Sick Olive Tree

Question from Ruth:
My olive tree has some kind of blight that is causing welts on the branches and some branches have died. What causes it and what shall I do to control this? I am very worried about my tree!

Answer from Pat:
Olives are subject to a number of pests and they sometimes also fall prey to several types of scale which then can cause black sooty mold. Control scale with oil sprays.

Among diseases, olive trees are subject to phytophthora root rot and to olive knot disease. It sounds as if your tree has fallen prey to olive knot disease.

Olive knot disease is caused by a bacterium (Pseudomonas sevastenoi pv. sevastenoi). This can be brought about by cool temperatures and damp or wet weather as we have had this year. The bacterium pseudomonas causes welts or knots on the branches. Bacteria then live inside these knots. The knots ooze in wet weather and can spread. This disease can kill twigs and branches and sometimes kills the entire tree.

The cure is to prune off what you can of the infected wood during dry weather. Also during dry weather, cut off the knots from the tree if they exist on the bark, with a sharp knife. This is if the bacterium has affected the tree. Be very sure to disinfect your pruning shears or knife every time you make a cut so you don’t spread the disease further. Then immediately after cutting and pruning, treat all the cuts and the bark of the whole tree with a product called Gallex®. Gallex® is a bacterium itself so it is an environmentally responsible cure. Be sure to follow all package directions carefully and follow all safety procedures recommended.

Take care not to wound the bark of the tree with machinery or tools or any restrictive thing such as wire or fencing. This bacterium gets going by infecting olive trees through wounded bark. Once the galls begin they then spread and enter other places on the tree through cracks and fissures in the bark. Vigilance and the use of Gallex® can eventually win out if you begin right away. Luckily fall weather gives you a chance to do this job now while the weather is dry before the rains return.