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04/01/2008 02:00:00 AM

Manage vineyards to reduce freeze-related grapevine injuries By Stephen Vasquez, Matthew Fidelibus and Peter Christensen
University of California

The cold, dry weather of the past two winters has increased crown gall infections in vineyards throughout the San Joaquin Valley. The bacterium Agrobacterium vitis lives in soil, and is systemic in the grapevine. It may form galls in vine tissues subjected to mechanical damage, especially freeze damage (Fig. 1).

Figure 1. Crown gall on young vines that experienced freeze damage. Photos courtesy of UC Regents

Galls on roots, trunks and cordons can disrupt the vine’s vascular tissues, and severe infections may result in yield reductions or vine death. The unusually low temperatures of December and January (2006-07), which occurred in the absence of precipitation, provided optimal conditions for crown gall. Many young grapevines were killed, whereas older vines lost canes, spurs, cordons or trunks. Permanent structures that were not killed outright displayed poor growth and galls (Fig. 2).

Local growers who notice galls on their vines should not panic. In the San Joaquin Valley, the galls usually dry up as warm weather develops. In such cases, the galls do not usually cause severe problems. However, the bacterium will remain within the vine, and in areas with optimal conditions, persistent galls may grow and girdle the vine over time.

Figure 2. Crown gall formation on Thompson seedless that was grafted to a new cultivar.

Normal low temperatures during the winter do not usually damage grapevines in the San Joaquin Valley. However, succulent green tissue is much more vulnerable to frost damage. Factors which determine the extent and severity of frost damage include shoot growth stage, the minimum temperature reached, and the duration of time that the tissues are at or below critical temperatures (Table 1).

A mild frost shortly after budbreak may only damage a few leaf cells, causing necrotic (brown to black) spots which will appear to be unevenly distributed throughout the leaf blade or shoot. If enough cells are damaged, the leaves will become distorted as they grow, and the entire leaf may die as the remaining shoot grows. As frost intensity increases, so will injury. It is not uncommon for moderate frosts to kill shoot tips and flower clusters on vines in low lying areas of the vineyard (Fig. 3).

Figure 3. Thompson seedless vineyard displaying significant frost damage. The vineyard middles were disked but were not packed and irrigated.

Severe frosts will kill entire shoots to the cane. When temperatures are low enough to kill whole shoots, the damage is often uniform throughout the vineyard. Shoots killed by frost will turn a dark brown to black color within a few days of freezing.

Unusually warm weather in January and February will advance budbreak and expose more green tissue to frosts which may occur in March and April. Thus, early budbreak can increase the risk of frost damage. Budbreak in Fresno County normally occurs on March 15 (+/- 7 days) for Thompson seedless, which is about when budbreak was noted in 2008. Since then we have had some exceptionally warm, pleasant weather. 

Such weather can give growers a false impression that the risk of frost has passed but early spring weather patterns frequently shift between highs and lows with devastating results. Growers should follow the weather forecast closely for extremes between highs and lows. In general, as growth progresses through spring, grapevines become more susceptible to frost but, fortunately, the likelihood of a severe frost decreases as the season progresses.

On clear nights when frost forms, the coldest air is found near the ground. As the night progresses, a cool layer of air continues to build and the shoots nearest the ground are the first to experience damage. Grapevines trained at one height are more susceptible than those trained at 12-15 inches higher.

For example, a Thompson seedless vineyard grown to a height of three feet will be more susceptible to frost than one trained on a open gable or overhead trellis with a head height of 48-54 inches. Previous research has shown that a temperature of 28ees Fahrenheit 1 foot above the soil can be 2 degrees warmer at 3 feet above ground. At 20 to 25 feet above the soil line the temperature will gain another 2 degrees. At some point the temperature will again begin to decrease, forming an inversion layer. The greater the temperature change between the soil line and 25 to 30 feet above ground level the greater the chance of avoiding frost damage.

When frost has been predicted, growers should take note of their vineyard’s growth stage so a strategy can be developed. Vineyards that have not yet begun to break may not require special attention. When green shoots are longer than six inches, soils may need to be prepped in advance of cold weather.  Table 1 shows the relative susceptibility of grapevine tissue at different growth stages and critical temperatures.

Table 1. Frost damage to various growth stages of grape.

Growth Stage Critical Temperature*
Buds

25-27˚F

Buds with wool (eraser stage) < 26˚F
Budbreak < 30˚F
Shoots < 6” in length < 31˚F
Shoots > 6” in length < 32˚F

*Critical temperatures are based on research under controlled environments. Vineyard characteristics (location, cultivar, etc.) may increase or decrease susceptibility to frost damage. These values should only be used as a point of reference when developing a frost protection program.

Soil Characteristics

In order to minimize damage caused by frost, vineyard soils should be prepared for maximum heat absorption during the day and release at night. Optimal conditions include soils that are free of vegetation, firm in texture, and moist. Moist dark soils improve their ability to absorb heat during the day and radiate it at night as ambient temperatures drop. Soil texture will also have an impact on heat absorption.

Vineyards planted to sandy soils are more prone to frost damage because they lack the ability to retain water. Additional water may be needed if winter precipitation has not been adequate to maintain soil moisture. Prior to a predicted frost, the goal should be a uniformly distribute irrigation, that allows for maximum heat absorption.

Table 2 shows the benefits of a bare, firm moist soil in contrast to the least affective frost conditions.

Table 2. Comparisons between optimal soil conditions for frost.

Soil Characteristics Vegetation Temperature benefit
Bare, firm, moist None Warmest Optimal
Moist Shredded cover crop 0.5 ˚F
Moist Low growing cover crop 1-3 ˚F colder
Dry, firm Freshly disked 2 ˚F colder
Dry to moist High cover crop 2 ˚F colder Least optimal

Soils that have been recently cultivated or disked need to be irrigated soon after the soil has been disturbed. Cultivated soils do not retain heat well because they are dry and have numerous air pockets. Native vegetation or cover crops that insulate soils from absorbing heat should be mowed or disked and followed by irrigation or significant precipitation.

Irrigating during a frost event can be beneficial. On nights that low temperatures are expected pumps should be turned on early enough that the entire vineyard is covered. Depending on size it may only be feasible to irrigate the most susceptible areas (low lying) where cold air tends to drain. Doing so will improve the chances of protection.

Vineyards that are drip irrigated should not have their row middles cultivated. Drip irrigation should be turned on to wet as much soil as possible. Growers will have to be especially vigilant to the weather forecast in order to start irrigating well in advance of the frost event. Cover crops or native vegetation should be mowed prior to budbreak and often as is necessary after the vines awaken. Row middles should not be cultivated unless a significant rain event has been predicted. Doing so could result in significant losses if frost should appear. It only takes a single frost event (one night at freezing or below) to experience a complete loss.

Stephen Vasquez is the UC Cooperative Extension viticulture adviser in Fresno County. Matthew Fidelibus and Peter Christensen are UC Cooperative Extension viticulture specialist and viticulture specialist, emeritus, respectively.

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