Differential Susceptibility of Olive Cultivars to Olive Knot Disease and Possible Involvement of Phenolic Compounds in Disease Tolerance

Abstract

Purpose: Olive knot disease caused by Pseudomonas savastanoi pv. savastanoi is among the most serious bacterial disease affecting olives in many olive growing countries. This study aimed to evaluate the susceptibility of olive trees cultivars towards Pseudomonas savastanoi pv. savastanoi, and to verify the involvement of polyphenols in disease resistance or tolerance. Method: The susceptibility of five olive cultivars against four strains of Pseudomonas savastanoi pv. savastanoi were evaluated by stem inoculation. The content of phenolic compounds was determined in different studied cultivars from shoots and Knots. Results: Evaluated Tunisian strains AW9 and AW8 showed a similar and intermediate virulence in each cultivar, TEK appeared the less virulent. While the Spanish strain IVIA 1628 was the most virulent. Our results revealed that cv. Zarrazi was very susceptible to the disease. Cultivars Arbequina and Chetoui appeared susceptible, cv. Chemlali exhibited an intermediate infection level. While cv. Oueslati appeared tolerant to the disease. Polyphenolic content from shoots increased significantly according to the strain’s virulence. For cvs. Zarrazi and Oueslati the polyphenol content of shoots inoculated with the most virulent strain IVIA1628 were (33.77 and 28, 69 mg GAE g−1 Ms), respectively. However, knot-polyphenol content increased significantly according to the virulence of the strains and cultivars susceptibility. The highest knot- polyphenol content value was recorded in the tolerant cultivar Oueslati inoculated with the most virulent strain IVIA 1628 (80.63 mg GAE g−1 Ms) while, cv. Zarrazi showed the lowest value with (30.16 GAE g−1 Ms). A negative correlation was observed between the polyphenol content in knot and the average of knot weight. Conclusion: The hight value of polyphenols found in small knots formed on cv. Oueslati suggest the implication of these compounds in the defence mechanism against the bacterial disease.