Q: Several oaks in my yard have died. I have noticed numerous growths on the limbs, which are supposedly galls. I want to know if they are killing my trees and if they will spread to the other trees. I have also lost two mimosa trees.
A: The galls are not causing the trees to die although they may look unsightly. It is of course sad to lose any tree but Mimosa trees are not long living trees and some are classified as invasive. After talking to you we finally discovered what I believe the true culprit of your oak death to be – mistletoe. People don’t realize this plant is a parasite. As you drive down A-1A you see many of the trees along the roadside contain mistletoe and I have seen many trees all around this county full of this parasite. Totally there are about 170 species of mistletoe. Seed dispersal is mainly done by birds. Even though the seed goes through the birds digestive system it retains sticky hair-like structures that enable it to remain attached to the tree. Once a seed germinates, it can penetrate the host plant by growing through the bark into the water conducting tissues. Symptoms of damage show up first in twig dieback above the point of the mistletoe attachment. It is interesting to note that the tree does not respond by compartmentalizing after the intrusion, which would normally occur if the tree was wounded or pruned. Mistletoe receives water and nutrients before the host tree so in times of drought, the mistletoe causes even greater stress to the host tree. It can live in the trees for many years but it is slowly usurping the strength out of the tree. All parts of the mistletoe contain toxic substances that inhibit protein synthesis in the intestines of humans, but the toxicity is rarely fatal. Mistletoe infections can be controlled by pruning infected branches. Branches must be cut one to two feet back of the visible infection area. Cutting only the top green portions of the plant can induce the development of many shoots in other parts of the tree so it is best to sacrifice tree limbs.