Grapevine crown gall: an old, emerging disease

Crown gall is considered one of the most important and widespread bacterial diseases of grapevine (Vitis vinifera L.) throughout the world. It is known in Europe for more than 150 years and can be still of great phytopathologic significance in the vineyards and nurseries, especially in cold-climate regions. The disease is predominantly caused by tumorigenic strains of Agrobacterium vitis, more rarely by tumorigenic A. tumefaciens and A. rhizogenes. Unlike A. tumefaciens and A. rhizogenes, that are broad-host-range pathogens, A. vitis is specific to grapevine. Crown gall reduces vigor and yield of grapevines and severe disease may cause partial or complete death of infected plants. High losses occur in nurseries where different graft combinations with visible symptoms are unmarketable and must be discarded. Typical symptoms of crown gall are tissue proliferation (tumors) formed mostly on the lower areas of the trunk and on aerial canes. Tumorigenic and nontumorigenic strains of A. vitis are also able to cause specific root decay and it has been hypothesized that both types may be factors involved in the “replant disease” syndrome. Wounds mainly caused by freezing temperatures or grafting serve as a crucial entry points for the pathogen and its complex infection process. During the infection process DNA fragment from the bacterial tumor inducing (Ti) plasmid is transferred and integrated into the plant genome (interkingdom gene transfer). This leads to the overproduction of the phytohormones auxin and cytokinin, resulting in an uncontrolled proliferation of plant cells and tumor formation. A. vitis is unevenly distributed within systemically infected grapevines and able to survive in vineyard soil, particularly in the vicinity of infected plants and in their debris. Another important aspect is the ability of the pathogen to be latently present within the grapevine, providing an important means of spread over short and long distances by asymptomatic propagation material. Management of grapevine crown gall is not easy considering that no effective chemical control measures are available. However, production of A. vitis-free grapevines is an essential prerequisite for an effective prevention of the disease, and great efforts should be done in this direction. For this reason, shoot tip propagation of grapevine and thermotherapy are available as control measures. Planting of crown gall and cold-resistant cultivars and rootstocks would be a good practice when establishing new vineyards. Biological control of crown gall is another promising approach in the control of the disease and several antagonistic bacterial strains have shown a certain level of efficiency in preventing tumor formation. Indexing of grapevines for the endophytic presence of A. vitis is a very important preventive measure. Differentiation and identification of tumorigenic strains can be rapidly assessed by PCR using primer combinations specific for bacterial Ti plasmid and chromosomal genes. However, a high level of genetic diversity among Agrobacterium strains may limit the efficiency of PCR. In our studies virC, virD, virF, pehA and 23S rRNA gene-specific primers (Bini et al., Vitis 47:181, 2008; Pulawska et al., Syst. Appl. Microbiol. 29:470, 2006; Suzaki et al., J. Gen. Plant Pathol. 70:342, 2004; Szegedi and Bottka, Vitis 41:37, 2002) were reliable in routine detection and identification of a broad range of Agrobacterium strains occurring in grapevine. However, there is necessity for development and standardization of indexing procedures including protocols of analysis and sampling methods. In the EU and many other European countries, A. vitis is not listed as a quarantine pathogen and is considered as a “quality organism” which significantly reduces the value of propagation material. Therefore, the importance of proper phytosanitary measures in grapevine nurseries and on commercial lots should be emphasized.