Peach Latent Mosaic Viroid (PLMVd) Therapy

W. E. Howell, L. J. Skrzeczkowski, J. Burgess, K. C. Eastwell,
NRSP-5/IR-2 and Washington State University, Prosser, Washington
E-mail: wehowell@wsu.edu

OBJECTIVE: Obtain PLMVd-free trees by shoot-tip propagation from infected trees grown at 4 to 10C.

PROCEDURES: Buds from 48 PLMVd-infected peach clones were chip-budded in the greenhouse to one-year old peach rootstocks, allowed to grow for 6 weeks and then move to a refrigerated room maintained at 4 to 8C. The plants were sustained by photoperiods of 16 hours at 750-foot candles throughout the experiment's duration. After 110 and 273 days under these growing conditions, 5mm shoot tips from these plants were excised and grafted to l year-old seedling trees. Additional trees were propagated and subjected to standard virus therapy conditions of heat treatment at 38C for up to 2 months. Shoot tips from these heat-treated plants were grafted to seedlings to serve as a standard therapy control. Trees were assayed for the presence of PLMVd by tissue print hybridization.

RESULTS: None of over 51 trees of 38 cultivars obtained by shoot tip propagation from PLMVd-infected trees grown at 4C for 110 days were free of PLMVd (Table 1). Extending exposure to cold growing conditions for another 163 days did not affect the results (tips from sixty-two plants of 48 cultivars propagated at this time tested positive for PLMVd). The control treatment for this experiment was the standard therapy of heat treatment at 38C followed by shoot tip propagation. PLMVd was successfully eliminated from most varieties by this method. It appears obvious that, despite general opinion within the scientific community that viroids in general prefer warmer temperatures, these results suggest therapy to remove PLMVd from peach and nectarine clones should be conducted at 38 rather than 4C. Interestingly, the data showed that time of exposure to heat treatment conditions was critical to the success of the therapy. Almost all shoot tips grafted after heat treatment for 29 days or more escaped the viroid. Most varieties were freed from PLMVd by this technique. A few were not. These were the clones too sensitive to survive the high temperature conditions for more than 29 days.

CONCLUSION: Cold therapy for PLMVd as conducted in this experiment did not work. Fortunately, through this research we established standards for obtaining PLMVd-free trees using heat treatment and shoot tip propagation. Improved therapy conditions are needed for cultivars too sensitive to survive heat treatment.

This project was originally proposed as a two-year study. However, we suggest the project be terminated because the preliminary evidence for cold therapy is convincingly negative. Based on the results we have established treatment standards for obtaining PLMVd–free plants (heat therapy for 30 or more days). In our laboratory this protocol is more applicable to a large range of cultivars than the in vitro micro-grafting technique described by Barba (1995).