Written Tobin Northfield, WSU Entomology; Scott Harper, WSU Plant Pathology; Bernardita Sallato, WSU Extension; Tianna DuPont, WSU Extension; Holly Ferguson, Yakima Valley College. July 17, 2019. New questions and answers added February 5, 2020.
As we update growers on the status and management of X Phytoplasma and Little Cherry Virus many questions have come up. Below are the answers to common questions.
X Phytoplasma (Western X) is one of two pathogens that cause little cherry disease, a problem of critical concern in Washington. Western X is at epidemic levels in the Columbia Basin with high incidence from Yakima, Benton and Franklin counties and present in Oregon around Hood River and in Chelan and Douglas Counties. Little Cherry Disease has devastated the industry in Canada and California in the past and it is critical that growers act now to aggressively manage Little Cherry Disease. After harvest is the time to sample for infected trees and remove trees that test positive. Manage vectors postharvest to help limit the spread of the pathogen.
Management
What is the most important thing I can do to limit the spread of Western X?
Remove infected trees! Do not just remove the parts of the tree with symptoms. The disease is throughout the tree and cutting a single branch will not help. Scout your orchards. Flag and sample suspect trees. It is paramount that infected trees are removed (and roots killed) because they are a source for the disease which will continue to spread from there. See https://treefruit.wsu.edu/crop-protection/disease-management/western-x/ for sampling and testing directions. In an early study, orchards where infected trees were removed as soon as they were observed, the disease incidence remained below 2% and decreased over time.
In California, in an abatement program between 1986 and 1990 found that tree removal was the most important factor for reducing new Western X infections and post-harvest insecticide applications only contributed a minor amount in suppression of new infections (1). They were using products with long residuals (14 to 42 days). Orchards in the abatement program were able to bring the percent of trees infected down from 1.7% to 0.3%. However, the leafhopper present in this study C. montanus is thought to move phytoplasma into orchards and not be very effective in dispersing it tree to tree.
What else can I do to limit the spread of Western X?
Monitor for leafhopper vectors and manage populations season long. Target late and early season. Leafhoppers may be present as late as November so the post-harvest period is critical.
Plant with pathogen-free planting sources and consider management of alternative hosts. See https://treefruit.wsu.edu/crop-protection/disease-management/western-x/
How do I remove infected trees?
Infected trees spread the pathogen to neighboring trees by insect vectors or via root-grafting from tree to tree. Treat for leafhoppers immediately before cutting. Leafhoppers are more likely to fly when it is warm, so treating early in the day when it is cool will reduce leafhoppers spreading from the tree while it is being treated. Treat stumps with herbicide immediately after cutting to ensure roots are dead. Generally, some trees immediately adjacent will also be affected because they were root grafted to the infected tree. These trees though they might not be symptomatic were likely infected via root grafting. Remember it takes 2 to 4 years for symptoms to become visible.
Generally, a concentrate of glyphosate is used. After cutting paint the concentrate on the stump particularly on the cambium layer (just under the bark). Don’t wait more than a few minutes after stumping to apply the glyphosate or the tree may not take it up into the cambium.
What is the best time of year to remove trees?
Removal is best as soon as you know the trees are infected. As long as infected trees remain in the field they are a potential reservoir for further spread. Trying to get another year of production from a symptomatic tree, while understandable, is not advisable because the disease will get progressively worse, and spread further. The team is working on herbicide related recommendations that may move this recommendation to the fall.
What should I do if my neighbor has infected trees, but seems to be blissfully unaware?
Talk to them about the disease, best management practices, etc. This problem affects everyone and requires community-wide management. All your efforts will be for nothing if your neighbor isn’t also controlling disease. Your pest board may also be able to help if neighborly awareness building is not enough.
How long can cut trees lie around before burning?
As long as they are dead, it doesn’t matter. The only cause for concern would be if leafhoppers are feeding on them, but if they’re dried out they won’t be appealing to the leafhoppers.
Are there any anti-microbial treatments for phytoplasma?
There are no products with a demonstrated mode of action against phytoplasmas that we are aware of.
When removing the complete block, would you apply herbicide in all the stumps before removing?
Probably not. Remove as many roots as you can when you pull out the trees. Rip thoroughly and wait sufficient time that remaining roots are dead before replanting.
If this was a problem not long ago in other states/countries, do we know how they managed to stop the problem?
In the 1950s Washington and Oregon survived through aggressive tree removal. In the 1980s California also survived through aggressive tree removal. The Californians conducted a statistical analysis to evaluate the effects of tree removal and insecticide use, measured as residual longevity. They concluded, “infected tree removal was primarily responsible for reduction in new [X disease] infections and that insecticide residual longevity contributed only a minor amount in suppression of new infections.” -Van Steenwyk et al., 1995 Plant Disease 79: 1025-1028.
What is the cost of the analyses?
$25 per pathogen, per sample at the WSU ELISA lab. So, $50 to test a single plant for both.
Pathogen
Does the LCV or Western X affect 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 little cherry virus be in the tree before it shows symptoms? And what about the Western X?
It can take a full growing season (~1 year) for symptoms to appear after infection for both Western X and LChV2.
Can the LCV 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 LCV Can LCV 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). There’s no cellular route for the pathogens to get in or out of pollen grains.
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.
Is the removal of the tree 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. We 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. We 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. However, the possibility for false negatives is high.
Can cherries with LCV or Western X be marketed?
Probably not, not only do the diseases affect the size and color, but also the taste.
Do we need to test the weeds?
No, incidence in weeds is low so it’s not likely to be economically viable or informative.
Should we be alarmed by June drop?
Yes. Premature fruit drop has many causes, but it could be associated with phytoplasma infection.
Is there more in organic versus conventional?
No, both organic and conventional producers are being hit by Little cherry virus-2 and X-disease phytoplasma at roughly equal rates.
Are the nurseries increasing the presence of the disease? Do California nurseries have LCD?
There is a potential risk for nursery blocks to get hit with random strikes by leafhoppers carrying X-disease phytoplasma, then distribute those trees to growers (given the latent period before disease is expressed, it wouldn’t be observed). While it is unknown whether little cherry is in California nurseries, the X-disease phytoplasma is present in that state, so the risk exists.
Is there a map of the peaches and nectarines that showed positive to X Phytoplasma?
Yes. Diseased peach and nectarines have been found in Yakima, Benton and Franklin counties.
How long the X phytoplasma and LCV can survive in the roots after removal?
As long as the root tissue is alive.
How long should we wait before replanting?
At least a year after removing the old trees. Let the field go fallow or plant a cover crop and look for any suckers that may be produced from live, broken roots. Also be aware of what is going on in neighboring fields.
Vectors
What is the main thing that moves Western X to new trees?
Root grafting is an important factor moving Western X to new trees. Remember it may be 2-4 years before symptoms become noticeable. Trees adjacent to removed trees which test positive the next year were likely already infected by root grafting previously. Grafting infected wood is a serious problem introducing the disease to new blocks. Leafhoppers are known to vector the disease.
Are there other possible vectors such as birds or squirrels?
The pathogens have a pretty specialized relationship with the insects that vector them. For example, pear decline is caused by a phytoplasma that is similar to X disease and can infect peaches. However, leafhoppers are unable to transmit Pear decline phytoplasma—only particular psyllids can. So, the range of insects for any phytoplasma is very limited.
Which leafhoppers vector Western X?
In a recent cherry orchard survey by Dr. Holly Ferguson, 12 potential vector species of leafhoppers were tested for the presence of Western X. Colladonus reductus and C. geminatus were the most abundant species in most orchards. Scaphytopius sp. and Osbornellus sp. were the most abundant species in a Selah orchard site. Over the entire study, for C. reductus, 123 of 891 samples (14%) tested positive for X while 111 out of 867 samples (13%) of C. geminatus tested positive for X. Other species less commonly found —Fieberiella florii, Osbornellus sp., Scaphytopius sp., and Euscelidius variegatus–also had 1-2 samples positive for Western X (2). For C. geminatus, 40 of 519 samples (8%) within orchards and 71 of 348 samples (20%) from extra-orchard habitats were positive for Western X. Likewise, for C. reductus,103 of 687 samples (15%) within orchard and 20 of 204 (10%) from extra orchard habitats were positive for WX. Results from transmission experiments conducted in 2018 provided evidence that both C. reductus and C. geminatus are capable of acquiring the Western X phytoplasma from infected sweet cherry plant materials and inoculating sweet cherry (Prunus avium) or a close relative Prunus tomentosa with WX. No Wester X transmissions by either species were observed from sweet cherry to celery or sweet cherry to radish. (Celery and radish served as herbaceous recipient plants). Additional transmission experiments need to be conducted with these and other leafhopper species that tested positive for Western X in the survey.
Previous work in several states found Colladonus geminatus, Fieberiella florii, Keonolla confluens, Scaphytopius acutus (now Scaphytopius delongi), and Colladonus montanus (Van Duzee) to be vectors of Western X with C. geminatus identified as the primary vector in Washington (3). Colladonas geminatus (Van Duzee) was thought to be important in X disease outbreaks in Washington in the 1940s-50s (4). In contrast, C. montanus and Fieberiella florii are the most important Western X vectors in California (5).
What is the best method to track leafhopper populations?
Yellow sticky cards placed at 3-5 ft in the canopy.
How do leafhoppers transmit the phytoplasma?
The cells of this pathogen are tiny. In an electron microscope, they look like small ovals, similar to a bacteria but without a rigid cell wall. When feeding the insects can acquire the phytoplasma. During the following three to five weeks the pathogen multiplies inside the insect invading its blood and eventually passing to the salivary glands where it can transmit the pathogen during feeding (7). In one older study in California, researchers found a one week acquisition time was sufficient for successful transmission with C. montanus (4).
Where do leafhoppers like to live?
We are still learning about the survival and dispersal patterns of the leafhoppers common in Washington. This may be very important to know. For example, of leafhopper Western X vectors common in California, C. montanus survives poorly on cherry, prefers herbaceous plants and is believed to overwinter outside of cherry orchards and introduce the virus to an orchard. In contrast, F. florii can reproduce on cherry and is an excellent vector (8). F. florii prefers to breed on ornamentals. Based on sticky trap and sweet net samples in the 2016-2018 survey, Ferguson noted that all of the potential vector species mentioned above spend at least part of the time in the cherry canopy and part of the time in the orchard floor vegetation. The orchard floor vegetation generally consisted of a grass and broadleaf mixture of plant species. C. geminatus and reductus are not known to be a grass inhabiting species. Ferguson observed greater numbers of these species in sweep net samples of orchard floor dominated by mallow and clover species. Outside of the orchards, C. geminatus and reductus were found on sagebrush, Purshia tridentata (antelope bush), and alfalfa. Alfalfa is a known host plant for Colladonus geminatus (9). In an Oregon study conducted in 1952-1953, leafhoppers were observed to disperse from alfalfa fields to cherry orchards in June and July as well as September and October (9). One of the orchard sites in the Ferguson study was next to an alfalfa field, which likely served as an overwintering site for leafhoppers.
How can I manage leafhopper populations?
Leafhoppers may be present as early as February and as late as November. Monitor leafhopper populations in early and late season, including postharvest in order to manage populations not controlled by your general insect management program. Monitor using yellow sticky card traps placed 3-5 feet up at the edge of the cherry canopy. University trials for postharvest management of leafhoppers are not available in the Pacific Northwest. University of California recommendations for postharvest treatment of leafhopper vectors of X disease include Esfenvalerate (eg Asana XL), Lambda-cyhalothrin (e.g. Warrior II) (1.28–2.56 fl oz/acre), or Thiamethoxam (e.g. Actara) (2–2.75 oz/acre) (Purcell 1987, UCIPM 2015). Each of these chemicals are toxic to bees, and should not be used around flowering plants.
What times of year do I need to manage leafhoppers?
Recent work by Ferguson et al. found the highest populations of leafhoppers in the fall with high populations in the spring in some orchards. In one orchard, populations exceeded 250 leafhoppers per card in October. Season long monitoring is useful but concentrate efforts in late season.
What is an example of a leafhopper program?
Manage leafhoppers when they are present – generally late season based on sticky card trap monitoring. If leafhoppers are present (1 or more per trap) rotate between pesticide groups.
There are multiple products and combinations that can be used. An example is a spray schedule every 21 to 30 days starting after harvest through late October based on monitoring yellow sticky card traps. They are rotating between four sprays including (1) a group 3 pyrethroid (e.g. Warrior, Actara), a (2) group 4a imidacloprid (neonicitinyl) (e.g. Midash Forte, Provado, Admire Pro) or a group 4c (e.g. Closer), (3) a new active product (e.g. Bexar group 21A) and (4) back to a group 3 or group 4.
Remember Washington product trials have mostly been tested on white apple leafhopper and the California trials centered on their common leafhoppers C. montanus and F. florii. Work with Washington common leafhoppers C. reductus and C. geminatus is needed.
What threshold should I use for leafhopper management?
An action threshold has not yet been studied in Washington. With pathogen vectors, even a low number can cause new infections. However, it is a probabilities game. Lower numbers, lower probability. Additional research is needed in this area.
Will removing covercrops or weeds prevent the presence of leafhoppers?
No, they won’t prevent the presence of leafhoppers.
What can we use to control leafhoppers for organic production?
This season we are trialing control methods (kaolin clay and reflective mulch) that would suit organic production, but they have not yet been evaluated.
Are these leafhoppers invasive species?
No, native species of leafhoppers vector X disease phytoplasma.
Is there any mating disruption for leafhoppers?
It is unknown what cues these leafhoppers use to find mates, so it is unclear if mating disruption would be effective.
Can leafhopper nymphs vector the disease?
Nymphs can take up the pathogen as they feed. Because they are not as mobile as adults they are less likely to vector to new plants. However, they will keep the pathogen in their system whereby they can vector the pathogen as they become adults.
How many generations of leafhoppers are there per year?
It is currently unknown how many generations there are per year in Washington. Previous research conducted in The Dalles, OR, area indicated that a C. geminatus life cycle from first instar nymph to adult took six weeks in the springtime (May) and about four weeks in the fall (9). Two generations a year were found for C. geminatus on alfalfa, their preferred host. Leafhoppers overwintered as eggs in alfalfa foliage. Leafhoppers moved from alfalfa and other host plants into orchards during the spring and fall. In these neighboring cherry orchards, C. geminatus numbers peaked in late May and November. In California, there are three periods of adult activity: mid-April through May, July, and September through October. Oregon and California abundance data are consistent with Washington data collected by Ferguson et al. Additional information is needed on the life histories of the Colladonus spp. in Washington cherry orchards.
Are fields close to sagebrush more likely to get new infections?
Not necessarily. The habitat may serve as hosts of the leafhoppers, but the weedy hosts that harbor the phytoplasma are unlikely to exist here unless the area has been disturbed. So, leafhoppers coming from this habitat are unlikely to bring the disease with them. However, particular care should be taken to test and remove diseased trees along these habitats that may spread disease to the rest of the orchard as leafhoppers pass through.
Will mowing the sagebrush in a strip around my field help reduce leafhopper populations?
This may actually increase, rather than reduce disease spread. Sagebrush habitat can host leafhoppers that spread into the orchard. However, sagebrush is unlikely to serve as primary sources of the phytoplasma that causes the disease. In contrast, removing these plants may allow weedy hosts that thrive in disturbed habitats to surround the orchard and serve as sources of disease. These disease hosts include puncturevine, tumble mustard, and flixweed, all of which do best in disturbed habitats.
What cherry varieties are not susceptible?
All sweet cherry varieties are susceptible to some degree, as are most sour cherries. Some ornamental cherries and pollinators such as Black Republican do not show disease symptoms, but they can be infected by and spread the X-disease phytoplasma. There are no known resistant varieties.
What don’t we know?
There is a lot that we don’t know about little cherry disease and about the vectors of Western X in particular. Important areas where we lack information include:
- How does disease occur?
- How quickly does Western X spread?
- How quickly do each of the leafhoppers acquire the phytoplasma?
- What are the lifecycles of the leafhoppers so that we know when eggs and other susceptible stages occur?
- What times of year do leafhoppers more easily acquire the phytoplasma? For example, in the spring when the concentration of the phytoplasma in the tree is low it may be too low for leafhoppers to acquire it.
- Can all of the leafhoppers in Washington breed on cherry? This will inform whether perimeters sprays may be effective.
It is important to remember that a lot of the information we have available about Western X is from the 80’s and 90’s in California. At the time they did not have all of the technology we have now to investigate these questions.
Additional information
Little Cherry Virus https://treefruit.wsu.edu/crop-protection/disease-management/little-cherry-disease/
X-Disease Phytoplasma (Western X) https://treefruit.wsu.edu/crop-protection/disease-management/western-x/
Grape Mealy Bug https://treefruit.wsu.edu/crop-protection/opm/grape-mealybug/
Apple Mealy Bug https://treefruit.wsu.edu/crop-protection/opm/apple-mealybug/
X-Disease and Little Cherry Virus Scouting and Sampling https://treefruit.wsu.edu/article/x-disease-phytoplasma-little-cherry-virus-scouting-sampling/
X-Disease and Little Cherry Virus Scouting and Sampling Guide https://treefruit.wsu.edu/crop-protection/disease-management/western-x/sampling-guide/
Another Bad Year for Little Cherry Disease https://treefruit.wsu.edu/article/another-bad-year-for-little-cherry-disease/
BMPs for tree removal for X disease and Little Cherry Virus infected trees https://treefruit.wsu.edu/article/bmps-for-tree-removal-for-x-disease-and-little-cherry-virus-infected-trees/
Little Cherry Disease Eligible for Tree Assistance Program https://treefruit.wsu.edu/article/little-cherry-disease-eligible-for-tree-assistance-program/
LCD Task Force Rolls Out New Online Form to Streamline Testing https://treefruit.wsu.edu/article/little-cherry-disease-task-force-rolls-out-new-online-form-to-streamline-testing/
Contacts
Tobin Northfield, Department of Entomology, Tree Fruit Research and Extension Center, 509-293-8789 or tnorthfield@wsu.edu
Scott Harper, Department of Plant Pathology, Washington State University, 509-786-9230 or scott.harper@wsu.edu
Tianna DuPont, WSU Extension, 509-293-8758 or tianna.dupont@wsu.edu
Bernardita Sallato, WSU Extension (509) 439-8542 b.sallato@wsu.edu
Ashley Thomson, OSU Extension (541) 296-5494 Ashley.Thompson@oregonstate.edu
Karen Lewis, WSU Extension (509) 760-2263 kmlewis@wsu.edu
References
- Van Steenwyk RA, Kirkpatrick BC, Fouche CF, Grant JA, Uyemoto JK. Evaluation of an Abatement Program for Western X-Disease in Sweet Cherry. Plant Dis. 1995;79:1025-8.
- Harper S, Ferguson H, Wright A, Eastwell K. The hunt for leafhopper vectors of Western X in Washington cherries. Washington State Tree Fruit Association Final Grant Report. 2018.
- Rice RE, Jones RA. Leafhopper Vectors of the Western X-Disease Pathogen: Collections in Central California. Environ Entomol. 1972;1(6).
- Suslow KG, Prucell AH. Seasonal Transmission of X Disease Agent from Cherry by Leafhopper Colladonas montanus. Plant Dis. 1982;66(1):28-30.
- Web Soil Survey [Available from: http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm].
- Purcell AH, Elkinton JA. A Comparison of Sampling Methods for Leafhopper Vectors of X Disease in California Cherry Orchards. Journal of economic entomology. 1980;73:854-60.
- Van Steenwyk RA, Havens DM, Freeman R. Evaluation of Trap Types for Two Vectors of Western X Disease: Colladonus montanus and Fieberiella florii (Homoptera: Cicadellidae). Journal of Economic Entomology. 1990;83(6):2279-83.
- Purcell AH, Uyemoto R, Van Steenwyk W, Schreader R, Suslow KG, Kirkpatrick B. Buckskin Disease of Cherry. California Agriculture. 1987;41:26-7.
- Nielson, MW. 1968. Biology of the geminate leafhopper, Colladonus geminatus, in Oregon. Ann. Entomol. Soc. Am. 61: 598-610.
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