Yard and Garden: Dealing with Unusual Growths on Trees

Trees add value to the landscape, so homeowners may become concerned when their trees develop unusual growths. Iowa State University Extension and Outreach horticulturists explain what causes some of these growths. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.

There are black growths on the branches of my chokecherry.  What are they? 

The black growths probably are black knot, a fungal disease that occurs on chokecherry, European birdcherry and several other wild and cultivated cherries and plums. The black growths (galls) can vary from a few inches to a foot or more in length.  

Black knot is caused by the fungus Apiosporina morbosa. Fungal spores produced on 1-year-old and older galls initiate new infections. The knots are soft when newly formed and later become hard and black with age.  

Black knot is difficult to control. If only a few galls are present, prune out the infected branches in late winter. When pruning, make the cut at least 3 to 4 inches below the gall. The pruned material should be removed from the area and destroyed. Several fungicide applications may help prevent future infections. The fungicide applications should begin just before bud break and continue until after fruit set.

Attempts to control black knot in badly infested trees are likely to be unsuccessful. When dealing with severe black knot infestations, the best options are to do nothing or remove the tree and replace it with a black knot resistant tree.

There is a large growth on the trunk of my maple tree.  What is it?

The growth on the trunk of the maple tree is likely a burl. Burls are abnormal swellings or growths that develop on the trunks and branches of trees. Burls can be found on deciduous trees and evergreens. The exact cause is unknown. Possible causes include bacteria, fungi, insects, wounds or environmental stress.  

Burls do not kill trees. However, they may reduce the tree’s vigor. On a positive note, the unusual swirling grain pattern found in burls makes them prized by woodworkers. Burls can be carved into bowls, furniture and other objects.

There are golf ball-sized growths on the branches of my pin oak.  Each growth has several horn-like projections.  What are they?

The growths on the pin oak are a type of gall. Galls are abnormal growths of plant tissue induced to form by mites, insects or other small organisms. The galls on the pin oak are called horned oak galls because of the horn-like projections that protrude from the surface of mature galls. Horned oak galls are caused by a tiny wasp (Callirhytis cornigera).  

The life cycle of Callirhytis cornigera is unique. It consists of alternating generations that give rise to two distinct types of galls produced by two groups of female wasps. In spring, wasps emerge from mature horned oak galls. At this time, all of the wasps are female. The female wasps deposit their eggs on developing leaves. This initiates the development of tiny leaf galls along the leaf veins. Adult wasps emerge from the leaf galls in mid-summer. This group of wasps includes both male and female wasps.

Mated females lay their eggs on twigs, initiating the development of horned stem galls. Each gall houses anywhere from 1 to 160 larvae. Each larva is housed individually in a cone-like structure within the gall.  Wasp development in twig galls takes almost three years. As the immature wasps reach pupation, the horns rise from within the gall to break through the surface and release the adult wasps.  

Pin oaks with small infestations of horned oak galls are not seriously harmed. However, heavy infestations can cause tree decline and even death. Fortunately, heavy infestations of horned oak galls on pin oaks in Iowa are not common. There are no effective control measures for home gardeners.