Epidemiological Basis for an Efficient Control of Pseudomonas savastanoi pv. savastanoi on Olive Trees

Pseudomonas savastanoi pv. savastanoi (Psv) is the causal agent of olive knot disease and its epiphytic presence and measures for the disease control have been evaluated in Spanish olive orchards.

Seasonal dynamics of Psv populations on stem and leaf surfaces from symptomless shoots of naturally infected olive trees, was monitored by direct isolation and enrichment- PCR in five olive orchards, located in two olive-growing regions of southern Spain. No significant differences were found between leaf and stems in respect to the number of samples where Psv was isolated or detected by PCR. Average Psv populations varied from 2 to 102 cfu/g of tissue and were highly variable among field plots. Significant differences in Psv populations were found between summer and the other seasons in one field plot. Average high levels of Psv populations were observed with warm and raining months and, on the contrary, low levels were generally found in hot and dry months.

Plants of ‘Picudo’ and ‘Arbequina’ cultivars were inoculated once with Psv after the plantation and several treatments (copper oxychloride, cuprocalcic sulfate plus mancozeb or acibenzolar-S-methyl, Bion) were applied during four years. The effect of the copper treatments on Psv was more significant in the case of ‘Picudo’ where the average Psv population densities on untreated control and on treated samples reached 104 and 102 cfu/g of tissue, respectively. Both copper treatments also reduced the proportion of the samples where Psv was isolated. No resistance to copper in recovered isolates of Psv was detected. Furthermore, the average knot number was significantly lower in the plants treated with copper than in the untreated plants.

The treatments with Bion did not affect either the Psv population densities or the number of knots. No treatment significantly increased the olive yield in the four years of study, probably because very young trees were used. In conclusion, monitoring the populations of the olive knot pathogen in the orchards provides information necessary to design an integrated management of the disease and copper treatments should be regularly used for an efficient chemical control.