Management of X-Disease of Stone Fruits
By Dr. Sharon M. Douglas
Department of Plant Pathology and Ecology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8601 Fax: (203) 974-8502
X-disease continues to be a problem for stone fruit growers in many regions of the United States as well as here in Connecticut. This important disease was first observed in the eastern U.S. in 1933 when Ernest Stoddard, a scientist with The Connecticut Agricultural Experiment Station, found it in a Connecticut peach orchard. In his pioneering work, Stoddard described and named this new disease. Concurrent with Stoddard’s work in Connecticut, workers in California described diseases of cherry and peach which had characteristics similar to X-disease. These eastern and western diseases were initially thought to be distinct diseases but we now know that they are strains of the same disease-causing organism.
In a variety of forms, X-disease has been reported in 26 states in the U.S. and in at least three provinces in Canada. This disease can be a problem in orchards with very different characteristics, including cultivars, locations, sites, and management practices. The incidence of X-disease in Connecticut is somewhat unusual since it is rarely found in orchards located near the coast or in the eastern portion of the state.
Causal Agent: For many years after its discovery, the agent causing X-disease was unknown and continued to be the "mysterious X." Although for a number of years the agent was believed to be a virus, it wasn’t until 1970 that the disease was associated with a mycoplasmalike organism. The name for mycoplasmalike organisms has recently been changed to "phytoplasmas" so the X-disease agent is commonly called the X-phytoplasma. This organism belongs to a relatively new class of disease agents which are similar to bacteria but lack cell walls and are limited to growing in the phloem of host plants. To date, no one has successfully grown the X-phytoplasma in artificial culture media outside of the plant hosts.
Plant Hosts: The economic hosts of X-disease are numerous Prunus species including peach, nectarine, sweet and sour cherry, and Japanese plum. Disease incidences as high as 60% and yield reductions ranging from 30% for mildly symptomatic to 80% for severely symptomatic trees have been observed in some commercial peach plantings in Connecticut. Direct losses are associated with reduced fruit quality and quantity. Indirect losses are associated with the costs of removing and replacing X-diseased trees and the subsequent 3-4 year wait for a commercial crop.
The key reservoir host is chokecherry, Prunus virginiana. This shrubby plant is common in woodlots and fence rows throughout the state. Recent studies have also implicated a number of weedy species in orchard ground covers as possible hosts.
Symptoms: Peach trees with X-disease usually appear normal at the start of the growing season although some trees may exhibit branch and twig dieback due to increased sensitivity to winter injury or may have slightly smaller leaves. However, by mid-summer, there is a fairly sudden and dramatic onset of symptoms. These might appear on a single branch as a "flag" in an otherwise healthy canopy or the entire tree might develop symptoms. Leaves develop yellow or reddish, irregular water-soaked blotches and often roll upward longitudinally along the mid-vein. The discolored areas become dry and brittle and the dead tissues drop out giving the leaf a tattered, shot-holed appearance. Highly symptomatic leaves drop off, often leaving a characteristic cluster of leaves at the tip of the branches.
Trees with X-disease gradually decline since the disease is chronic and progressive due to the systemic distribution of the X-phytoplasma in the phloem. Infected trees eventually die within 2-6 years depending upon the age of the tree at the time of infection.
Both the quality and quantity of fruit on diseased trees are affected. There is a gradation of symptoms ranging from abortion, premature coloring and ripening, and bitter flavor, all of which reduce marketability.
Symptoms of X-disease on chokecherry typically develop on the entire shrub. Leaves show premature bright yellow to red fall coloration in late May or early June and have substantially shortened internodes. Diseased chokecherries usually die within 1-3 years after exhibiting symptoms.
Insect Vectors: Since the X-phytoplasma lives inside the phloem of the plant host, it cannot spread by wind or rain as is the case with many other diseases. Instead, it is carried from plant to plant by insect vectors. Over ten species of leafhoppers have been reported to vector the X-phytoplasma in the U.S. and six species have been identified in Connecticut. These include Colladonus clitellarius, Fieberiella florii, Gyponona lamina, Norvellina seminuda, Paraphlepsius irroratus, and Scaphytopius acutus. Leafhoppers are small piercing and sucking insects that feed in the phloem of plants. As they feed in the phloem of infected leaves they acquire the X-phytoplasmas. When they fly to another tree, they probe into the phloem of that tree and inadvertently inoculate the tree with the X-phytoplasmas.
Numerous studies on the biology of these leafhoppers have been conducted at the Experiment Station over the years. Several of the key findings include: the types of plant species in the ground cover of an orchard influences both the total populations and species of leafhoppers in the orchard; higher leafhopper populations are associated with more feeding damage to peach leaves; and greater feeding damage to peach leaves provides greater potential for transmitting X-disease.
Cycle of Disease: The cycle of X-disease is complex. Leafhoppers overwinter as eggs in the leaf litter primarily at the edges of orchards. In Spring, the eggs hatch and nymphs develop on weedy and woody plants in wood lots and fence rows. These leafhoppers are thought to feed on nearby X-diseased chokecherry where they acquire the X-phytoplasmas. In June, seasonal changes in the feeding and egg laying preferences of the leafhoppers subsequently encourage dispersal of the adults into the orchard and also stimulate movement among various ground cover species and peach trees.
Numerous observations have provided evidence for spread of X-disease from chokecherry to peach. X-disease has been found to occur with higher prevalence in peach blocks located adjacent to woodlots containing X-diseased chokecherry when compared with blocks which are isolated from nearby chokecherry. Evidence for the spread of X-disease from peach to peach is limited although it is believed to occur through root grafts and occasionally by leafhopper transmission.
Strategies for Disease Management: The most effective method for managing a disease as complex as X-disease requires an integrated approach which considers as many aspect of the disease as possible. The first aspect of control involves cultural methods which eliminate sources of the X-phytoplasma. Removal of chokecherry within 500 ft of an orchard block continues to be the most important and effective practice for reducing the spread of disease. Additionally, removal of diseased peach trees and pruning of affected limbs or branches are helpful.
Vector management and control is the second important line of defense. This is accomplished by manipulating the attractiveness and availability of plant species which the leafhoppers use for food and for egg-laying. Frequent mowing or using herbicides to control plant hosts at the edge of the orchard, mowing of the orchard floor, control of the plant species which comprise the ground cover in the orchard (by planting species which are unsuitable hosts for the leafhoppers), and herbicide strips under the trees all help to discourage leafhopper activity both in and near the orchard. Although there is evidence that insecticide sprays directed at woody vegetation at the orchard edge and ground cover can reduce leafhopper populations, no effective compounds are presently registered for use. Additionally, since the economics, effects on non-target and/or beneficial insects and mites, and damage thresholds have not been established for the use of insecticides in this manner, this approach has significant practical limitations.
The final line of defense involves the use of antibiotic therapy which targets the X-phytoplasma. At present, the only compound registered for use in the U.S. and Connecticut is an oxytetracycline calcium complex available as Mycoject®. This product consists of pressurized capsules which contain 4.22% active ingredient. The compound is injected into the phloem of the tree where it knocks down the population of X-phytoplasmas but does not completely eliminate or cure the tree of the organism. Injected trees typically exhibit remission of X-disease symptoms for 1-3 years which helps to prolong the tree’s productivity for a finite period of time.
Before using this strategy for control, there are several factors that should be considered. The first factor focuses on the severity of symptoms exhibited by the tree. Research has shown that trees with less than 50% of the canopy exhibiting symptoms respond best to injections. In part, this is due to the overall vigor of the tree and to the efficiency of the uptake and distribution of the antibiotic within the tree. When trees with 50% or more of the canopy showing symptoms are injected, results are not as promising. In general, the less severe the X-disease symptoms, the greater the success with symptom remission following injection. The issue of whether or not to inject apparently healthy, asymptomatic trees is difficult to address since the costs and potential problems with injections need to be factored into the decision. However, the evidence against injecting trees on a preventative basis appears much stronger than the evidence for use of the antibiotic in this manner.
The second factor for consideration is the timing of the treatment. This should be done from late August until early October, after fruit are harvested (Mycoject® has a 60 DHI) and when the flow of nutrients is directed from the canopy to the roots. If trees are injected too late in the season, uptake and distribution of the antibiotic will not be effective.
A third factor is the method of treatment. The number of capsules per tree is calculated on the basis of tree size and trunk diameter. Holes are drilled to a specified depth and at an even spacing around the trunk. Capsules are inserted into the holes according to the instructions provided with the antibiotic.
The fourth and final factor for consideration addresses possible problems associated with this treatment. Reports of phytotoxicity and poor wound healing of drill holes can lead to significant secondary problems. These issues raise concerns about the possibility of invasion of poorly callused wounds by common pests and/or fungi present in the orchard such as those associated with Valsa or perennial canker. Other concerns surround the fact that studies to evaluate repeated treatment of individual trees and the cumulative effects associated with the use of this system have not been conducted.
Because of the complexity of the X-disease cycle we cannot expect to effectively control this disease with simple solutions. However, use of an integrated approach for managing disease should allow orchardists to continue to grow stone fruit in Connecticut.
Additional Reading: Douglas, S. M. and M. S. McClure. 1988. New integrated approach for controlling X-disease of stone fruits. CT. Agr. Exp. Sta. Bulletin 854. 10 p.
X-disease of stone fruits continues to be a limiting factor for orchardists throughout Connecticut and the U.S. The disease is caused by a phytoplasma and is transmitted by several species of leafhoppers. This fact sheet discusses the symptoms, disease cycle, and strategies for managing this economically important disease.