Salicylic and Jasmonic Acid Synergism during Black Knot Disease Progression in Plums

Black knot (BK) is a deadly disease of European (Prunus domestica) and Japanese (Prunus salicina) plums caused by the hemibiotrophic fungus Apiosporina morbosa. Generally, phytopathogens hamper the balance of primary defense phytohormones, such as salicylic acid (SA)–jasmonic acid (JA) balance, for disease progression. Thus, we quantified the important phytohormone titers in tissues of susceptible and resistant genotypes belonging to European and Japanese plums at five different time points. Our previous results suggested that auxin-cytokinins interplay driven by A. morbosa appeared to be vital in disease progression by hampering the plant defense system. Here, we further show that such hampering of disease progression is likely mediated by perturbance in SA, JA, and, to some extent, gibberellic acid. The results further indicate that SA and JA in plant defense are not always necessarily antagonistic as most of the studies suggest but can be different, especially in woody perennials. Together, our results suggest that the changes in phytohormone levels, especially in terms of SA and JA content due to BK infection and progression in plums, could be used as phytohormonal markers in the identification of BK-resistant cultivars.