Crown Gall of Grapes Following Winter Injury

Winter injury was very obvious throughout the summer in Ontario vineyards. Some vines that sustained "minimal" winter damage leafed out as normal and appeared to be fine, however, through the spring and summer, some of these vines "collapsed". The vines overcame the damage early in the year when soil moisture was plentiful and temperatures were moderate but as the vines began to experience the normal stresses of summer, their damage became evident. Their leaves and tender shoots wilted and then part or all of the vine died. It took a few days or weeks for the symptoms to clearly develop throughout the vine. Damage was cultivar dependent but also highly affected by microclimate and even minor differences in elevation.

Crown Gall

Crown gall disease occurs worldwide in grapevine and is a serious problem since it is difficult to control. It is largely disseminated in grapevine propagative material, and unfortunately at this time there is no program for certifying that grape nursery stock has been tested for freedom from the pathogen. The disease is most severe in temperate climate areas where winter injury is common. Studies have shown that Vitis labrusca cultivars generally have less crown gall than interspecific hybrids or V. vinifera. When wound conditions occur, which in cool climate growing areas is often the result of cold injury, infection will occur in contaminated nursery stock and galls develop.

The causal agent, Agrobacterium vitis, is an opportunistic pathogen that takes advantage of freeze-damaged cells. A. vitis can survive systemically in the vascular system of apparently healthy vines for many years until some cell damage occurs. The bacterium then invades plant cells associated with the plant’s natural wound-healing process. Callus cells are generated in the presence of auxin, a plant hormone, when it is transported from growing shoots to injured cells in early summer. When these callus cells are infected by A. vitis, their normal process of regenerating functional vascular tissue at the wound sites is subverted and instead, rapid, localized, disorganized cell growth occurs. These cells are stimulated by the bacterium to increase their production of growth hormones, resulting in the formation of galls (Figure 1). In early summer these galls are smooth and cream coloured, but with age become dark, hardened, brittle and corky. In southern Ontario, typically galls are seen at and above the graft union to the first wire (Figure 1). Crown gall formation from last winter’s damage may take several years to appear. In the nursery, crown gall is typically seen at the graft union, and can in the early stages of infection be confused with callusing (Figure 2).

Figure 1: Crown gall formation on grape vine

Figure 2: In southern Ontario, typically galls are seen at and above the graft union to the first wire

The effect of crown gall on vine growth, longevity, and production will depend on the severity of the galling. Galls may girdle the trunk preventing flow of nutrients to the vine. Severely galled vines are usually severely stressed and are more susceptible to winter kill. If the vine is clearly dead or it is obvious that it will not recover, then it should be removed. If possible, new trunks can be brought up to replace those that are affected. In some cases, galls may affect only a portion of the cambium and the vine will continue to bear fruit. However, once a vine is infected, galling will likely continue to occur in subsequent years reducing the vigor of the infected vine.

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Since the development of crown gall is closely correlated with the occurrence of freeze injury, management practices that reduce injury are most useful in managing the disease. Replanting with disease-free, crown gall resistant varieties is important. Cold susceptible varieties should not be planted into marginal sites. Low areas in the field where frost pockets and poor soil drainage may occur are typically where crown gall expression is first apparent. Hilling of vines with soil above the crown will help protect new shoots from winter injury that may be needed for trunk renewal.

The bacterium can remain in decaying root pieces in the soil for several years. This can be a source of inoculum for reinfection of new vines, although the significance of soil as a source of primary inoculum needs further study. It is important when removing infected vines to try to remove as much of the root material as possible to lower the chances of re-infection of replanted vines. Since A. vitis survives only on grape, planting disease-free stock into sites that have not been planted to grapes within the last 10 years should keep the vines free of the disease.

Since the pathogen survives systemically in vines, it is easily spread in propagation material. Hot water treatment of vines before planting is an effective method to reduce the amount of crown gall bacterium in grape vines. It will kill much of any bacteria present in the vines but it is not a definitive guarantee of disease-free stock. However, this is best left to nurseries experienced with the techniques involved. Cultivars vary with respect to heat sensitivity and cuttings also vary in heat sensitivity depending on when they are treated during the dormant season.

Chemical treatment such as copper sprays, topical applications of Gallex or kerosene may be effective in killing gall tissues, but new galls frequently develop at the treated sites in subsequent years. In other words, there are no effective chemical treatments for this disease.

Significant new research on the use of biological control to protect vines from crown gall disease is in progress in several laboratories and looks very promising.

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