Maintaining just the right amount of boron, an essential micronutrient for grapes, can be tricky for growers in some parts of California, but new research in foliar and drip-irrigation application helps in striking the knife-edge balance.
Bill Peacock, Tulare County farm advisor, says once boron amounts are determined by tissue samples and stabilized, a cornerstone for a good maintenance treatment with drip is one pound per acre every three to four years, or one-quarter pound to one-third pound each year.
He cautioned that annual applications at the one-pound rate for three or four years will cause excessive levels in the tissue.
Peacock told of his trials with boron on grapes from 1988 to 2001 during the recent San Joaquin Valley Table Grape Seminar at Visalia. Collaborating in the trials were Pete Christensen, University of California viticulture specialist emeritus, and Bob Beede, Kings County farm advisor.
The acceptable range for soil-content boron in grapes is indeed narrow, from deficient at less than 0.4 parts per million to toxic at 1 ppm or more. Irrigation water with a concentration of 1 ppm boron can also cause problems. In vine tissue, less than 30 ppm can be deficient, while anything greater than 100 ppm can be toxic.
Too little boron can lead to irregular ripening shot berries and poor production, even though vines later overcome symptoms of foliar speckling.
Too much, perhaps when applied through unneeded foliar or soil applications, causes toxicity with cupped, irregularly shaped foliage.
Natural boron low
Boron content of granite and other igneous rock of the eastern slopes of the SJV is relatively low. That leads to some deficiencies on that side of the valley, particularly in sandy soils, low spots, or around irrigation valves where excessive leaching occurs.
Toxicity along the east side is rare but may occur with over-fertilization with materials containing boron. Too much boron, however, can reach vineyards on the west side of the valley and in coastal counties where lakebed soils have accumulated material from sedimentary rock of the coastal range. Excesses in soil must be leached out by using high-quality, boron-free water.
Peacock said the studies on a mature Thompson Seedless vineyard in Tulare County showed that one pound of boron per acre could be applied safely through drip, even in a single application. Uptake of drip-applied boron is very rapid and response can be seen in about three weeks.
“For maintenance,” he said, “boron can be applied any time of the year. However, to correct a diagnosed deficiency, apply before dormancy, if possible, to correct boron deficiency in dormant bud tissue.”
Research in 1998 and 1999 on foliar applied boron focused on a furrow-irrigated Thompson vineyard near Kingsburg that had severe boron deficiency in the 1997 season. Treatments of 1 pound per acre were applied as a 20.5 percent boron soluble product. Timing was during the fall, prebloom, and bloom.
Correction of a deficiency was not always immediate because of restricted mobility within the plant and continued development of deficient tissues.
“In severely deficient vineyards,” he said, “pre-bloom and bloom sprays were only partially effective in preventing fruit symptoms. Fall foliar sprays increased boron content of dormant bud tissue and are more effective in eliminating cluster and berry symptoms the following year.”
In a summary of the application results, he said leaves are more tolerant of boron applied in the fall, and the micronutrient can be applied to foliage at a one-pound-per-acre rate in a single application without consequence.
Another seminar speaker, Walt Bentley, UC IPM specialist at Parlier, said the vine mealybug can be eradicated in vineyards where it is newly established, but that depends heavily on monitoring for infested vines and follow up with chemical treatment.
“Growers must be able to recognize this pest. Have someone, an irrigator or a harvesting crew member, look at every vine and mark each one that is suspicious,” he said.
The individual vine approach and early detection make eradication possible, but eradication cannot be done on a large-scale basis. Once an infestation is established, usually in three years, treatment must be made annually.
Signs of the pest, typically on young vines, include heavy concentrations of crystallized honeydew and waxy deposits on canes and clusters. Distinct from the better-known grape mealybug, the insect itself resembles citrus mealybug.
Vine mealybug, first identified in the Coachella Valley in 1994 and Arvin in 1998, has since spread through the SJV, North Coast, and foothill districts. One difficulty in control is it retreats to roots or hides under bark, protected from sprays, during certain life stages.
Movement of the vine mealybug has been attributed to nursery stock, and California nurseries are using a hot-water treatment, already in use against nematodes and phylloxera, to disinfect cuttings.
Bentley said fall and spring treatments with Lorsban prior to bud break have been effective, but he stressed that regulations for the product vary by counties and local ag commissioner offices should be consulted before use. Other effective treatments are Admire through drip and foliar sprays of Applaud and Dimethoate.
A pheromone product has been used to attract male vine mealybugs. It can be used as a basis for a more intensive survey to detect infested vines. Research on mating confusion to be linked with chemical control is also in progress.
The materials being used, Bentley added, are compatible with biological controls, and an Anagrus parasite is being reared for use in raisin vineyards, although guidelines for releases have not been developed.