Written by Tianna DuPont, Cody Molnar WSU Extension. updated Dec 2020.
Sampling and removal of infected trees is critical to slowing the current outbreak of Western X and Little Cherry Virus. Infected trees can not be cured. The disease is spread throughout the tree even when symptoms are obvious only in one section. Infected trees must be removed. Any infected trees that remain in the orchard are a reservoir of disease for your orchard and your neighbor’s.
Little Cherry Disease is caused by Little Cherry Virus 1 and 2 and X-disease is caused by X-disease phytoplasma. In addition to insect vectors, root grafting can move the diseases from tree to tree and threaten new infections in replacement trees.
Before removing infected trees treat with insecticide so that leafhopper/ mealy bug vectors do not move to uninfected trees.
It is recommended to apply glyphosate herbicide to trees at the time of tree removal or test adjoining trees to detect latent infections early. Herbicide application can also help kill roots of infected trees so that they can no longer infect new roots via root grafting.
Treat the interface of the bark with the wood, the area called the cambium or sapwood (the outer ring of wood, next to and including the bark) (Figure 1). This is the living part of the tree which moves water and nutrients and will be most likely to move the herbicide. Treat when the tree is actively growing. Herbicide must be applied immediately after cutting.
Multiple glyphosate labels allow application in stone fruit and as a ‘cut stump treatment’ (eg Glystar pg 29, Buccaneer pg 24). Check your glyphosate label. Most labels say to apply a 50 (mixed with water) to 100% solution of the glyphosate product. Applications can be made with a small hand/backsprayer or a paint brush to fully cover the entire cambium (Figure 2). Some labels also allow a frill/injection application (eg Glyphosate 4DS pg 28) (Figure 3). A hatchet, chainsaw or drill can be used to notch the tree.
Tree Removal Trials
In 2019 and 2020 four replicated trials were conducted to identify rates, timing, method and efficacy of herbicide application when used for tree removal of X-disease or Little Cherry Virus infected trees. Results from these initial trials found that:
August and May applications can result in root death. September timings did not result in root death in the year of application.
Frill applications performed best across trials. Cut stump applications were slightly less effective.
Glyphosate may need to be more than 1 oz per tree (assuming 30 in tree circumference).
Herbicide applications during tree removal resulted in tree injury of adjoining trees in Mazzard rootstocks with a large rooting area but not smaller Gisela 12. Herbicide applications are most important on large (Mazzard) type rootstocks.