Client's Request: I'm a trapper in the Ag Department. We have been looking for plum bud gall mites in the county. I recently came across this plum tree with a huge growth or gall, it's 8" long and 7" wide on the underside of a branch, the tree is around 15 years old. Homeowner wants to know what it is and what should be done about it. The tree is in Pittsburg. Any ideas?
MGCC Help Desk Response: Thank you for contacting the UC Master Gardener Program Help Desk with your question about a very large gall on this 15 year old plum tree in Pittsburg.
I believe this is a gall caused by the bacterium Agrobacterium tumefaciens commonly referred to as crown gall mite or possibly Plum Bud Gall Mite (Acalitus phloeocoptes (Nalepa)). Crown gall, as its name implies, typically appears at the crown of a plant, where the trunk enters the soil but it also can infect the main trunk and side branches. Plum trees are among a long list of plants commonly infected by crown gall including many popular home garden fruit trees (pluots, etc.).
The surface of crown galls and wood underneath is the same color as healthy bark and wood. However, when cut with a knife, crown galls are softer than normal wood and lack the typical pattern of annual growth rings. Galls can be tiny and smooth on young plants but usually are rough and sometimes massive on older trees.
Crown gall appears to have a relatively minor effect on most older plants which is the situation you found. Since this tree is 15 years old and the gall appears to encompass a major part of the main branch I would suggest leaving it be. If the distal portion of the branch starts to decline then the branch can be removed back towards the trunk thereby removing the gall.
Without further inspection of the gall here in our office, another possibility is the Plum Bud Gall Mite (Acalitus phloeocoptes (Nalepa)) which are well described in the Santa Clara County UCCE link below.
Following is a link to a UC Integrated Pest Management website that explains crown gall http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/DISEASES/crowngall.html
and here is a Santa Clara County UCCE link to Plum Bud Gall Mite:
http://mgsantaclara.ucanr.edu/files/300211.pdf
We could give a more conclusive response if you would bring a sample to our offices during our office hours described at the bottom of this response.
I hope your homeowner will be relieved. Good luck.
Help Desk of the UC Master Gardener Program of Contra Costa County (EDC)
Notes: Contra Costa MG's Help Desk is available almost year-round to answer your gardening questions. Except for a few holidays (e.g., last 2 weeks December), we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 2380 Bisso Lane, Concord, CA 94520. We can also be reached via telephone: (925) 608-6683, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/. MGCC Blogs can be found at http://ccmg.edu/HortCoCo/ You can also subscribe to the Biog.