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).