Crown gall causes round galls to form on stems or roots, often near the soil line of the plant. Galls may vary from the size of peas to over an inch in diameter. When young, the galls can be white or cream colored and spongy or wart-like; as they age, they become dark and woody. Galls can interfere with the plant’s ability to move water and nutrients through the stem, which may result in stunting or decline of the plant. Crown gall can infect nearly all dicotyledonous plants, and is most common in euonymus, Prunus spp., brambles, rose, willow, and grapes.
Signs and Symptoms
Crown gall is especially interesting because it results from nature’s own genetic engineering. The crown gall bacterium, Agrobacterium tumefaciens, enters the plant through a wound. It then inserts a portion of its own DNA into the DNA of the plant. Once this bacterial DNA is incorporated into the plant DNA, it induces the plant to overproduce plant hormones that stimulate cell division, resulting in a gall that is a perfect home for the bacterium. The bacterial DNA also causes the plant to produce special food called opines that only Agrobacterium can utilize. Scientists have used the crown gall bacterium to do their own genetic engineering of plants.
Crown gall is usually introduced into a location on infected planting stock, so it is crucial to buy only disease-free plants. Avoid unnecessary wounding to prevent infection. If galls are present, they cannot be cured and the plant should be removed. No chemical sprays are effective against crown gall, but a biocontrol agent made from the bacterium Agrobacterium radiobacter can be used as a dip for propagative cuttings to prevent infection by the crown gall bacterium. This biocontrol is marketed as Galltrol A, Norbac 84C, Nogall, or Diegall.