DR. JOSE RAMON URBEZ-TORRES 

Don’t be fooled if anybody tries to tell you they don’t have crown gall. Everywhere we plant grapes we can find this disease.

Allorhizobium vitis (A. vitis) (formerly Agrobacterium vitis) is a gall causing bacteria specific to grapes. The disease causes tumors or galls in the trunk of grape vines, as well as necrosis in the roots. The galls form above ground, often at the graft union, but can be higher up the trunk or even in the cordon. 

Crown gall is one of a few significant bacterial diseases in grapes, but it can be devastating. It is true that it is more prevalent in areas subject to freeze events, so this is a timely topic given the severe cold spells we have had in the last two winters here in the Okanagan. 

The A. vitis bacteria can be asymptomatic in the grape vine and depending on the amount of bacteria present in the plant, either within or on the surface of the vine, it will not cause a gall. But if there is a sufficient threshold of bacteria, and the vine sustains a wound, either through pruning, machine damage or cracking caused by freezing, the bacteria has a way into the plant. 

The wounded tissue releases a signal and the bacteria is attracted to those wounds.

This could be described as a ‘cancer-like process.’ Genetic information in the bacteria causes the plant to form the galls. 

The gall interferes with water and nutrient movement through the vine. It may not kill a mature vine but affected vines will have reduced vigour, delayed ripening and reduced sugar content (brix).

Young vines are particularly susceptible as they have smaller trunk diameter than mature vines. If the gall is able to girdle the young vine it will choke it off completely and it is likely to die within a year of being infected. We are seeing the most mortality in vines within five to six years after planting.

Leaves turning red may be the first indication of a problem while scouting your vineyard. I find there is a strong correlation between foliage symptoms and the presence of a gall so the next place you would look would be the graft union. 

The bacteria arrives at the vineyard inside plant material and unfortunately is not the subject of pre-screening. A. vitis is a non-regulated pathogen. It is already present in Canada and the Canadian Food Inspection Agency (CFIA) does not require testing or quarantining.

A primary source of introduction will be with symptomless plant material coming from the nursery. But root debris left over from removing vines in the vineyard may also harbour the bacteria. Studies have also shown A. vitis can remain in the soil for up to two years, so soil can be an inoculum source as well.  

Some current methods of testing are very time- consuming, but recently Drs. Tanja Vogel and Louise Nelson from the University of British Columbia (UBC) have used Droplet Digital PCR testing to identify as little as one or two A. vitis bacteria in grapes. This testing method is very, very, robust and it has shown us that the bacteria is not evenly distributed through the vine. 

This uneven distribution may be a key to propagating clean plant material. If we are able to detect clean tissue in parts of the vine, we can use tissue culture to propagate clean plants.

Unfortunately, there are no products registered in Canada for controlling crown gall in grapes. But there is hope for biological controls. The best research we have right now is actually with biological controls because it has been shown that they can work. 

Work is being done in Australia and Japan to commercialize biological products. We are also working with Dr. Vogel at UBC. We have identified local isolates here in B.C. that are as promising at inhibiting the growth of crown gall and may be more adapted to our local conditions.   

Without a means of control, the first line of defence is to maintain optimum vine health with adequate nutrition and water. Controlling fall vigour and giving vines adequate time to go into dormancy will make them more resilient to freeze events. Studies have shown that hilling vines can insulate the graft union to protect from freeze damage and practising multi trunking to allow for replacement and renewal of damaged trunks is also a strategy for freeze prone locations.

Right now, there are no sources of vine material that are certified A. vitis -free But I do believe that it is possible to develop clean vines and that will go a long way to controlling crown gall.