Distribution of Agrobacterium vitis in Grapevines and Its Relevance to Pathogen Elimination

Health Status of Ready-to-Plant Grapevine Nursery Material in Canada Regarding Crown Gall and Description of the First Allorhizobium vitis Strain OP-G1 Isolated from British Columbia

Crown gall disease of grapevines caused by Allorhizobium vitis causes significant damage to vineyards in cold-climate viticulture areas such as Canada and the northern United States. Introduction of the disease into vineyards occurs mainly through planting of infected but asymptomatic nursery material. Because A. vitis is not a regulated pest for import into Canada, no information on the health status of nursery material destined for import into Canada has previously been collected. This study evaluated the health status of ready-to-plant nursery material from domestic and international nurseries in regard to crown gall by determining the abundance of A. vitis in different plant sections via Droplet Digital PCR technology. In addition, different rootstocks from one nursery were compared. Results showed that A. vitis was present in planting material from all nurseries tested. The bacteria were nonuniformly distributed in dormant nursery material, and there was no difference in abundance between the rootstocks tested. In addition, the first A. vitis strain OP-G1 isolated from galls in British Columbia is described. Results showed that a minimum of 5,000 bacterial OP-G1 cells were needed for symptom expression, suggesting that the initiation of symptom development is not based on presence of bacteria in nursery material alone; a minimum threshold is needed, and environmental conditions need to be met.

Development of a DNA-Based Real-Time PCR Assay To Quantify Allorhizobium vitis Over Time in Grapevine (Vitis vinifera L.) Plantlets

Allorhizobium vitis is the primary causal pathogen of grapevine crown gall disease. Because this endophytic bacterium can survive as a systemic latent (symptomless) infection in grapevine, detecting and monitoring its development in planta is of great importance. In plant bacteria studies, plate counting is routinely used as a simple and reliable method to evaluate the bacterial population level in planta. However, isolation techniques are time-consuming and present some disadvantages such as the risk of contamination and the need for fresh samples for research. In this study, we developed a DNA-based real-time PCR assay that can replace the classical method to monitor the development of Allorhizobium vitis in grapevine plantlets. Primers targeting Allorhizobium vitis chromosomic genes and the virulent tumor-inducing plasmid were validated. The proposed quantitative real-time PCR technique is highly reliable and reproducible to assess Allorhizobium vitis numeration at the earliest stage of infection until tumor development in grapevine plantlets. Moreover, this low-cost technique provides rapid and robust in planta quantification of the pathogen and is suitable for fundamental research to monitor bacterial development over time.

The Ecology of Agrobacterium vitis and Management of Crown Gall Disease in Vineyards

Agrobacterium vitis is the primary causal agent of grapevine crown gall worldwide. Symptoms of grapevine crown gall disease include tumor formation on the aerial plant parts, whereas both tumorigenic and nontumorigenic strains of A. vitis cause root necrosis. Genetic and genomic analyses indicated that A. vitis is distinguishable from the members of the Agrobacterium genus and its transfer to the genus Allorhizobium was suggested. A. vitis is genetically diverse, with respect to both chromosomal and plasmid DNA. Its pathogenicity is mainly determined by a large conjugal tumor-inducing (Ti) plasmid characterized by a mosaic structure with conserved and variable regions. Traditionally, A. vitis Ti plasmids and host strains were differentiated into octopine/cucumopine, nopaline, and vitopine groups, based on opine markers. However, tumorigenic and nontumorigenic strains of A. vitis may carry other ecologically important plasmids, such as tartrate- and opine-catabolic plasmids. A. vitis colonizes vines endophytically. It is also able to survive epiphytically on grapevine plants and is detected in soil exclusively in association with grapevine plants. Because A. vitis persists systemically in symptomless grapevine plants, it can be efficiently disseminated to distant geographical areas via international trade of propagation material. The use of healthy planting material in areas with no history of the crown gall represents the crucial measure of disease management. Moreover, biological control and production of resistant grape varieties are encouraging as future control measures.

Quantification of Agrobacterium vitis from Grapevine Nursery Stock and Vineyard Soil using Droplet Digital PCR

Current detection methodologies for Agrobacterium vitis, causing crown gall of grapevines, are time intensive and lack the ability to quantify pathogen abundance in nursery stock and soil. Information on pathogen abundance is a key component to develop management strategies. The aim of this study was to develop a rapid and sensitive quantification assay for grapevine nursery stock and vineyard soil via droplet digital polymerase chain reaction targeting the virA gene. DNA isolated from roots of dormant grapevines originating from nurseries in Germany, California, and Ontario were tested for virA abundance. Bacterial numbers varied with grapevine origin; plants from California had the highest numbers. In addition, rhizosphere soil from two vineyards in the Okanagan valley in British Columbia was tested over a growing season. Sampling time during the season did not affect virA gene abundance. The older vineyard had higher soil A. vitis populations than the younger vineyard. The assay developed here has potential for use in national clean plant programs to prevent import of infected grapevine nursery stock and to test vineyard soil for abundance of the pathogen before planting.

The Impacts of Tumorigenic and Nontumorigenic Agrobacterium vitis Strains on Graft Strength and Growth of Grapevines

The effects of tumorigenic and nontumorigenic strains of Agrobacterium vitis on graft strength and growth of grapevines was studied. A procedure was developed for inoculating graft interface surfaces with A. vitis and for measuring the force required to break grafts at different time points. Cuttings were soaked in an aqueous suspension of bacteria, about 10⁶ CFU/ml, and bacteria were spread onto the graft interface during the grafting procedure. Tumorigenic strain CG49 caused reduced bud germination and increased callus (crown gall) at the graft union and at the base of cuttings at 30 days postinoculation (dpi) and significantly reduced shoot growth by 60 dpi whereas, at the same time points, nontumorigenic strain F2/5 inhibited callus formation but did not affect bud germination or shoot growth. Graft strength was enhanced at 30 dpi with CG49, presumably because the crown gall callus served to secure the union; graft strength was weakened by F2/5 over the same period. Between 30 and 60 dpi, the greatest increase in graft strength was observed in the water control. Following graft union inoculations, the A. vitis population increased more than 1,000-fold within 5 days.