Based on what he was seeing in the field and reports from colleagues in the second week of June, George Zhuang, University of California Cooperative Extension viticulture farm advisor for Fresno County, expected veraison to begin in California’s raisin-type grapes around the middle of June. That would put the onset of veraison about a week earlier than last year.
On June 11, clusters Zhuang sampled from a vineyard near Kerman showed Brix levels of 6.4º in Thompson Seedless and 8.0º in Selma Pete. That would indicate Selma Pete berries beginning to soften in-mid June, with Thompson seedless, a later-maturing variety, starting veraison near the end of June.
Also in the second week of June, Fiesta, Diamond Muscat and Selma Pete blocks in vineyards at the UC Kearney Research & Extension Center, Parlier, were very close to veraison, he notes.
As Zhuang points out, the period between the start of veraison and mid-July is a critical one for grape development. “During this time the berries start to accumulate sugar very quickly,” he says, “So, it’s very important to minimize severe water stress so that the leaves can continue producing sugar through photosynthesis.”
Proper vine nutrition also plays a major role in keeping canopies healthy and productive, he notes. A petiole analysis at this time can reveal the need to correct any deficiencies in levels of magnesium, phosphorus and, more importantly, potassium.
Keeping powdery mildew under control during veraison is another key to encouraging development of good sugar levels in the berries. If severe enough, this fungal disease could also infect the rachis and leaves to hinder the sugar flow and photosynthesis.
Growers who have been diligent in alternating between fungicide applications and sulfur dust treatments have been able to keep any powdery mildew outbreak pretty much under control, Zhuang notes. He recommends using the Powdery Mildew Risk Assessment Index available at http://www.ipm.ucdavis.edu/calludt.cgi/GRAPEPMVIEW1 to determine disease pressure and how often treatment is needed to protect the vines.
Just about all of California’s raisins are produced within a 100-mile radius of Fresno. Here, in vineyards receiving adequate water, the raisin grape vines are looking pretty good, Zhuang reports. In other cases, though, vines are showing signs of water stress.
For mildly stressed vines, shoot growth slows and internodes shorten, he explains. As water stress increases, the angle of the leaf blade to its petiole decreases. Under severe water stress, shoot tips and tendrils dry and may die on the primary and lateral shoots.
“To maintain grape production with less water, don’t start watering until you need to,” he advises. “And, when you do, only apply what you need.”
Several tools are available for monitoring water stress in vineyards, he adds. They include pressure bombs to measure water stress in the vines and tensiometers and neutron probes to determine available moisture in the soil.
“If you have enough experience, you can gauge the amount of water stress in a vineyard by how the vines look,” he says. “If the leaves are a nice green color, the shoots are growing well and the clusters are looking good, your vines probably are receiving enough water.”
Water hasn’t been the only concern for raisin grape growers this season. During the spring Zhuang received a number of calls from growers about the numbers of vine mealybugs they were seeing.
It wasn’t that populations were unusually high. Instead, this pest became active earlier than normal, says David Haviland, UCCE entomology farm advisor for Kern County.
“The rather warm weather in February and March gave the vine mealybug a head start on the season,” he says. “The insects began moving up onto the vine earlier than typical, resulting in more generations than usual for this time of year. By early June, we were already seeing vine mealybugs in the canopies. Normally, they don’t get that far up the vines until mid- to late-June.
Mid-June is the tail-end of the period when insecticide sprays are effective in controlling the vine mealybug, Haviland adds. After that, the denser canopies and tighter clusters limit the ability of sprays to penetrate through and reach the insects. Also, this is when adult females move under the bark, safe from sprays, to lay eggs.
“Vine mealybugs aren’t very mobile,” he says. “If you had them last year, you have them again this year. And, if you didn’t have them last year, consider yourself lucky and hope they don’t show up this season.”