By Tianna DuPont, WSU Extension; Scott Harper, WSU Plant Pathology; Tobin Northfield, WSU Entomology. March 7, 2024.
Which plants are alternative hosts for X-disease phytoplasma?
Between 2021 and 2023 the WSU Harper lab surveyed 24 orchards and areas adjacent to orchards from Wenatchee, Washington, to Hood River, Oregon, in an effort to identify which plant species, besides cherry or peach/plum/nectarine were significant hosts of the X-disease phytoplasma and could contribute to disease spread.
Of the species tested, 52 of 77 had phytoplasma positives. Plants from five families stood out with significant numbers of positives, these being members of the Asteraceae (Dandelion), Amaranthaceae (Goosefoot/Lambsquarter and Pigweed), Malvacea (Mallow), and Brassicaceae (Flixweed, Tumblemustard, Hoary Bittercress, and Shepherd’s Purse), and Fabaceae (Clover). All these species are common components of the orchard floor or are present around orchard borders. Also, the Cooper lab at the USDA-ARS identified these plants are commonly fed-on by the Colladonus leafhoppers that vector the X-disease phytoplasma. Twelve other plant families had a small number of positives for X-disease phytoplasma, these being Apiaceae (eg. Queen Anne’s Lace and Wild Carrot), Caryophyllaceae (Chickweed), Fabaceae (Clover), Geraniaceae (Redstem Filaree & Wild Geranium), Lamiaceae (henbit, purple dead nettle), Oxalidaceae (oxalis, ground sorrel), Plantaginaceae (Plantain), Poaceae (Downy Brome), Polygonaceae (Knotweed), Rosaceae (wild rose), Solanaceae (Nightshades), and Tribulus (Puncturevine)[1].
How important are weedy hosts?
Broadleaf weeds are a preferred feeding host for the Colladonus leafhoppers that transmit the X-disease phytoplasma in Washington, and orchards that have a lot of weeds tend to have higher leafhopper numbers than in those orchards where weeds are controlled. As we found, many of these weeds are also capable of hosting the X-disease phytoplasma, so the presence of positive weeds indicates that leafhoppers are actively transmitting this pathogen in the orchard. The weeds themselves though, are marginal hosts that carry only low concentrations of the phytoplasma, and the likelihood of a leafhopper acquiring the pathogen from an infected weed is lower than acquiring it from an infected tree. So, weeds infected with X-disease are like a ‘canary in a coal mine’ signaling that transmission is likely happening in an orchard or region, but aren’t as significant in driving spread as infected trees.
Management recommendations
To reduce the risk of transmission, removal of infected trees is key. Infected trees (and their root suckers) are most likely to have high enough concentrations for leafhoppers to acquire the X-disease phytoplasma and spread it to neighboring trees or orchards.
It is also important to suppress or manage weeds present in your orchards, because the leafhoppers are attracted to the weeds, which they like to feed on. The WSU Northfield lab has found that practices which reduce the percentage of broadleaf weeds in the orchard, including dense grass plantings in the drive row and herbicide applications (tree row and grass strip) can help reduce weedy hosts that leafhoppers favor and alternative hosts for the phytoplasma.
Combining tree removal, weed control, and insecticide applications is the best way to reduce the risk of X-disease phytoplasma spread, taking out sources of inoculum and making the orchard unappealing to the leafhoppers.
For more information
X-disease Phytoplasma Factsheet
Contacts
Tianna DuPont
WSU Extension
(509) 293-8758
tianna.dupont@wsu.edu
Scott Harper
WSU Plant Pathology
scott.harper@wsu.edu
Tobin Northfield
WSU Entomology tnorthfield@wsu.edu
Fruit Matters articles may only be republished with prior author permission © Washington State University. Reprint articles with permission must include: Originally published by Washington State Tree Fruit Extension Fruit Matters at treefruit.wsu.edu and a link to the original article.
[1] Families with positives samples where more than 5 total samples were observed in orchards.
