Summary written by Corina Serban (WSU Extension), Scott Harper, Madalyn Shires, and Cody Molnar (WSU Plant Pathology). July 8, 2022.
When researchers talk about ‘Little Cherry’ they are using a colloquial term for three different pathogens: Little cherry virus-1 (LChV-1) and Little cherry virus-2 (LChV-2) which cause Little Cherry Disease, and Candidatus Phytoplasma pruni, the X-disease phytoplasma (XDP) that causes X-Disease. It is difficult to distinguish which pathogen is causing disease, for all three causes the infected cherry trees to develop small, poorly colored cherries that make them unmarketable.
Little cherry virus-1, Little cherry virus-2, and X-disease phytoplasma can be transmitted by all types of grafting, including top-working and root-grafting between neighboring trees. Little cherry virus-2 is transmitted by apple (Phenacoccus aceris) and grape (Pseudococcus maritimus) mealybugs, while little cherry virus-1 has no known insect vector. The X-disease phytoplasma is transmitted by leafhoppers. 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. The two leafhoppers most often found to carry phytoplasma in Washington are C. m. reductus and C. geminatus.
Below are questions and answers about Little Cherry Virus and X-disease phytoplasma transmission.
Q1. Can either pathogen (Little cherry viruses or X-disease phytoplasma) be transmitted to healthy cherry trees by deer or mice feeding on infected cherry leaves or tree roots? If this would be possible, could the deer/mice carry the scent in their poop? How about mites and thrips feeding into infected cherry tree?
A: No, these pathogens are not known to be transmissible by animal grazing. The vector relationship is specific to certain leafhopper species, their host preferences, and feeding behaviors. Non-leafhoppers have different host preferences and feeding behaviors so are unlikely to be vectors.
It is unlikely that plant or pathogen produced volatiles would survive passage through an animal digestive tract.
Q2. Can either pathogen (Little Cherry Virus or X-disease phytoplasma) be transmitted to a healthy cherry tree by pruning tools used to prune or collect samples from infected trees in the orchard? Should growers do anything to disinfect the pruning tools between the trees?
A: They cannot be transmitted on pruning tools, which is what virologists call mechanical transmission, for a couple of reasons:
- It requires transfer of phytoplasma or virus-containing sap into the phloem on the next tree and injuries are large as pruning cuts disrupt phloem connections, so it is extremely unlikely to spread. Experiments using highly concentrated virus, far higher than would normally be found in a plant, to deliberately mechanically inoculate viruses have an extremely low success rates (>1:10,000) and that was using a scalpel which makes a fine cut that can callus over and have the phloem underneath knit and flow.
- The phytoplasma in particular is a very fragile organism and tends to rupture under stress. Sap dries rapidly, and this would likely kill the phytoplasma.
- However, cleaning tools between trees is a good management practice to prevent spread of viruses that are mechanically transmissible, such as Prune dwarf virus and Prunus necrotic ringspot virus. A solution of 10% bleach is cheap and effective, soaking the blades for a few minutes before washing with water, though it is hard on the tools.
Q3. What is the difference between the virus and phytoplasma in relation to the pathogens that cause Little Cherry Disease?
A: While the symptoms produced are similar, the two viruses and the phytoplasma are very different organisms in terms of their biology, which is summarized in the table below:
| Pathogen | LChV-1 | LChV-2 | Ca. P. pruni |
| Host range | Prunus sp. | Prunus sp. | Wide |
| Causes disease on | P. avium & P. cerasus | P. avium & P. cerasus | Most Prunus sp. |
| Insect vectors | Unknown | 2 mealybug species | 7 leafhopper species |
| Insect vector host range | Unknown | Prunus, apple, grape. | Wide |
Q4. What are the requirements that are necessary for Little Cherry Disease to occur in cherry trees regarding the host, pathogen, and environment?
A: First, all commercial cherry cultivars are susceptible to both of the viruses and the phytoplasma, and all will eventually develop disease. Environment affects the rate at which disease occurs after infection, and symptoms severity. For example, cold winters or sudden freezes like this year can slow phytoplasma accumulation and in some cases, like this year, this reduces symptom expression. On the other hand, the viruses are not heat tolerant, so high temperatures like summer 2021 can temporarily suppress virus concentrations. Finally, there are genetic differences within the phytoplasma, and within the viruses. What effect the genetic differences have on disease development is something we’re looking into.
Contact:
Scott Harper
Assistant Professor
WSU Department of Plant Pathology
509-786-9230
scott.harper@wsu.edu
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