My son just bought a house and in the back yard is a tree with these round growths all over the branches. Leaves are not on the tree yet, so don't know what kind of tree it is. After researching on the internet, it seems that these could be phomopsis galls. Do these spread to other trees? Do they have wasp in them, as my daughter-in-law is highly allergic to wasp stings. Thanks for any help you can be. Jeannie
My guess is that the tree is an Oak (probably a Pin Oak). They are very
susceptible to gall forming insects. These tiny gall makers can indeed be a type of wasp but are not a danger to humans. They are way too small.These wasps are parasites of insects and can be considered beneficial.
Lightly infested pin oaks can live for many years without apparent harm but outbreak infestations can disfigure trees causing extensive branch dieback and even tree death. The clusters of dried out galls, which are especially noticeable after autumn leaf fall, remain aesthetically disfiguring for years after the wasps have emerged and abandoned them. The gouty oak gall, which is similar but lacks the horns, is produced by the wasp C. quercuspunctata. The complex life cycle of these wasps thwarts control efforts.
FYI - Phomopsis gall is caused by a disease organism, usually on woody shrubs.
More information on oak galls: http://www2.ca.uky.edu/entomology/entfacts/entfactpdf/ef408.pdf
Jefferson Co Cooperative Extension Service
810 Barret Ave.
Louisville Ky, 40204