Although Merced County vines are running late, due to the unseasonable cool weather this spring, the crop seems to be developing pretty well, says Maxwell Norton, University of California Cooperative Extension farm advisor for Merced County.
He describes early June conditions of wine grape vineyards in the county as “unremarkable,” despite the frost that nipped vineyards in the lowest areas of the county when shoots were coming out,
“Growers were really worried at the time, but we’ve seen no extensive damage,” Norton says.
Because powdery mildew thrives in the mild temperatures experienced this spring, he cautions growers to remain vigilant and continue treating vines, as needed, to control the fungal disease. With the right type of temperatures, it could remain a threat into early summer, he says.
The continuing rains this spring prompted some growers to start spraying for phomopsis cane and leaf spot disease.
“It’s found throughout California and can be a problem in years like this when we get a lot of rains as new shoots are developing,” Norton says. “In the past, I’ve seen it cause really significant damage.”
Phomopsis is easy to control with preventive applications of fungicide, he notes. But, now that canopies have started to close and protect the outsides of the vines from most of the rain, the threat is pretty much over for this year.
Three insects are causing the most concern for wine grape growers at this time. They include the European grapevine month. Sightings of the insect prompted state and federal agricultural officials to establish a 108-square-mile quarantine area in the northeastern-most part of Merced County last year.
“There have been no additional outbreaks of the insect in the northern San Joaquin Valley, and the few growers in the quarantine area here are hoping it will be lifted,” Norton says.
However, last month a quarantine covering 103 square miles was declared farther north in Nevada County after European grapevine moths were detected in two traps, one in a vineyard in western Nevada County and the other in the Nevada City area. That’s reason for concern, he says.
Although some growers in San Joaquin County and in areas of various coastal counties are treating to control the light brown apple moth, the insect hasn’t been spotted in his county, Norton says. Neither has the latest threat to California farmers — the brown marmorated stink bug.
“We’re really concerned about it,” he says. “All it takes is a few adults in a load of grapes to damage quality.”
Meanwhile, some Merced County wine grape growers may still be feeding nitrogen to their vineyards. Powdery mildew spraying and cultivating and mowing to control weeds also continues, along with some berm sprays to clean up any weeds before the canes get too low.
“It’s been pretty quiet in the vineyards the past few weeks, as usual at this time of the season,” Norton says. “We’re just watching the vines grow. Although sometimes, like this year when the temperatures aren’t always consistent, figuring out how much irrigation to apply can be tricky.”