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Little Cherry Virus and Western X Disease: Q&A with Scott Harper

Written by Bernardita Sallato and Scott Harper, WSU, February 2019

Scott Harper, WSU Professor and Director of the Clean Plant Center in Prosser, alerted growers and stakeholders about Little Cherry Virus LChV and Western X Disease at the 2019 Cherry Institute Meeting. Here are most frequent questions and answers.

 

  • Does the LChV or Western X affects other crops?

Western X phytoplasma can infect cherries, peaches and other stone fruit, as well as chokecherry, and some weeds. As far as we know, Little cherry virus-2 only infects sweet and sour cherries.

  • How long can the LChV and Western X be in the tree before it shows symptoms? 

It can take a full growing season (~1 year) for symptoms to appear after infection for both Western X and LChV2.

  • Can the LChV and Western X be in my tree without showing symptoms? Can it be detected?

Yes it can, this is common early on in the infection cycle (< 1 year). Also, both Western X and Little cherry virus-2 can cause symptomless infections in certain cultivars and can therefore only be detected by molecular assays. The problem is that these symptomless ‘carriers’ are sources of spread to nearby trees, orchards, etc.

  • Can LChV and Western X be transferred by pollen?

No, neither pathogen can. Spread is by grafting infected material and by insect vectors (mealybugs and leafhoppers respectively).

  • Can any of them be spread by pruning in the winter?

No, the titer is too low during winter, and mechanical transmission, while not impossible, is unlikely to be an effective means of spread.

  • Can the virus be root grafted to the neighbor trees? What are the recommendations?

Yes, both Western X and Little cherry virus-2 can be spread by root grafting from tree to tree. Stone fruit are prone to forming root grafts, therefore when removing trees I suggest painting the stumps with herbicide and looking for herbicide injury on neighboring trees – it’ll show you which trees are connected.

  • Can the infected trees be treated with something? Fungicide? Pesticide?

No. There are no compounds effective against the causal agents of little cherry disease. Once a tree is infected, it is infected for life.

  • The removal of the tree is enough to get rid of the virus?

Neither Little cherry virus-2 or Western X can survive outside of living host tissue. But, when removing trees keep in mind that you may be leaving broken roots behind, which can graft to new trees or send up suckers the following year. I suggest fumigation after removal, and/or treating suckers with herbicide.

  • How long can the virus stay in the roots after removing the tree? What are the recommendations after removing the tree?

As long as the tissue is alive. I suggest fumigation after removal to kill live broken roots, and/or treating any suckers that appear with herbicide.

  • Are there any differences between cultivar or rootstocks susceptibility?

Little cherry virus-2 seems to affect darker cultivars more severely, while Western X affects most commercial cultivars. Some pollenators, such as Black Republican, may be symptomless. Rootstock selection, with the exception of Mahaleb, which is sensitive to Western X, doesn’t have a great effect of disease progression, though I have noticed that trees on Mazzard perform better when infected with Western X than those on Gisela rootstocks.

  • Can the vigor affect the susceptibility of the tree?

A vigorously growing tree may take longer to show symptoms; we have seen this when comparing trees on Mazzard vs. Gisela, but long term, the disease will progress regardless of vigor.

  • Are there any tools to evaluate it in the field? What is the cost to evaluate?

There is a field detection kit for Little cherry virus-2 available from Agdia.

  • Can cherries with LChV or Western X be marketed?

Probably not, not only do the diseases affect the size and color, but also the taste.

  • Other ways to prevent the appearance of the LChV and Western X in my orchard?
  1. Regular sprays to suppress vector numbers, and/or deter them from feeding.
  2. Plant/Replant using stock that has been tested for Little cherry virus-2 and Western X – you don’t want to introduce the pathogens into your orchards.
  3. Ensure that you and your neighbors are doing both of the above, and scout for diseased trees – the earlier you find and remove them, the better.

For more information, visit http://cpcnw.wsu.edu/

Contact

Dr. Scott Harper

24106 N. Bunn Rd.
Prosser, WA 99350

509-786-9242

cpcnw@wsu.edu

Washington State University