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ProMED-mail



LITTLE CHERRY AND X DISEASES, STONE FRUIT - USA: (NORTHWEST)
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International Society for Infectious Diseases
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Date: Tue 10 Dec 2019
Source: Good Fruit Grower [abridged, edited]
https://www.goodfruit.com/x-disease-devastation-strikes-pacific-northwest/


Across the Pacific Northwest, the cherry industry is grappling with epidemic levels of X disease, caused by a phytoplasma pathogen that's commonly called Western X in the Northwest. It's spreading faster and farther than anyone expected: The Clean Plant Center Northwest reports the percentage of positive samples has more than doubled, and it's increasingly reported in North Central Washington too. This year's [2019] sample data showed even higher incidence than was expected: about 60% of the samples came back positive for X disease. That's up from 24% last year [2018].

Growers responded by increasing their scouting for the signs of infection that can only be seen near harvest, removing symptomatic trees and spraying religiously for the insect vectors, well into what used to be the off-season.

As for little cherry virus 2, the other primary pathogen that causes little cherry disease symptoms, it accounted for just 1% of the samples sent in, down from 14% last year [2018]. Most of those cases were from North Central Washington.

Washington State University's Scott Harper said, "Active removal efforts have gotten rid of a lot of inoculum," pushing the risk down. That's what the industry wants to see for X disease as soon as possible, but it's a little more complicated.

"Little cherry virus is stuck in cherry," Harper said, with no other hosts. The X disease phytoplasma can be found [also] in other stone fruit, ornamental trees, and even weeds, meaning there are a lot more reservoirs. In fact, peach orchards in Washington are seeing rates of infection similar to cherries, he said. Plus, the vectors hop around. "It's a landscape problem, not a crop problem," he added.

[Byline: Kate Prengaman, TJ Mullinax]

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[X disease
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Phytoplasmas of the 16SrIII taxonomic group cause X diseases (such as Western-X, Canada-X; _Candidatus_ Phytoplasma pruni) of stone fruit (_Prunus_ spp). Symptoms and host specificity differ for members of the group. X-disease symptoms in cherry (also called cherry buckskin) vary with the cultivar and rootstock combinations but are often similar to viral little cherry disease.

Phytoplasmas can be spread by sap-sucking insects (mostly leaf hoppers), by grafting, with contaminated tools (such as during pruning of trees), and with tissue cultures. They survive in asymptomatic crop hosts or non-crop reservoir species. Disease management of phytoplasmas is difficult and generally includes vector control, removal of infected plants and other sources of inoculum, as well as the use of certified clean planting and propagating material. Such measures require reliable molecular diagnostic methods and coordinated efforts on a regional scale, especially when dealing with long-lived tree crops.

Another member of 16SrIII has been shown to be associated with frog skin disease of cassava (for example see ProMED-mail post 20071130.3864).

Little cherry
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Little cherry disease (LCD) is a serious problem for sweet and sour cherry producers in parts of northern Europe and North America. It can be caused by 2 closteroviruses, _Little cherry virus 1_ (LChV-1, genus _Velarivirus_) and LChV-2 (genus _Ampelovirus_), in single or mixed infections. Symptoms vary with host cultivars but may include reduced size and quality of fruit, premature leaf reddening, stunting of seedlings, and some loss of tree vigour and yield. All commercial cherry cultivars are susceptible; ornamental flowering cherries can be asymptomatic reservoirs of LChV-1.

An insect vector for LChV-1 is not known. LChV-2 is transmitted by at least one species of mealy bugs, which can be spread by wind and with packing material in their nymph stages. Both viruses can be spread by both shoot and root grafting and with infected plant material. Disease management for LCD includes use of certified clean planting stock and budwood, elimination of inoculum (removal of infected trees), and vector control for LChV-2.

Maps
USA:
https://www.mapsofworld.com/usa/large-us-map.html
Individual states via:
http://www.mapsofworld.com/usa/usa-state-and-capital-map.html

Pictures
X-disease phytoplasma symptoms on cherry:
https://i.ytimg.com/vi/8G4SFWb-auU/maxresdefault.jpg and
http://treefruit.wsu.edu/wp-content/uploads/2017/02/Western-X_Ken-Eastwell.png
X-disease on peach leaves:
https://pnwhandbooks.org/sites/pnwhandbooks/files/plant/images/peach-prunus-persica-x-disease/056.jpg
X-diseases, photo gallery:
https://www.forestryimages.org/browse/subthumb.cfm?sub=11219
LCD symptoms:
https://www.agric.wa.gov.au/sites/gateway/files/Cherry%201_0.jpg,
http://treefruit.wsu.edu/wp-content/uploads/2016/01/little-cherry-disease-andrea-bixby-brosi.png,
http://www.bctfpg.ca/files/leaf(1).jpg, and
http://agriculture.vic.gov.au/__data/assets/image/0006/273867/LChV2_leaves_web.jpg
LCD affected cherry seedlings:
http://www.forestryimages.org/images/768x512/0656032.jpg
LCD photo gallery:
https://www.forestryimages.org/browse/subimages.cfm?sub=11205

Links
Information on X diseases of stone fruit:
https://www.cabi.org/isc/datasheet/39166,
http://www.padil.gov.au/pests-and-diseases/pest/pdf/136663,
https://pnwhandbooks.org/plantdisease/host-disease/cherry-prunus-spp-x-disease,
http://www.agr.gc.ca/eng/science-and-innovation/agricultural-practices/agroforestry/diseases-and-pests/western-x-disease/?id=1367501444568,
http://ipm.ucanr.edu/PMG/r105100111.html, and
https://www.ct.gov/caes/cwp/view.asp?a=2815&q=376862
X-disease phytoplasma group (16SrIII) taxonomy and species list:
https://www.uniprot.org/taxonomy/85623
Phytoplasma resource centre:
https://plantpathology.ba.ars.usda.gov/phytoplasma.html
Taxonomy and information for phytoplasma vectors (with pictures) via:
http://www.psyllids.org/index.htm
Information on little cherry disease and viruses:
http://www.plantwise.org/KnowledgeBank/Datasheet.aspx?dsid=16072,
https://www.agric.wa.gov.au/plant-biosecurity/little-cherry-disease-pest-data-sheet,
http://www.bctfpg.ca/pest_guide/info/128/, and
http://www.slideshare.net/treddout/little-cherry-virus-2
Virus taxonomy via:
https://talk.ictvonline.org/taxonomy/
Information on mealy bugs:
http://mrec.ifas.ufl.edu/LSO/Mealybugs.htm (with pictures)
- Mod.DHA]

See Also

Phytoplasmas, stone fruit - USA 20190224.6334529
2018
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Little cherry & X diseases, cherry - USA: (WA) 20181108.6132710
Cherry viruses - USA: spread 20180108.5545889
2014
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Little cherry disease - Australia (02): LChV-2, 1st rep (TS) 20140304.2309952
Little cherry disease - Australia: (TS) susp 20140205.2255550
Little cherry disease - USA: (WA) 20140127.2233739
2009
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Phytoplasmas, cherry - Lithuania: new pathogen (KS) 20090525.1948
2007
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Frogskin disease, cassava - Colombia: phytoplasmas 20071130.3864
2004
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Cherry virus A, little cherry virus-1 - Poland 20040822.2337
2002
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Little cherry virus, sweet cherry - Canada 20021201.5940
.................................................sb/dha/tw/dk