Thanks for bringing in samples of the problem, this made it much easier to diagnose. A few years ago, we were inundated with malformed growths called witches’ broom or galls on several of our ornamental landscape plants such as ligustrum, loropetalum, viburnum, bush roses, pittosporum, and oleander.
We were really surprised when we found some of the galls on the oleander in the demonstration garden. Our ornamental plant demonstration garden is at the James S. Page Governmental Complex in Yulee. Samples of the galls found on various plants were sent to UF/IFAS from Extension faculty working in the Northeast area. UF/IFAS and the Florida Department of Plant Industry (DPI) suspected the plant damage might have been caused by herbicides being sprayed nearby or possibly thrips, but they were unable to isolate the culprit until recently.
However, some of the sites (like our demonstration garden) do not use herbicides. Herbicide spray can easily cause damage to plants when applied near other plants. The process of getting herbicides onto a plant not specifically targeted by the spray is called drift. Landscapers avoid drift by spraying when there are no wind gusts or by adjusting the droplet size of the nozzle on their sprayers.
Dr. Adam Dale was one of the UF/IFAS researcher specialists who pinpointed the pathogen as the Eriophyid mite. This eriophyid mite belongs in the genus Aceria. Although most of the mites in this genus are plant host specific, the mite found on our plant material does not seem care which plant it feeds on, as you can see from the long list of ornamental plants forming galls in its presence. Our best management practice is to prune out the damage. When we cut off the two oleander limbs a few years ago, we have seen no other occurrences. Please throw the stems away in garden trash bags, do not leave them on the ground. Keeping the area debris free is a good horticultural practice as it helps remove the pest from the site – whether it is an insect or a disease. We currently do not have a publication on this mite, but we expect it to be out next year.
The attached UF/IFAS publication contains information written by Marjorie Hoy on rose damage caused by the Eriophyid mite: https://edis.ifas.ufl.edu/pdffiles/IN/IN99900.pdf