- Author: Tony Nunes, UC Master Gardener
By Tony Nunes, U.C. Master Gardener of Napa County
Knowing basic botany and grapevine structure and selecting a variety and rootstock suited to the location can help vineyard owners succeed. Master Gardeners call it putting the right plant in the right place.
We are fortunate to live in a region with soil types and microclimates that produce world-class wines. Just like commercial vineyard managers, home vineyard owners need to make decisions based on their unique site, goals and conditions and make adjustments driven by Mother Nature throughout each growing season.
It starts with a vineyard management plan for the season. Napa Valley has kicked off the year with rainfall amounts and low temperatures not seen in many years. Determining pruning dates is the first step. In some places, pruning has already started as vineyard managers allocate their resources throughout the valley.
Home vineyard owners may have the flexibility to delay pruning, which in turn delays bud break and minimizes the potential for damage from a late frost. Some people make two passes, first to remove last year's old canes, then to make the final command decision: the pruning of buds created during the 2016 growing season that will produce the crop for this year's harvest.
As our growing season progresses, days get longer, soil temperatures rise and grapevine sap begins to flow. This awakening from winter dormancy is one of many milestones of the growing season.
One element of integrated pest management is a plan for controlling powdery mildew (Uncinula necator) and botrytis (Botrytis cinerea) from bud break to veraison, the color change in grapes that signals the onset of ripening. The spores that cause powdery mildew or botrytis are ubiquitous in Napa Valley vineyards.
Powdery mildew infections from the previous year appear as a blue fungus on individual berries or clusters. Botrytis affects the internal tissues of the berry or the entire cluster. Berries split and fungal spores can grow out of split skins. The spores from infected clusters are distributed easily by wind. Spores can survive the winter on the grapevine bark and cause infections the following year.
Early in the growing season, vine shoots grow rapidly. Like a jack-in-the-box, buds produce shoots and a self-pollinating flower blooms to produce the year's crop. Bloom and fruit set are critical periods driving yield and fruit quality. Temperature swings and rain are not welcome during this time.
Grapevines do not required special fertilizers but should be kept in balance with nutrients available in the soil. Too much nitrogen in the soil can cause vines to produce excess canopy, creating issues with pest management and crop quality.
When applying fertilizer, first ask yourself why. Testing can help you determine what the vine needs. The first test of the growing season, petiole testing, should be performed after fruit set. Petioles connect leaf blades to the vine shoot. Samples of 25 to 50 petioles are selected for each block and tested to determine what nutrients might be deficient or present in excess. Fertilizers are then applied according to the specific needs of that block.
Grape clusters need leaves and sunshine to ripen the crop. Additionally, new buds require sunshine to produce energy for next year's crop. Managing the amount of leaves and their health is referred to as canopy management.
Removing leaves at the base of the canes surrounding grape bunches allows for better sunlight penetration and airflow. To guide decisions about leaf removal, growers observe weather conditions and how the vineyards are oriented to the sun. Leaf removal on the west side facing the afternoon sun is limited during July and August to minimize sunburn and bunch rot. Home grape growers may also need to observe their microclimates or placement of buildings when determining how to manage the canopy.
Irrigation decisions also affect canopy health. Vineyard managers monitor soil moisture levels and visual clues from leaf blades and tendrils during the early season's rapid shoot growth, flowering and fruit set. When irrigation is needed, it is best to irrigate deeply according to the site's soil-moisture holding capacity.
Roots don't grow in dry soil. The goal is to encourage roots to grow as deeply as possible given the site's soil conditions, with just enough water stress to produce high-quality fruit.
Home vineyard owners are generally optimists or at least try to be. Each year, they look forward to a successful harvest, drawing on the knowledge gained from previous growing seasons.
Master Gardeners are volunteers who help the University of California reach the gardening public with home gardening information. U. C. Master Gardeners of Napa County ( http://ucanr.edu/ucmgnapa/) are available to answer gardening questions in person or by phone, Monday, Wednesday and Friday, 9 a.m. to Noon, at the U. C. Cooperative Extension office, 1710 Soscol Avenue, Suite 4, Napa, 707-253-4143, or from outside City of Napa toll-free at 877-279-3065. Or e-mail your garden questions by following the guidelines on our web site. Click on Napa, then on Have Garden Questions? Find us on Facebook under UC Master Gardeners of Napa County.
But we are also aware that with bud break come all the other concerns in managing our vines: frost protection, rain at the wrong time and powdery mildew. And more powdery mildew. The last few years have taught us to be flexible and not anticipate what nature has in store.
Managing the vineyard is less stressful if we know we have done all we can to improve the chances of a successful fall harvest. Powdery mildew is the most serious and widespread disease in California vineyards in terms of cost of control and losses in yield. Wine quality can be affected when as few as 3 percent of the berries are diseased; severe mildew may cause berries to crack, allowing rot-promoting organisms to enter. Surprisingly, powdery mildew is not the same mildew we get on roses.
All succulent tissues on a grapevine are susceptible to mildew, and the fungus begins to show shortly after bud break. Usually within one week of the first spring rain you can see colonies on the underside of leaves. The fungus forms a white, web-like mat over the leaf tissue and draws nutrients from the plant tissue. This leaves develop a dusty appearance as the spores spread.
How does mildew get started? It cannot grow on dead or dormant grape tissue. It survives the winter under infected buds; or when autumn rains wash the spores off of leaves and shoots and onto the bark of upper trunks and cordons. In the spring, rain and sprinklers release the spores and wind blows them onto newly emerging shoots and leaves. If you had a lot of mildew the previous year, you might see red, stain-like scarring on vine canes. This is dormant mildew. Buds near this scarring will start to be infected.
In mild weather, spores germinate in less than five days. Mildew really loves temperatures between 70°F and 85°F. High temperatures kill the spores. And temperatures above 95°F for 12 continuous hours cause the mildew to stop growing. Temperature plays a larger role in the disease development than moisture and humanity. Rain, dew and irrigation water can actually cause poor germination of the mildew spores, and they can sometimes even be washed away. Powdery mildew is spread by windborne spores and they can travel a long distance.
The infection in the fruit can continue until certain sugar levels are reached in the grapes. The damage can cause stunted berries, decreased yield, delayed ripening, scarring and off flavors in the wine. Some varieties are more susceptible to mildew, including Chardonnay, Cabernet Sauvignon, Carignane and table grapes.
To control powdery mildew, it is best to apply preventatives and keep applying them. It is difficult to eradicate and you need to start early. Many materials can be used. For advice, visit www.ipm.ucdavis.edu/PMG/crops-agriculture.html.
Protect vines and new buds and leaves before your vines become infected. Fungicides will inhibit spore development and germination. Sulfur is an effective and economical control and in most years provides adequate protection. Other options are also available.
New vines do not need treatment, but in the second year, you should monitor for mildew on leaves. After year two, start the regimen.
To summarize, if you grow grapevines, you will have to treat for mildew every year. The control is a preventative. Some products provide longer protection than others. Be sure to watch the weather.
Workshop: On Saturday, April 6, from 9:30 a.m. to noon, Napa County Master Gardeners will present a workshop on “Small Home Vineyards” at the Yountville Community Center. The fee is $15. Call 707-944-8712 to register or register online at www.townofyountville.com/index.aspx?page=274. Call the Master Gardener office for more information at 707-253-4221.
Master Gardeners are volunteers who help the University of California reach the gardening public with home gardening information. Napa County Master Gardeners (http://cenapa.ucdavis.edu) are available to answer gardening questions in person or by phone, Monday, Wednesday and Friday, 9 a.m. to Noon, at the U. C. Cooperative Extension office, 1710 Soscol Avenue, Suite 4, Napa, 707-253-4143, or from outside City of Napa toll-free at 877-279-3065. Or e-mail your garden questions by following the guidelines on our web site. Click on Napa, then on Have Garden Questions?