Garden Word of the Day
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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. It is 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 they 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 leaves and flower buds of oleander. The P. savastanoi pv. savastanoi twin only affects olive, causing galls on twigs, stems, and at 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. Infection normally occurs in autumn and winter, but symptoms do not appear until late spring or early summer. The bacteria live in the galls they form, and can all be found on the surface of twigs, leaves, and fruit. The bacteria are commonly spread by wind and rain.
To prevent olive knot in your garden or foodscape, be sure to plant disease-free root stock, 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 fall, and two in spring) can help prevent and treat olive knot infections, but there is no sure cure. Luckily, this bacterial disease is not a serious threat to your olive tree’s health. It is spreading, however, and becoming more of a problem. This is due, in part, to the popularity of a particularly susceptible cultivar, the Manzanillo olive
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