Effect of Heat Treatments on Grape Bud Mortality and Survival of Agrobacterium vitis in vitro and in Dormant Grape Cuttings

No differences were observed in the heat sensitivity of four strains of Agrobacterium vitis , the cause of grape crown gall. At 50°C, the calculated D values of cells that were grown to stationary growth phase in LB broth were about 2.5 (time in minutes required for population to be reduced by ninety percent). The effect of heat treatments on survival of A. vitis in dormant cuttings was also measured. A. vitis -infected dormant grape cuttings (containing 5.0 x 10 2 to 2.7 x 10 4 cfu per unit of sampled tissue) were submersed in a water bath at 50°C for 30 or 60 minutes, 30°C for two hours followed by 50°C for 90 minutes, or at 55°C for 30 minutes. Treatments significantly reduced the numbers of cuttings with detectable A. vitis ; however, none of the treatments totally eradicated the pathogen. Therefore, A. vitis cells surviving in cuttings appear to be more heat-tolerant than cells grown in vitro . Using heat treatment to reduce systemic A. vitis in cuttings may be a good strategy for managing crown gall in vineyards. The 50°C for 60 minutes treatment always resulted in improved bud germination as compared to untreated buds. The effect of treatments of 50°C for 90 minutes and 55°C for 30 minutes on bud germination was variable depending on cultivar.