Increasing Incidence of X-Disease in Ontario

X-disease is an economically important disease of stone fruits including sweet and sour cherry, peach, nectarine, and Japanese plum. X-disease has been reported primarily in the Great Lakes region, and its distribution corresponds with the occurrence of wild chokecherry (Prunus virginiana L.) which is the principle reservoir of this disease. Recently, an increasing incidence of X-disease has been seen in southern Ontario, likely a result of the expansion of chokecherry into stone fruit production areas. Once established in an area, X-disease can be very destructive to peach and cherry orchards. More recently, in Virginia , X-disease has been found in grapevine co-infected with aster yellows disease and has been referred to as North American Grapevine Yellows disease (NAGY). NAGY is a lethal disease of Vinifera, and is vectored by a number of leafhopper and psyllid insect species. In Ontario, both X-disease and aster yellows are naturally occurring in grapevine, although NAGY has not yet been reported.

Symptoms:

In peach, symptoms are not usually seen for 6 to 9 months after infection, often in the next growing season. This is largely due to the fact that the pathogen spreads mostly in late summer or fall, when both pathogen concentrations are high in leaves and leafhoppers are most abundant. In mid-summer, infected leaves develop irregular yellow spotting which becomes reddish purple with upward rolling of the leaf at the margins. Necrotic areas soon develop and drop out leaving a shot-hole effect and tattered leaves (Fig.1). As the season advances, there is a progressive defoliation of stems from the base up, with only a rosetted tuft of leaves remaining at the shoot tips. Fruit on infected branches is smaller, lacks flavor often with a bitter taste, and may drop before ripening. Diseased branches are more susceptible to winter kill. Symptoms are generally more severe during hot summers. The most characteristic symptom within the first two years of infection is the presence of both healthy and infected symptomatic branches on the same tree. Usually by the third year after infection, most branches will show symptoms. Young trees die within 1 to 2 years after the first symptoms appear, and older trees gradually decline in vigor and often die from winter kill or other opportunistic diseases.

Infected cherry trees on Mahaleb rootstock are usually killed midsummer or early the following year. This is the result of rapid necrosis of rootstock cells just below the graft scion. Foliage turns pale with a reddish tinge and curls upward. Trees on Mazzard rootstock decline slowly over many years. Often the only symptom is on the fruit, which may be smaller, bitter and pink at harvest.

Grapevine infected with NAGY exhibit leaf yellowing, die back of shoot tips and fruit abortion. Infected grapevines often die within months of the onset of symptoms and significant losses of vines have been observed in Virginia.
Leaves on infected chokecherry turn yellow by mid June with gradual reddening far in advance of normal fall colouration (Fig 1). Infected bushes are usually stunted with shortened internodes and die back of branches increases each year. Infected chokecherry usually dies within 1 to 3 years after exhibiting symptoms.

Causal Agent:

X-disease is caused by a phytoplasma, a small parasitic organism smaller than many bacteria, and lives in the phloem cells of plants. The phloem is a network of nutrient conducting tissues moving food manufactured in the leaves to other parts of the plant. The phytoplasma also infects the leafhopper vector where it multiplies and remains for the life of the insect, ensuring its transmission to susceptible feeding hosts of the leafhopper.

Transmission:

Several species of leafhopper transmit X-disease between different susceptible plant species. Leafhoppers acquire the X-disease pathogen while sucking juices from the leaves of infected plants. After the pathogen has multiplied in the insect for 2 or 3 weeks it can be injected through saliva into healthy leaves during leafhopper feeding. Movement of X-disease can occur from infected sweet and sour cherry, although chokecherry is often the principle reservoir. Other reservoirs of X-disease include certain weeds such as clover species, dandelion, and several rosaceous species including strawberry and blackberry. Maximum spread in stone fruits occurs from mid August through October when high concentrations of the pathogen are present in the leaves and leafhopper populations are increasing in orchards. The most significant spread of X-disease is from cherry to cherry, from cherry to peach, or chokecherry and bitter cherry to either cherry or peach. Spread of X-disease from peach to peach by leafhoppers appears to be of minor significance.

Control:

Wild chokecherry should be removed from within 250 m of susceptible orchards and vineyards. Chokecherry is commonly found in fence rows, along edges of woods, unused road allowances, overgrown meadows and abandoned fields. Brush killers provide the cheapest and most effective control with both summer and autumn spray applications. Treated areas must be re-examined annually during the growing season to ensure complete eradication. Infected cherry trees should be removed near peach orchards, as X-disease is most severe in young peach orchards planted next to old X-disease infected cherry blocks. In nurseries and orchards, all infected trees should be removed and destroyed as soon as they are found. New trees may be safely replanted the following spring. Control of leafhopper vectors throughout the growing season is not practical.

Legend: Upper left: X-disease infected chokecherry in a fencerow, Upper right: Foliar chlorosis and reddening on foliage of X-disease infected chokecherry, Bottom: Reddish spots and shot-holing of peach leaves infected with X-disease.