X Disease

Also known as cherry X disease, peach X disease, and cherry buckskin disease, X disease makes fruit small, pale, and bitter. It can affect stone fruits, such as apricots and cherries. Once a tree is infected with X disease, it must be removed to prevent the infection from spreading to other trees.

Cherries on the left infected with X disease (Plantpathfinder) CC BY-SA 3.0

What causes X disease?

X disease is caused by phytoplasma. Phytoplasmas are microscopic, one-celled bacteria that have no cell walls or nuclei. To me, that’s just weird. Animal and plant cells and most bacteria have clearly defined cell walls that hold their insides in, where they belong, and a central nucleus that runs the show. I have a hard time wrapping my brain around these phytoplasmas. Maybe they are like parasitic amoebas, even though amoebas are animals. But I digress.

So how does a cell with no wings, legs, feet, or brain find its way to our fruit trees?

The answer is phoresy. Phoresy describes the relationship between two organisms in which one player is a hitchhiker but not a parasite. There are some pretty bizarre examples of phoresy. You may want to check it out.

Phytoplasmas catch rides inside sap-sucking leafhoppers, planthoppers, and psyllids without harming their hosts. When a cherry leafhopper feeds on an infected plant, the phytoplasma responsible for X disease is sucked into the insect’s gut with the sugary sap. When that vector moves to an uninfected plant and begins feeding, the pathogen is transferred to the new plant, causing infection.

What does X disease look like?

This disease can take up to 9 months to appear after infection occurs. Eventually, trees infected with X disease produce pale fruit that is small and leathery. Symptoms are commonly seen on only one branch at first. The leaves on infected branches may appear bronzed and small. Older leaves tend to fall off. Beyond that, symptoms can vary, depending on the tree species.

Cherry tree infected with X disease (H.J. Larsen, Bugwood.org) CC BY 3.0 US

X disease is often mistaken for root rot. To figure out which it is, look closely at the graft union. That’s where the rootstock was grafted onto the fruiting stock. Pits and grooves are visible near the graft union if it is X disease. Cut into the wood and see brown areas in the phloem. These other symptoms of X disease are also common:

• Chlorosis
• Purple leaf spots that create a “shot hole” effect
• Sparse leaf cover
• Tip dieback
• Upward leaf curling

The fruit from infected trees tends to be pale, pointed, small, and nasty.

X disease hosts

There are two types of X disease hosts: those that don’t mind the infection (reservoir hosts) and those who succumb to the disease (non-reservoir hosts). Reservoir hosts are a problem because they look perfectly healthy but provide a source of infection to many important fruit and nut trees in our landscapes. Almond trees, chokecherries, sweet and sour cherries, Japanese plums, clovers, and dandelions are all reservoir hosts. [Dandelions?!!? Yes, dandelions.]

Nectarines and peaches can also become infected with X disease but are non-reservoir hosts. This means that they can catch the disease but not spread it. Scientists don’t yet understand how that happens, but they’re working on it.

X disease management
 
Since scientists have been unable to reproduce the phytoplasmas responsible for this disease in the lab, there are currently no treatments for this fatal disease. To reduce the likelihood of X disease occurring in your landscape, do your best to manage the insect pests responsible for carrying the pathogen.

The insects that spread X disease are often found on beets, burclover, ceanothus, curly dock, hawthorn, and pyracantha. Because of this, it is a good idea to maintain some distance between these plants and vulnerable trees.

If you suspect X disease has infected one of your trees, get help. Contact your local Master Gardeners or the Department of Agriculture for verification. They can also help you figure out the safe removal and disposal of infected trees.