We occasionally receive samples in the Plant Disease Clinic of euonymus and other plants with large, round galls growing on the stems. Often, these plants show the results of an interesting disease called crown gall.
Symptoms of crown gall include small to large round or knobby outgrowths (galls) of tissue on the stem or roots, especially near the soil line. These galls may disrupt the flow of water and nutrients through the plant, resulting in an overall weakness and loss of vigor. In some cases, galls cause only aesthetic damage. Galls range in size from pea-size to several inches in diameter. When they are young, galls are light colored and smooth, but as they mature they become dark and woody and may crack and slough off. A crown gall is a solid mass of tissue, in contrast to many insect galls that are hollow.
A wide range of over 600 non-grass herbaceous and woody plants are susceptible to crown gall, but it is most common on euonymus, rose, grape, Prunus species (cherry and plum), apple, and willow. Crown gall is probably most important in the nursery industry, where the pathogen can be spread on knives used during propagation, resulting in significant losses after transplanting.
Crown gall is caused by a bacterium, Agrobacterium tumefaciens, which survives in the soil. The bacterium enters the plant through wounds. The fascinating part of the bacterium's life cycle begins once it is in the plant, where the bacterium injects a portion of its genetic material into the plant's genetic material. This genetic transformation causes the plant to produce excessive hormones and compounds called opines. The hormones cause the plant to overproduce new cells, forming a gall that the Agrobacterium can live in. Opines are a special food that only Agrobacterium can use. Crown gall is nature's own genetic engineering.
The best way to manage crown gall is to avoid it. Do not purchase plants with abnormal growths on the stem. Avoid unnecessary wounding of plants. If a plant in the landscape does develop crown gall, it should be removed. Since the bacterium survives in the soil, infected plants should be replaced with plants that do not get this disease, such as grasses, for a few years.