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.
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Things you need
- Pruning tools
- Alcohol or bleach
- Garden hose or irrigation system
- Nitrogen fertiliser
- Oxytetracycline micro-injections
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.
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