Location: Crops Pathology and Genetics Research
2013 Annual Report
ARS scientists in Davis, California, conducted experiments to establish reproducible inoculation methods for testing all the Olive Cultivars maintained in the National Clonal Germplasm Repository (Davis, California) for their sensitivity to the olive knot pathogen, Pseudomonas savastonoi (P. savastonoi).
Three inoculation methods were tested. The methods included: 1. Petiole is removed from the branch followed by inoculation with P savastonoi. (no blade wounding) 2. The petiole was removed from the branch followed by stab wounding of the petiole scar using a blade containing P. savastonoi. 3. Stab wounding of the internodal region using a blade containing P. savastonoi.
Data collection from these experiments is ongoing. Seventy-three (73) plants were inoculated representing seven commercially available CV’s. All plants were maintained under greenhouse conditions. Up to 5 months post inoculation no symptoms were visible. Olive Knots were first observed 7 months post inoculation. Galls were scored in April, 2013, on a scale of 0-4 with 0 being no reaction and 4 being severe galling. All three approaches were considered unacceptable in terms of speed of symptom development.
We also conducted, “conventional “Stab Method” inoculations using a standard blade immersed in Pseudomonas inoculum and jabbed into the stem at the base of the seedling which produced rapid reproducible results. To conduct the olive knot screening using the stab method, we inoculated each tree at three positions ranging from the soil line up to approximately 16 inches above the soil. We termed these the Top, Mid and Bottom positions. In cases where the trees were not large enough, we had two inoculation positions. It is interesting to note, there does not appear to be a position effect in terms of sensitivity to olive knot development. This means, in subsequent screening trials, we will use average olive knot ratings from all positions to generate a single resistance rating score. We observed significant diversity in olive knot susceptibility in the 7 cultivars examined. Ratings ranged from 1 to 4. No CV’s were completely devoid of olive knot. Most galls displayed an “open phenotype” (i.e. rating of 1).
Plants rated a 1 or 2 were considered to display a higher level of resistance and were retained for further study, e.g., Cultivar Corantina exhibited the highest level of resistance with 90% of the plants tested showing a rating of 2 or less. Cultivar Arbosana was rated highly susceptible.
We have generated plantlets from greater than 60 olive germplasm accessions which includes 15 which we hypothesize to be the most olive knot susceptible genotypes and 15 we hypothesize to be the most olive knot resistant.