’ll probably still be in drought conditions for 5-10 years,” Meadows said, adding, “I know that’s not what anybody wants to hear.”

He said the crux of the drought problem is “the deficit.” On average, a year of rainfall is missing over the three-and one-half-year drought statewide, and about a two-year deficit in the San Joaquin Valley. The rain will not be easily made up, he says, considering that much of what falls is lost.

“We get rain, but we can’t catch it. It goes out to the Pacific and I don’t see any dents in the hydrologic drought.”

Meadows says it will be difficult to recharge aquifers.

Steady rains needed

Asked what it would take to cut the length of the drought, he said more steady rains – “not monsoonal, not three inches at a time” – could help.

He said the El Niño conditions bring warmer temperatures, the potential for more hail, and a chance of flooding and soil erosion.

“Despite common thought, flooding can occur during a drought,” Meadows said.

Grapevine canker

Speakers at the symposium reviewed a wide range of topics - some related to the down side of rains and over-irrigation.

Doug Gubler, a professor with the University of California (UC), Davis’ Department of Plant Pathology, explained how pruning wounds can help spread grapevine canker, including Eutypa and Botryosphaeria.

Gubler says canker pathogens release spores during rainfall, heavy fog, or sprinkler irrigation, and pruning wounds are the main infection ports. The disease is more severe when vines are water stressed and cane pruned vines generally have less disease.

Rally and Topsin are among fungicides that can be applied, but are effective for only about two weeks. If used with a “paint” called Vitiseal, effectiveness can increase to three months.

To reduce vulnerability, Gubler recommends pruning late – in mid-to-late February or early March or double pruning, first with a tractor mounted rotary saw and a second pruning in late winter.

Nutrient uptake

Mark Battany, UC viticulture farm advisor for San Luis Obispo and Santa Barbara counties, discussed vineyard soil, water, and tissue analysis, plus different factors that affect nutrient uptake.

He said excessive soil moisture – flooding – can result in potassium deficiency.

“I was in a Pinot Noir vineyard and knew something was wrong when I heard all the toads croaking,” he said, adding that a tractor got stuck there in July.

Battany says sampling soils provides a benchmark for soil amendments, helps in evaluating site uniformity, can increase efficiency of fertilizer use, can help avoid the buildup of harmful elements, and can increase profitability.

It’s important, he says, to take into account moisture, soil type, and soil depth. He recommends taking a composite soil sample from about 20 locations throughout a block.

According to Battany, nutrient values vary by time of year; for example, potassium, phosphorous, and magnesium values are likely higher in the spring.

He also recommends an irrigation water analysis, since irrigation water chemistry can alter the chemical characteristics of soil. Tissue sampling is the most reliable method for nutritional analysis.

“Regular analysis can help identify problems before they lead to yield losses,” Battany said.

Vineyard weed management

Kurt Hembree, UC weed farm advisor, Fresno County, talked of vineyard weed management and the relatively new herbicide Mission, combined with Roundup, which he found particularly effective against some stubborn weeds, including horseweed, hairy fleabane, panicle willow herb, and burning nettle.

He warned that weeds can sap water, nutrients, and sunlight and can become a habitat for rodents and insect pests.

Pointing out that seeds from a single plant can range from the hundreds up to one million, Hembree said, “Don’t add to the seed bank,” citing an old saying “One year’s seeding means seven years weeding.”

He emphasized the importance to check how sprayers perform, spray coverage, treatment timing, and spray tip performance. He added that labels on herbicides can provide a wealth of information.

“Read the label,” Hembree said, noting it’s important to diversify the mode of action of herbicides to avoid resistance.

Red blotch disease vector

Kent Daane, Extension specialist with UC Berkeley and UC Kearney Research and Extension, said the search continues to find an insect that could be a vector for red blotch disease.

He says Movento continues to be the best product for controlling vine mealybug, adding that application methods for optimal performance may vary, and researchers are seeking to better understand the movement of Movento.

Biological controls help, he added, but can be incomplete. Mating disruption can help suppress the population, but may best be used in an area wide, annual program.

Daane says grape leaf roll movement can be suppressed using rouging and an area wide mealybug control program. Yet, this can be expensive and it does not guarantee there will be no future losses.

The vine mealybug, a vector for leaf roll, is considered the worst among mealybug pests since it has more eggs and causes more damage.

George Zhuang, a UC farm advisor for Fresno County, discussed bud dissection to guide winter pruning and estimate yield. By winter, the number of clusters each shoot will bear in the spring has been set. He said bunches per vine accounts for 60 percent of yield, berries per bunch 30 percent, and berry weight just 10 percent.

He collects bud wood for sampling – 25 canes per site with about 15 nodes each or 100 spurs per site with about two nodes each. Then he dissects the buds to see the potential number of cluster primordia.

Bud dissection can measure bud fruitfulness right after harvest, says Zhuang. Seasonal yield variations largely come from bud fruitfulness, accounting for more than 60 percent. 

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