If your olive tree has strange knobs forming on the stems, it may be olive knot.
Olive knot is a bacterial disease of olive and oleander caused by Pseudomonas savastanoi pathovar (pv.) nerii and P. syringae pv. savastanoi. These bacteria are like evil twins that don’t cause serious harm but are still a pain to be around.
Symptoms of olive knot
The knobby bit you see is called a gall. Galls are swollen, distorted, woody growths that can girdle stems and twigs. The P. savastanoi pathovar (pv.) nerii bacteria is the one that causes the galls on both oleander and olive twigs. It also forms galls on the leaves and flower buds of oleander. The P. savastanoi pv. savastanoi twin only affects olive, causing galls on twigs, stems, and leaf nodes.
Olive knot lifecycle
The bacteria responsible for olive knot enter the tree through pruning wounds, leaf scars, frost cracks, and sun scald wounds after traveling on wind and rain. Infection occurs in autumn and winter, but symptoms do not appear until late spring or early summer. The bacteria live in the galls they create and on the surface of twigs, leaves, and fruit.
To prevent olive knot in your garden or foodscape, plant disease-free rootstock, avoid overhead watering, and prune off any infected material during the dry season, disposing of infected material in the trash. Be sure to sanitize your tools afterward.
Fixed copper sprays (one in the autumn and two in spring) can help prevent and treat olive knot infections, but there is no sure cure. While this bacterial disease is not dire, it is spreading and becoming more of a problem. Due, in part, to the popularity of a particularly susceptible cultivar, the Manzanillo olive.
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