Bacterial infection causes distorted swellings on olive, Olea europaea, and oleander, Nerium oleander.
Identification
Pseudomonas savastanoi pathovar (pv.) nerii causes distorted, swollen, knotlike growths on stems and bark of oleander and olive. On oleander it also galls flower buds and leaves. Galled parts often die back, but overall plant health is usually not seriously threatened.
Pseudomonas savastanoi (=syringae) pv. savastanoi infects only olive, galling woody parts, especially the leaf nodes (bud development sites) on branches. Olive knot disease has become more common and serious, in part due to increased plantings of the Manzanillo olive, a highly susceptible cultivar, and cultural practices that can promote disease development.
Life Cycle
The gall-inducing bacteria persist in cracked, distorted, and rough bark. During wet weather in winter and early spring, the bacteria reproduce and are spread by splashing rain and contaminated hands and tools. Spring growth becomes infected through fresh wounds, such as pruning cuts and frost cracks. Additionally, olive can be infected through leaf scars on branches for several days after leaves have dropped.
Solutions
To prevent olive knot or oleander gall, plant only disease-free nursery stock. If you purchase oleander and olive during late spring through fall you can see whether plants are galled; plants acquired during the rainy season may be infected but have not yet developed galls. Avoid overhead watering, which spreads the bacteria.
Regular monitoring and properly pruning off galled parts help to control the disease, but prune only during the dry season, when the bacteria are inactive, and dispose of infected tissue away from hosts.
It may help to limit pathogen spread if you scrub pruning blades clean and disinfect them in a 10 to 20% bleach solution before each cut, especially if plants must be pruned during the rainy season. See Pest Notes: Olive Knot for more information.
