X-disease phytoplasma (Western X)

The X-disease phytoplasma (‘Candidatus Phytoplasma pruni’) is the primary cause of small, pale, tasteless and unmarketable cherries in Washington state. In peaches, plums, and nectarines, X-disease symptoms are typically yellowed, curled and shot-holed leaves, and small-deformed fruit. X-disease is present across North America, throughout Washington State, and at high incidence in Yakima, Benton, Franklin, Grant, and Chelan counties. It is also present in Douglas, Okanogan, Klickitat counties, and Oregon in The Dalles area.

Background

X-disease also known as Western X, Cherry Buckskin, and Peach X is not a new disease, but Washington is experiencing new complications. It was first identified in cherry trees of Washington State in 1946. In a 1947 survey, about 1% of cherry trees were found to be infected, and it has remained present ever since, fluctuating in frequency. However, it has increased in prevalence dramatically in recent years, with pathogen strains, vectors, and environmental conditions that were not present in the previous outbreak.

Symptoms

Infection reduces fruit size and quality in sweet cherries. In contrast to little cherry virus 2 where fruit are often bitter, fruit from X-disease infected trees are often tasteless. Fruit from X-disease infected trees have reduced fructose, glucose, and sorbitol content and in some cases total phenolic content increases (Wright et al. 2020). The primary strains present in the Pacific Northwest do not generally express foliar symptoms on cherry, whereas strains in other parts of the country can produce enlarged styles and witches’ broom.

Symptoms in cherry

• Small and misshapen fruit.
• Poor color development.
• Fruit lacking in flavor/ bitter.
• Symptoms can be confused with unripe fruit until close to harvest.

• Symptoms are restricted to one/a few branches unless trees have been infected for multiple years.

Symptoms in peaches, plums, and nectarines

• Yellowed curled leaves.
• Leaf shot hole.
• Small-deformed fruit.
• Leaf yellowing symptoms on infected peaches and nectarines begin to appear about 2 months prior to harvest, and get progressively worse, with shot holes appearing as the season progresses.

Symptom progression

1. Year 1-2: small fruit may be restricted to one branch, or cluster, fruit color may develop normally, or individual pale to white fruit may be observed.
2. Years 3-4: systemically infected tree, small fruit observed on multiple or all limbs, and poor color development is pronounced.
3. 5+ years: cultivar dependent, but characterized by reduced fruit yield, and dieback of limbs.

Causal Organism

X-disease phytoplasma is not a virus, but instead is a type of wall-less bacterium known as a phytoplasma. The X-disease phytoplasma lives and replicates in the vascular phloem of infected trees, interfering with tree growth and development. Five major strains of ‘Ca. P. pruni’ have been found to cause X-disease in commerically grown Prunus species (Molnar et al. 2024). These strains differ in geographic distribution, severity, symptoms and vector transmissibility. Unfortunately, the dominant strains in the Pacific Northwest cause fruit symptoms, but not leaf symptoms or stunting, making them more difficult to scout for.

Occurrence

X-disease is present across North America, throughout Washington State with high incidence in Grant, Yakima, Benton, Franklin, and Chelan counties, and present in Okanogan, Douglas, and Klickitat County and Oregon in The Dalles area (Molnar et al 2022).

Impacts

More than 238,856 trees equivalent to 974 acres of sweet cherries and 33,082 peach, nectarine, plum and apricot trees representing 81 acres were removed due to X-disease and Little Cherry Disease between 2015 and 2020 according to a recent survey conducted by WSU/ OSU Extension with 81 respondents who collectively manage 15,420 acres, 26% of Washington and Oregon total cherry acreage (Molnar et al 2022). Removed trees reduced revenue to the industry by an estimated $30 million in 2020 and $65 million between 2015 and 2020. Over the seven-year re-establishment period estimated lost revenue and establishment costs to growers is approximately $115 million.

Host Range

X-disease phytoplasma infects most Prunus species, ex. cherries, peaches, nectarines, almonds, plums, and chokecherry. X-disease phytoplasma also infects a wide range of broadleaf plants. Previous work found dandelion, knotweed, goosefoot (lambsquarter), sagebrush, chickweed, mallow, alfalfa, puncture vine, tumble mustard, and flixweed were hosts (Jensen 1971). In a 2021 to 2023 survey 52 of 77 species tested were positive for X-disease phytoplasma (Harper et al in prep). Plants from six families had significant numbers of positives including members of the Asteraceae (dandelion), Malvacea (mallow), Amaranthaceae (goosefoot/ lambsquarter & pigweed), Polygonaceae (knotweed), Brassicaceae (flixweed, tumblemustard, hoary bittercress, & shepherd’s purse), and Plantaginaceae (plantain). All these species are common components of the orchard floor or are present around orchard borders. Twelve other plant families were less prevalent in orchards and had a small number of positives for X-disease phytoplasma including, Apiaceae (eg. queen anne’s lace & wild carrot), Caryophyllaceae (chickweed), Fabaceae (clover), Geraniaceae (redstem filaree & wild geranium), Lamiaceae (henbit, purple dead nettle), Oxalidaceae (oxalis, ground sorrel), Poaceae (downy brome), Rosaceae (wild rose), Solanaceae (nightshades), and Tribulus (puncturevine).

Transmission

Grafting

X-disease phytoplasma is readily transmitted by all types of grafting.

Vector

Leafhoppers are the only known vectors. Seven leafhoppers are known to transmit X-disease phytoplasma: Colladonus reductus, Colladonus geminatus, Euscelidius variegatus, Colladonus montanus, Fiebriella florii, Scaphytopius acutus, and Paraphlepsius irroratus (Jensen 1969, Purcell and Elkinton 1980). In tests of over 2000 individuals no other leafhoppers were found positive for X-disease phytoplasma (Marshall et al., in prep). The only species found actively transmitting in that survey were C. m. reductus, and C. gemintus. Colladonus reductus is the most common X-disease leafhopper vector in Washington followed by E. variegatus with low frequency of C. geminatus (Orpet et al, in prep). Low numbers of S. acutus were found in Washington surveys from 2020 to 2022 (Marshall et al., in prep, Northfield and Nottingham 2020). Research in the 1960s suggests E. variegatus appears to be a less effective vector than C. reductus, with a longer latency period (approximately 50 days compared to 30-35 days) (Jensen 1969). This finding aligns with our finding that field-collected E. variegatus never had enough phytoplasma in their mouthparts for transmission (Marshall et al., in prep).

Transmission by leafhoppers will vary depending on the level of infection in the block as well as numbers of leafhoppers. For example, in managed orchards with some X-disease incidence 10-15% may have acquired phytoplasma, while up to 2% of Colladonus spp. leafhoppers are able to transmit (Marshall in prep). However, in an unmanaged orchard with high levels of infection (50 of 50 trees tested positive) 40% of C. reductus leafhoppers had acquired the phytoplasmas.

Life Cycle of the Organism

The X-disease phytoplasma replicates in the phloem tissue of the tree. Over winter the phytoplasma numbers in the aerial parts of the tree decline but do not die out completely, with higher levels of phytoplasma surviving in the roots. In the spring, the aerial portions of the tree become re-infected as the phytoplasma moves up the phloem of the tree, usually following the same general route as in the previous year. As a result, you may see symptoms in one limb for a year or more, but symptoms will eventually appear in additional limbs. Removing a symptomatic limb does not eliminate the phytoplasma since it is already in the root system before symptoms appear.

Vector Biology

In Washington C. m. reductus has three periods of peak adult abundance, June, August, and September/October (after harvest), indicating a potential for three generations (Marshall et al 2024). In California C. reductus was found to have a generation time of 56 days (Severin and Klostermeyer 1950). C. reductus fed on a wide range of hosts in feeding trials including mallow, alfalfa, cherry, peach, white clover, and dandelion (Northfield and Cooper 2020). Current phenology of C. geminatus is largely unknown due to low abundance in recent years. Research in the 1940s and 50s suggested it had generation times of 60 and 56 days in Oregon and California, respectively, with adults emerging in May and September in Oregon (Nielson 1966), which is generally supported by recent research.

Sampling and Testing for X-disease

Material to sample:

Submit four five-inch cuttings from the diseased limb(s) including young woody tissue (pencil size previous season wood), leaves and fruit stems.

Woody material should be from the previous season or older because we have found that current season growth can have variable pathogen concentrations. New research shows that leaves may test negative even though the wood they’re attached to is positive, therefore leaves by themselves are not recommended. Pencil sized wood from the previous season and fruit stems are the best tissue to sample.

Where to sample:

Trees with symptoms: Sample from symptomatic limbs. Trees with no symptoms: Sample from each leader. *Samples only needed in non-confirmed blocks/ adjoining trees. See flow chart.

When to sample:

The week before harvest to mid-August.

Sample condition:

Keep tissue moist and cool (e.g. package with a cold pack). Old or dried tissue is more likely to have false negatives.

Download sampling fliers:

Scouting flier vf (2021.02.04)  

Muestreo de X Fitoplasma y Little Cherry Virus flier WSU OSU (2021.02.04)

Controls

There is no cure and an infected tree will remain infected for the rest of its life. There are no commercial products that have been shown to have an effect on the phytoplasma in scientific studies. Management requires a combination of these four strategies:

Pathogen-Free Planting Sources

Replacement trees must be obtained from pathogen-free planting stock. Nursery trees can be free of symptoms and still be infected. Use certified trees. Manage your risks – if in doubt, have the material tested before you buy or plant.

Identify and Remove Infected Trees

Primary control measures rely on identification and removal of infected trees. Remove infected trees following postharvest treatment for leafhoppers. Infected trees spread the pathogen to neighboring trees by insect vectors or via root-grafting from tree to tree. Treating stumps with herbicide immediately after cutting or injecting into trees before cutting trees (frill treatment) can help to identify adjoining root grafted trees and ensure roots are dead. Several glyphosate products are labeled, see BMPs for Tree Removal. 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 (Van Steenwyk 1995).

Cultural Controls

Kaolin. Recent research has shown that kaolin clay (Surround) applications can reduce leafhopper vector populations in high pressure blocks by 48% percent and reduce leafhopper movement into the canopy (Marshall et al 2024). Kaolin not only deters leafhoppers from orchards it also repels leafhoppers from trees.

Extenday. Extenday can reduce populations in high or low pressure blocks by over 80% (Marshall et al 2024). The effectiveness of Extenday is likely due to removing access to alternative and reproductive hosts in the drive row of the orchards, such as dandelion and other broadleaf weeds.

Grass in drive rows. Planting grass in drive rows can reduce peak leafhopper numbers by approximately 50% compared to weedy drive rows (Northfield unpublished 2023). Leafhoppers cannot survive on grasses (Northfield and Cooper 2020). The most common plants fed on by the leafhopper vectors of X-disease phytoplasma are broadleaf weeds such as dandelion, mallow, lambsquarter, chickweed, knotweed, and plantain (Cooper et al. 2022).

Remove suckers. Root suckers provide easily accessible feeding sites for leafhopper vectors. Root suckers also tend to have higher levels of phytoplasma earlier in the season, likely due to movement of the pathogen from the roots where it overwinters (Wright et al.2022). Removing root suckers may reduce movement of leafhopper vectors into the canopy and reduce transmission (Marshall unpublished data 2023).

Monitor and Manage Vectors

Consider timing. Both leafhopper population numbers and X-disease phytoplasma concentration in the tree are likely to be higher after harvest. When phytoplasma concentration in the tree is higher leafhoppers are more likely to acquire and transfer the pathogen. Concentrate monitoring and management efforts when risk is highest after harvest.

Monitor. Monitor leafhopper populations early and late season, including postharvest in order to manage populations not controlled by your general insect management program.

• Use yellow sticky cards or sweep nets (Purcell and Elkinton 1980).
• Deploy post-harvest.
• Hang at sticky traps 2-4 feet from the orchard floor.
• Place traps on orchard borders, in areas of concern in your block and throughout block. Approx. 1 trap per two acres.
• Monitor every 1-2 weeks.
• Use presence (an average of 1 leafhopper per trap) as a threshold to spray.[i]
• Identify leafhoppers that vector X-disease phytoplasma.

Rotate leafhopper products when populations are present. Manage leafhoppers when they are present – generally after harvest through October based on monitoring. If leafhoppers are present spray rotating between pesticide groups. With the residual of common (conventional) products sticky cards will likely show 21-30 days of control necessitating 4 to 6 after harvest sprays per season.

For example, rotating between:

• group 3 pyrethroid (e.g. Warrior)
• group 4 neonicotinoid (e.g. Actara)
• a new active group
• back to a group 3 or group 4
• group 1 (e.g. Carbaryl) late in season when leaf-drop is not a concern.

Remember it takes several weeks after feeding on an infected plant for a leafhopper to be able to transmit the phytoplasma. The phytoplasma must pass through the insect gut, into the ‘blood’, and to the salivary glands before it can be excreted into a new plant with the saliva. Every two to three weeks sprays should be the shortest interval needed. More frequent sprays will mean you likely run out of legal applications before the end of the season when transmission is likely to be highest. See table below.

Manage alternative hosts of the phytoplasma and of the leafhoppers

Reducing broadleaf weeds in orchards can help reduce leafhopper numbers and potentially reduce transmission. Grasses are poor hosts for key vectors and are not a common host for phytoplasma. Apply broadleaf herbicides to the drive row as well as tree row. For example, in a recent study, herbicide applications reduced leafhopper vector numbers by up to 50%, likely by reducing the preferred habitat for the leafhoppers (Northfield unpublished data). Healthy weed-free grass strips compete with broadleaf weeds and supply a non-phytoplasma host environment.

Summary

Finally, control of this disease requires a community-wide effort. What your neighbor does or doesn’t do, affects you (and vice versa). The key to ending the current X-disease epidemic relies on reducing the amount of pathogen present in the state. This can only be done by removing infected trees because it is from those trees that the leafhoppers are acquiring and spreading the pathogen.