Very few bacterial diseases affect olive trees, the most common of which are bacterial gall or "olive knot" and bacterial leaf scorch. If your olive trees have a bacterial disease that causes brown spots, the culprit is most likely bacterial leaf scorch, also called Oleander leaf scorch, which is caused by the bacterium Xylella fastidiosa. Bacterial leaf scorch infects the xylem or water-conducting plant tissues, via insects called sharpshooters.
Diagnose the bacterial disease that's causing brown spots on your olive trees. Most noticeable during spring and summer, bacterial leaf scorch causes the leaf edges to turn yellow and the leaves to droop, and then the leaf margins will turn brown. The leaf discolouration spreads from the tips and margins inward, causing the leaves to die.
Prune away all infected parts of your olive trees that are showing symptoms to help control bacterial leaf scorch. Remove and destroy these tree parts to help contain the spread of the bacterium. Disinfect your pruning tools with a solution of one-part bleach or alcohol and 10 parts water before and after pruning the diseased olive trees to prevent spreading the bacterium to other olive trees or landscape plants.
Control the insects that are responsible for spreading the bacterial leaf scorch disease. The glassy-winged sharpshooter or Homalodisca vitripennis, smoke-tree sharpshooter or H. liturata and the blue-green sharpshooter or Graphocephala atropuntata are the most common types of sharpshooter insects to spread the bacterium. You can apply an insecticide that's labelled for these insects to your olive trees to discourage the insects from feeding on the xylem and spreading the bacterial disease. Follow the application instructions carefully on the pesticide label.
Water and fertilise your olive trees properly to help extend their lifespan and vigour in spite of the bacterial disease. Provide your olive trees with at least a monthly deep watering during summer or dry periods, soaking the soil down to and around the roots. Feed your olive trees once each year with a nitrogen-based fertiliser prior to flowering or once every two years with an organic fertiliser in spring.
Treat the infected olive trees with micro-injections of oxytetracycline, an antibiotic that may help reduce the disease symptoms. The use of oxytetracycline injections for treating bacterial leaf scorch is experimental and won't necessarily cure the disease, however. Consult your local agricultural extension service or a licensed arborist about oxytetracycline injections.
Olive knot is another common bacterial disease that affects olive trees, caused by the bacterium Pseudomonas syringae. Olive knot infects olive trees through wounds in the wood during wet weather, causing galls or knots on the olive tree's bark and stems, followed by twig and branch dieback. You can control olive knot by pruning away all infected parts of the olive tree, or by spraying the tree with a copper-based fungicide or Bordeaux mixture during autumn and spring. Keep in mind that bacterial diseases can sometimes resemble fungal infections of your olive trees, because both can produce similar symptoms. Common fungal diseases of olive trees include verticillium wilt, Phytophthora root rot and olive peacock scab.
Beware that you may need to remove the infected olive trees to ultimately stop the spread of the bacterial leaf scorch. Pruning away the symptomatic parts of the olive trees may or may not actually prevent the spread of the bacterium, because it might've already spread throughout the tree without showing more extensive symptoms. Also, controlling sharpshooter insect populations won't necessarily stop the bacterium's spread either.