UC Riverside

Pseudomonas savastanoi pv. savastanoi (Psv) is a wound pathogen causing olive knot disease in California olive production. Sanitation of field equipment with sodium hypochlorite to reduce pathogen dissemination is a strategy to manage the disease but is highly corrosive. In laboratory assays, quaternary ammonium compounds (QACs) were highly toxic to Psv at low concentrations (≤5 µg/ml) and brief exposure times (≤60 s). In field trials, pruning equipment contaminated with Psv was effectively disinfested with a QAC, greatly reducing disease incidence on cutting wounds. This QAC was registered for use against olive knot on orchard equipment in 2015.

Copper-based bactericides are used as wound protectants against olive knot. Applications made within 24 h of wounding significantly reduced disease. Copper-treated wounds inoculated after 7 days reduced disease incidence by >50 % demonstrating good persistence. Untreated wounds inoculated after ≥10 days resulted in <20% disease, indicating wound healing. ‘Arbequina’ olives were less susceptible to Psv than ‘Manzanillo’, but disease was still high on both cultivars.

In-vitro evaluation of 147 California Psv strains detected three copper-resistant strains that grew at >150 µg/ml metallic copper in growth media. Copper-treated olive wounds inoculated with a copper-resistant strain resulted in reduced control compared to a -sensitive strain. Copper-sensitive strains were more virulent, but this depended on the type of wound inoculated and inoculum concentration.

Baseline sensitivities were established for kasugamycin and oxytetracycline as copper alternatives. Minimum inhibitory concentrations for 147 strains ranged from 1.86 to 11.52 µg/ml and 0.13 to 0.40 µg/ml for kasugamycin and oxytetracycline, respectively. In field studies, kasugamycin (200 µg/ml) performed equally to copper (1,260 µg/ml metallic copper) in reducing disease on lateral wounds of olives inoculated with a copper-sensitive strain and was better than copper using a -resistant strain. Oxytetracycline (200 µg/ml) was not as effective.

The genetic variability among 152 California strains based on rep-PCR with primers BOX, ERIC, and REP was high within regions, although overall genetic variability among strains was limited (≥82% similarity). Phenetic analyses identified several genotypes, but most strains belonged to one of two groups. The three copper-resistant strains had distinct fingerprints from the other strains.