X-disease phytoplasma (Western X)

X-disease phytoplasma, Little cherry virus 2 (LChV2) and Little cherry virus 1 (LChV1), cause small cherry symptoms often described as ‘X-disease’ and ‘Little Cherry Disease.’ Diseased cherry trees produce small, poorly colored, unmarketable fruit. In peaches, plums, and nectarines X-disease symptoms are typically yellowed curled leaves and shot hole as well as small-deformed fruit. X-disease is present across North America, throughout Washington State, and at epidemic levels in the Columbia River basin, with high incidence in Yakima, Benton, and Franklin counties, and present in Oregon in The Dalles area.

Background

X-disease (Western X) is not a new problem. It was first identified in cherry trees of WA 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.

Symptoms

Infection reduces fruit size and quality in sweet cherries. In contrast to Little cherry virus 2 where fruit often has little flavor, fruit from X-disease infected trees are often bitter. Fruit have reduced fructose, glucose, and sorbitol content and in some cases total

phenolic content increases [1]. In addition to fruit symptoms you can see reduced growth/extension of infected limbs, sometimes leading to leaves crowding into dense clusters (rosette) when trees have been infected for multiple years.

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 2-3: systemically infected tree, small fruit observed on multiple or all limbs, and poor color development is pronounced.
3. 4+ 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.

Occurrence

X-disease is present across North America, throughout Washington State, and at epidemic levels in the Columbia River basin, with high incidence from Yakima, Benton, and Franklin counties, and present in Oregon in The Dalles area.

Host Range

X-disease phytoplasma infects most Prunus species, ex. cherries, peaches, nectarines, almonds, plums, and chokecherry. X-disease phytoplasma also infects weeds: ex. Dandelion, knotweed, goosefoot (lambsquarter), sagebrush, chickweed, mallow, alfalfa, puncture vine, tumble mustard, and flixweed [2].

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 have been 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 [27]. 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.

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 montanus montanus, Fiebriella florii, Scaphytopius acutus, Paraphlepsius irroratus, Colladonus montanus reductus, E. variegatus and Colladonus geminatus [3, 4]. The two most common in Washington are C. reductus, C. geminatus and E. variegatus. Low numbers of S. acutus and were found in WA surveys in 2020 and 2021 [5]. Research in the 1960 suggests E. variegatus appears to be a less effective vector than C. reductus, with a longer incubation period (approximately 50 days) [3], but this work needs to be validated with Washington phytoplasma strains.

_{*Note Colladonus reductus is a subspecies of montanus named C. montanus reductus.}

Life Cycle of the Organism

The X-disease phytoplasma replicates in the phloem tissue of the tree. It is believed that the phytoplasma either ceases to replicate or dies in the aerial parts of the tree as the branches go dormant during the winter months, but active, living phytoplasma cells overwinter 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

Biology of C. geminatus is largely unknown, but 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 [6]. In California C. m. reductus was found to have a generation time of 56 days [7]. Recent research suggests that C. m. reductus has three generations in Washington with two occurring after harvest. C. m. reductus fed on a wide range of hosts in feeding trials including mallow, alfalfa, cherry, peach, white clover, and dandelion [8].

Sampling and Testing for X-disease

Material to sample:

Submit four five-inch cuttings from the diseased limb(s) including young woody tissue, leaves, and FRUIT STEMS.

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.

Where to send samples:

See labs page

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 proven in scientific studies to have an effect on the phytoplasma. Management requires a combination of these four strategies:

1. 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. Plant certified trees. Manage your risks – if in doubt, have the material tested before you buy or plant.
2. 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 ensure roots are dead and identify adjoining root grafted trees. 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.
3. Monitor and Manage Vectors:

Consider timing. Both leafhopper population number 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 [4].
  • 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 has to 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.

Consider cultural controls

Recent research has shown that kaolin clay applications can reduce leafhopper vector populations in high pressure blocks by 47-78% percent and appears to reduce leafhopper movement into the canopy, and Extenday can reduce populations in high or low pressure blocks by 81-91% (28).

4. Manage alternative hosts of the phytoplasma and of the leafhoppers: clovers, dandelions, curly dock, bitter cherry, chokecherry. Grasses appear to be poor leafhopper hosts and are not a host for phytoplasma. Apply broadleaf herbicides. Healthy weed-free grass strips compete with broadleaf weeds and supply a non-phytoplasma host environment.

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.