Comparison of Chilling Requirement in `Ta Tao' Budded Peach Trees by Shoot Forcing

Abstract

Peach trees exposed to `Ta Tao' vegetative chip bud grafts have been shown to have physiological changes that include bloom delay, delayed maturity, reduced shoot vigor, and early autumn defoliation. `Ta Tao' is known to be infected with the graft transmissible agent Peach Latent Mosaic Viroid (PLMVd). PLMVd has been associated with bloom delay and reduced shoot vigor. `Coronet' peach trees planted in a high-density, Y-trained orchard system were treated with vegetative chip buds from `Ta Tao'. Transmission of PLMVd was confirmed in the treated trees by cRNA probe hybridization. A shoot forcing study was set up to determine if exposure to `Ta Tao' would alter the chilling requirement of `Coronet' peach. Terminal fruiting shoots were collected periodically during the dormant season from 8 Jan. 1999 to 19 Feb. 1999, which represented a range of chill hour accumulation from 574 to 927, respectively. Shoots were forced in a greenhouse, and chilling requirement was considered complete when 50% of the flowers opened. Chilling requirement was not changed by exposure to `Ta Tao' chip buds. The number of days required for shoots to bloom was significantly and consistently longer for the `Ta Tao' treated trees. The number of shoots responding to forcing conditions was significantly less in the treated trees. The data suggest that the graft transmissible effects from `Ta Tao' buds increased the growing degree hours required for `Coronet' leaf and flower bud emergence after rest completion under greenhouse forcing conditions.