Field evaluation of olive (Olea europaea) genotypes for resistance to Pseudomonas savastanoi pv. savastanoi

Current commercial olive (Olea europaea) cultivars are susceptible to olive knot disease caused by Pseudomonas savastanoi pv. savastanoi. Here we report on the examination of the olive tree germplasm collection maintained at the USDA-ARS National Clonal Germplasm Repository (NCGR) in Winters, California for the incidence of olive knot disease. Over a two year period we evaluated olive knot disease incidence on 506 trees representing 144 unique accessions consisting of 104 named cultivars originating from the Mediterranean Basin, South America, and California, including several hybrids, and related subspecies e.g. Olea europaea ssp. cuspidata. All genotypes were replicated in four blocks with trees in these blocks ranging in age from 6 to 35 year old. The disease index was higher in 2014 (43.5%) than in 2013 (29.4%). The percentage of infection varied significantly (P < 0.05) between the different blocks in the study orchard. Eleven genotypes exhibited greater than 30% disease severity with Tragolea (Greece), Barnea (Israel) and Arbusana (Spain) being the most susceptible to olive knot disease. Eighty-six genotypes exhibited less than 10% disease severity with 17 genotypes of this group showing zero olive knot incidence over the two year observation period.