With many California wine grape growers heading into the home stretch of this year’s harvest, Paul Verdegaal, University of California Cooperative Extension farm advisor for San Joaquin County offers these post-harvest vineyard management tips:
• Irrigate to help vines recover
Irrigating vines soon after harvest is especially important in a dry season, like this one, he notes. The amount of foliage still on the vine at this time can help both earlier and later-maturing vines make good use of water in recovering from their efforts of producing the crop.
Because of the early harvest, Verdegaal doesn’t anticipate problems irrigating any varieties other than the very latest one. In many years, he cautions, care is needed to avoid giving the trees too much water too late in the season.
• Rebuild nutrient levels
Depending on time of harvest, vines can benefit from fall-applied nitrogen, if used correctly. That means allowing enough time after application for soil microorganisms to convert nitrogen into the nitrate form — the form that the roots can take up easily — before the vine goes dormant. Otherwise, nitrogen leach below the root zone, potentially threatening water quality and hurting profits.
“Some nitrogen formulations take longer than others to break down into the plant-available form,” Verdegaal says. “In fields with varieties that mature later, it may be too late and growers should wait until next spring to put on the needed nitrogen.”
It’s a different story with potassium. Much more stable in the soil than nitrogen, potassium can be taken up as long as the leaves remain active, he says. It can also be applied in the spring. The two main types of potassium fertilizers are the sulfate or chloride forms. Newer formulations can be applied through drip systems for more efficient and cost-effective fertilization. Interest in foliar application of potassium continues to grow, but it’s a more expensive option for providing vines relatively small amounts of the nutrient, Verdegaal notes.
The post-harvest period is also a good time to amend soil with materials like gypsum to improve soil texture and water infiltration or and lime or soil sulfur to correct high acid levels in the soils.
• Check vines for viruses
Despite increased availability of virus-clean rootstocks and efforts by the industry to rid nurseries of viruses, some still manage to make their way into California vineyards. “Viruses are something you need to be on the guard for all the time,” Verdegaal says.
Common disease-causing organisms include:
• Leafroll, which develops gradually over the season, inducing a reddish color in the leaves of red varieties. In both red and white varieties the leaf margins roll downward and often the leaves become thick and brittle. Infected vines usually are less fruitful, may have smaller clusters and poor sugar levels or color development.
• Fanleaf, which can seem to improve growth, causes foliar distortion, vein banding, poor fruit set and reduces the productive life of the vineyard. The dagger nematode, a vector for the virus, spreads the disease by feeding first on the roots of diseased vines and then on the roots of healthy vines.
• Red blotch, which was first recognized in 2008 in a Napa Valley vineyard. It discolors grape leaves in fall and lowers sugar levels in grapes. Symptoms are similar to leafroll, without the rolling of leaf margins, and include blotches of pink or red veins on green leaves in the fall, when grape leaves would normally be turning a uniform gold color. Some grapes may never mature fully.
“Until recent years, the most common vector in grapevines was the grower who used cuttings from infected plants to produce new vineyards,” Verdegaal says, “But all mealybug species transmit one or more of the known leafroll viruses.”
He advises marking any suspicious vines with colored tape or paint or any semipermanast marker for continuing observation or removal.
• Survey any weed problems
Fall is a good time to identify and record location of weeds that may have escaped your control program. Such information is useful in selecting appropriate herbicides and rotate modes of action for next year, he says. Also, this survey can be used to note any species developing resistance to herbicides and any noxious weeds that may be establishing in the vineyard.
• Repair any harvest damage to stakes and trellises
• Update regulatory compliance records
Verdegaal recommends using this time to review paper work showing you are current in meeting requirements of various regulatory programs, such as Water Coalition membership , air pollution plan permits (if you’re farming contiguous blocks of 100 or more acres) and pesticide use reports.
• Check for vertebrate problems
Also, Verdegaal suggest using this post-harvest period to note any populations of gophers, voles (meadow mice), rabbits, coyotes or birds that may require action early next year to mitigate damage to your vineyard.