Growing & Winemaking


Vineyard Management Following a Wet Winter

May 2017
by Glenn McGourty

We are all delighted it finally has rained enough in most of California to end drought conditions. It was a long five years. In many regions, rainfall is 50% to 75% above average. Snowpack in the Sierra was 175% to 200% above normal by late March. On my property along the upper Russian River in Mendocino County, we had five flood events—a record in my lifetime! Most reservoirs are full around the state.

While we all enjoy having water again, the abundant rainfall can cause some profound effects in your vineyard.

Flooded areas
Flood damage can be minor or major depending where your vineyard is located. A portion of my vineyard is in the Russian River floodway, and I try to take down all of my deer fencing and gates in the fall if it looks like it is going to be a wet year. Problems arise when you don’t get your fences down on time: They make great nets to catch water-borne debris. Anything in your fencing system made of wood (box braces, line posts, etc.) is likely to be broken if fences are still up. Trellises also catch debris, especially floating wood and straw. Cleanup and repairs can be time consuming and expensive.

Flooding is also the way that nematodes, phylloxera and new weed seed find their way into your vineyard. Be aware of potential new problems that may have floated in with the flood.

Hopefully your pumps and controls are above the water line or were removed before the winter when flooding is likely to occur. If your pumps did get flooded, do not operate them before having them serviced to remove silt and moisture. They will quickly be destroyed from short circuits if there is moisture in the armature. Silt in the bearings will also cause those to stop working, and they will need to be replaced. Switches and circuit breakers should be inspected, serviced or replaced if they have been underwater.

Vineyard floors
Water-logged soils can be a problem for vine root health. During the winter when vines are dormant, there is generally little worry about vine roots, as they can stay submerged for weeks with little damage (many rootstocks have genetics from vines that grow in riparian areas). Once the vines have leafed out, however, if they are in wet areas they may grow poorly and exhibit nutrient deficiencies. Generally all will be well once the ground dries out, and shoots will once again start to grow.

The one rootstock to worry about is 5BB, which is susceptible to Phytophthora root rot. Serious damage may occur to vines planted on this rootstock, and it should not be used where saturated soils are likely to occur.

If you planted cover crops, you will probably have abundant growth following a wet winter. This is positive for soil health, as exudates from the roots encourage soil microflora that helps aggregate soils, cycle nutrients and suppress soil pathogens. Similarly, the cover crop’s above-ground portions are a source of carbon and nitrogen (especially legumes).

Getting cover crops mowed down and incorporated into the soil may be challenging due to the extra biomass. Extra mowing and disking passes may be required. There inevitably will be wet sections of the vineyard that can sink your equipment. Try to work around those areas as best you can. It can also be a problem for no-till vineyards, as the wheel ruts will stay there forever, and you may need to re-till, level and reseed those areas at some point. In vineyards managed as no-till, you will probably have to mow extra into the late spring and summer, as the extra soil moisture will keep things growing longer.

Beneath the vine, weed control may also require extra attention. If you are doing it with cultivation, you may have some problems getting weeds under control. The early passes normally done in late November or December were not possible this season. This means that when you eventually get into the vineyard, there will be much larger weeds that require multiple passes. Those fortunate enough to have sheep grazing during the dormant period will have a much easier time, as the sheep can feed when the vineyard is too wet for equipment to operate, and they keep weeds fairly well grazed down, requiring less tillage.

If you are using herbicides, application timing will determine how well your weed control is likely to perform. Early applications are likely to lose their effectiveness due to biological and chemical degradation (yes, bacteria and fungi can use herbicides as a food source), and there might also be some leaching. A second application may be required for summer weeds. Applying herbicide later makes sense in higher rainfall years, but timing can be difficult. Ideally sprays should be made before the overwintering weeds bolt so that there is not a lot of dead plant residue beneath the vines. Best timing for these sprays would be late January or early February.

Canopy management and fungal disease
In general, high soil moisture early in the season will cause abundant vegetative growth. This can affect crop set during the current season and the next season if there is dense foliage around the flowers and clusters. Shading affects the next season’s crop when buds are differentiating on the basal buds of the new green shoots. Pre-bloom canopy management helps lessen this problem. Do it early if you can! Thinning sterile shoots on trunks and cordons will help. Tuck shoots when they are relatively short; cane damage is much more likely to happen with large shoots.

Pulling leaves around the shoots at flowering time can help set a smaller crop if it appears that large flowers are likely to form large clusters. (This can be risky: If the weather gets wet, you are likely to have a poor set anyway, so don’t do this unless you are sure bloom weather is going to be good. Wait until the last possible practical time to get the job done.) Similarly, if there is a lot of shoot growth and you are worrying about poor fruit set due to shatter, tipping the vines prior to bloom may help set a better crop. It will also cause lateral buds to break, making canopy-management more difficult due to lateral shoots. Mechanical hedging is the best way to maintain a canopy that starts to get out of control. You may need multiple passes in a year like this.

Powdery mildew tends to be a problem following a wet year as often spring weather conditions fall into the ideal temperatures for disease: 70° to 85° F. High humidity may also exacerbate fungal problems.

The foundation of a strong powdery mildew program is early season sprays with wettable sulfur or Stylet oil when vines have 3 to 6 inches of growth. This controls the overwintering form of the disease and the first infections. When done properly, it may represent 95% of a control program according to the UC Grape Pest Management book (UCANR Press Publication 3343). Follow-up applications can be made every 6 inches of growth or 14 days. If you are alternating between the two materials, wait at least three weeks, as phytotoxicity is likely when used too closely together. Never mix these materials together!

Pulling leaves after fruit set is also going to help reduce mildew in developing clusters, as lower humidity and sunlight exposure lessen disease development. Also, fungicides can reach the fruit with better coverage and more effective powdery mildew control.

Botrytis shoot tip and bunch rots often are a problem. Normally it isn’t very practical to spray for shoot tip rot control. It happens when conditions are very wet, so getting spray rigs into the vineyard isn’t easy. Vines aren’t seriously affected in most cases, but there will be more lateral shoot growth. You may also start a source of inoculum that will cause further infections if rains continue. Botrytis bunch rot can be a problem, with early infections happening at bloom. If you have had bunch rot problems in the past, consider a fungicide program that would include sprays during bloom and again before bunch closure. There are fungicides registered for bunch rot control for both conventional and organic/Biodynamic growing systems. If you are growing varieties with large berries and/or tight clusters, you might want to use fungicides as odds are cluster weights and berry sizes will be larger than usual, predisposing fruit to Botrytis infections. Leaf removal around the clusters after bloom will also be important.

Insects and mites
If weather is cool following bud break, shoots tend to grow slowly. Willamette mites and western flower thrips can become a problem, as they are feeding on the limited amount of green tissue that concentrates injury. Stylet oil can be used as both a fungicide and insecticide at this point, but be sure that you have a safe interval from any sulfur sprays. Once the weather warms up and shoots begin to grow, the damage will usually be insignificant. Many wineries do not want Stylet oil to be used on the fruit after set, as it makes the fruit look shiny and not normal. Also, they do not want wettable sulfur applied after fruit set, as they are worried about sulfur odors in the wine.

Large, lush canopies provide ideal conditions for leafhoppers. The thinner cuticles on leaves are easier to feed on and lay eggs into, and leafhopper populations build up quickly. The key to leafhopper control is to start early, before large populations become damaging. Often biological control is adequate, but you may need to spray if populations are building quickly. This is particularly the case with Virginia creeper leafhopper, for which biological control is only now becoming introduced into the North Coast. There are both organic and conventional insecticides available for controlling leafhoppers.

Yield and quality
Crop yields are often good if pollination happens without any mishaps (rain, mist or other conditions that might affect fruit set). In fact, fruit thinning might be needed for red varieties if there is a large crop load. Similarly, with white varieties, fruit thinning would be a good idea if you have “cluster on cluster” crop loads, as this will delay ripening and set up ideal conditions for fruit rot if the summer is cool, and fall rains come early. Thinning should be done around bunch closure for the best effect.

Practicing regulated deficit irrigation is highly advisable. Don’t apply water until the vines absolutely need it. You will already have vigorous vine growth, and you don’t want to promote more unnecessary growth. The amount of irrigation you need will probably be much less than normal. Watch shoot tips, or use other monitoring if available (soil moisture monitoring, pressure bombs, etc.)

Every season has challenges, and when we have a winter with abundant rain, plant disease during the growing season is likely to be a problem. Similarly, extra money often has to be spent on canopy management as the vines become extra vigorous. Watch for powdery mildew and botrytis bunch rot, as they can quickly ruin a crop. Finally, vegetation on the vineyard floor is going to grow longer and stronger. Keeping your vineyard looking neat will require extra effort. Good luck producing the 2017 vintage!
Glenn McGourty is the University of California Cooperative Extension winegrowing and plant science advisor for Lake and Mendocino counties. He tends a 1-acre vineyard of the aromatic Italian wine grape variety Arneis on his property along the Russian River near Ukiah, Calif.

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