Written by Tom Auvil, Tree Fruit Research Commission
A recent article authored by Marc Fuchs of Cornell University published in the summer issue of the New York Fruit Quarterly affirmed the concerns that some popular new scion varieties may have virus(s) that impairs tree performance and may induce tree death when budded onto G.935. Growers and nurseries are encouraged to exercise caution when choosing scions to graft/bud onto G.935. One suggestion is to use budwood that has done very well on G.935 over time or ‘certified virus tested’ (CVT) material which was tested by more rigorous testing than by ELISA methodology. Symptoms of the recent problem with virus infected scions on G.935 include good tree growth in the nursery, sub-par growth in year one in the orchard, but then trees have obvious decline or collapse in the second growing season in the orchard.
There are several apple viruses that may be unobserved or latent until weather stresses the trees and/or a susceptible scion or rootstock is grafted on to the diseased scion or rootstock.
G.935 has been tested with observable viruses such as apple mosaic and unknown viruses that killed other Geneva rootstocks in a trials in Vantage and Brewster with no loss of vigor or fruit quality. More sensitive are G.16, which succumbs quickly to nearly any virus and G.814 which grows quickly into a full canopy then enters into a sharp decline in the 3rd and 4th season. G.814 has done well with CVT scion wood.
Viruses may be unnoticeable and latent in CVT rootstocks or scions if the certification program only uses a few ELISA tests, which are inexpensive but not 100% accurate. ELISA typically seeks only one virus per test. A combination of viruses can accumulate unnoticed by budding multiple generations of trees in the variety evaluation process. Then, if a susceptible rootstock or scion is budded/grafted with the contaminated wood, symptoms become noticeable.
There is a deep virus DNA sequencing technique which can identify a broader range of virus diseases in apple. The industry will be able to avoid virus related problems as more sources of rootstocks and scions receive deep sequence virus screening for viruses and cleanup to obtain robust CVT status. European trials comparing productivity of CVT trees compared to virus infected trees to have sustained yields 15% above the ‘standard’ non-certified trees.
It is important to consider that this problem is a rootstock sensitivity problem related to latent viruses that have not been a visible problem in fruit production unless budded onto sensitive rootstocks such as G.16, G.814 or G.935.