Olive Knot Management in California
Both the introduction of super high density orchards for oil production and the advent of mechanized harvest of table olives contribute to a growing need to reevaluate strategies for management of olive knot disease in California orchards. In California, disease management has focused exclusively on sanitation, mainly the removal of galls from infected trees, and plant protection with the use of copper (Cu) (2).
Source of Bacterial Inoculum
While old galls serve as a reminder of disease pressure, their potential to supply primary inoculum is unknown. Additionally, recent studies suggest that primary inoculum is not limited to bacteria residing in galls, but rather includes epiphytic bacterial populations on leaf and twig surfaces (1). Consequently, research focused on product screening for disease management should include assessment of product impacts on the epiphytic pathogen population rather than simply an assessment of disease incidence and severity.
The potential for pathogen translocation in xylem also suggests that P. savastanoi may survive in planta rather than just in galls or on plant surfaces. In planta populations may be less affected by topical applications of bactericidal compounds.
Disease Management: Cu Applications
Continued use of Cu exerts a selection pressure on the bacterial population, and Cu-resistant isolates of P. savastanoi have already been observed in commercial olive orchards in CA (B. Krueger and B. Teviotdale, personal communication). Alternatives to Cu application for disease management may help reduce the selection of Cu-resistant isolates and extend the longevity of Cu-efficacy in disease management.
Disease Management: Latex Polymer Plant Coatings
Industry observations suggest that plants coated with a latex polymer have reduced sensitivity to freeze-injury (John Slaughter, personal communication). Several table and oil olive growers in Tulare, Kings, and Fresno Counties are utilizing a latex polymer product for frost protection, and some growers combine the latex polymer with copper in a tank mix to enhance olive knot control. In addition to preventing freeze-induced wounds that serve as infection courts for P. savastanoi, plant coatings may provide a physical barrier to infection. Additionally, adding a film coating to a Cu application may extend foliar Cu persistence. Studies focused on assessing the efficacy of film-forming polymers (in the presence or absence of Cu) for mitigation of disease incidence and severity in California are currently underway
1. Quesada, J.M., Renyalver, R., Pérez-Panadés, J., Salcedo, C.I., Carbonell, E.A., López, M.M. 2010. Dissemination of Pseudomonas savastanoi pv. savastanoi populations and subsequent appearance of olive knot disease. Plant Pathology 59:262-269.
2. Teviotdale, B.L., Krueger, W.H. 2004. Effects of timing of copper sprays, defoliation, rainfall, and inoculum concentration on incidence of olive knot disease. Plant Dis. 88:131-135.