Effects of Thermotherapy Treatments on Systemic Agrobacterium vitis in Dormant Grape Cuttings

Agrobacterium tumefaciens biovar 3 (A. vitis) was eradicated or reduced below the level of detection in dormant cuttings of grape [Vitis vinifera cultivar (cv.) Thompson seedless] and rootstock NAZ3 (V. vinifera · V. rupestris) by hot-water treatment (exposure to 50 C for 30 min). Strains of A. vitis varied in their sensitivity to heat, but were generally more sensitive than strains of A. tumefaciens biovar 1 or biovar 2. Populations of about 103 CFU/ml in broth were killed by a 30-min treatment. Biovar 1 strains were apparently unaffected by 50 C, even when exposed for 30 min. Non-tumorigenic biovar 1 strains were recovered from hot-water-treated cuttings. An assessment was also made of the effect of treatment on growth parameters of cuttings in a field nursery after 9 months. The effect of hot-water treatment on the vitality and growth of vines varied with different cultivars or rootstocks. The number and length of canes, root dry weight and length and diameter of trunks, increased in most instances. Treatment and time usually did affect bud survival and, in most cases, increased the level of callus formation at the base of the cuttings. Hot-water treatment may offer a simple, effective, economical and environmentally safe means of eradicating A. vitis from dormant grape cuttings