Black Knot (BK) is a deadly disease of European (Prunus domestics) and Japanese (Prunus salicina) plums caused by the hemibiotrophic fungus Apiosporina morbosa. After infection, the appearance of warty black knots indicates a phytohormonal imbalance in infected tissues. Based on this hypothesis, we quantified phytohormones such as indole-3-acetic acid, tryptophan, indoleamines (N-acetylserotonin, serotonin, and melatonin), and cytokinins (zeatin, 6-benzyladenine, and 2-isopentenyladenine) in temporally collected tissues of susceptible and resistant genotypes belonging to European and Japanese plums during of BK progression. The results suggested auxin-cytokinins interplay driven by A. morbosa appears to be vital in disease progression by hampering the plant defense system. Taken together, our results indicate the possibility of using the phytohormone profile as a biomarker for BK resistance 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 phytohormones titers in tissues of susceptible and resistant genotypes belonging to European and Japanese plums at five different time points. The results suggested auxin-cytokinins interplay driven by A. morbosa appeared to be vital in disease progression by hampering the plant defense system via perturbance in SA, JA, and gibberellic acid. The results further point out that SA and JA in plant defense are not necessarily antagonistic always, especially in woody perennials. Results also suggest that the changes in SA and JA content following BK disease infection and progression in plums could be utilized as phytohormonal markers in the identification of BK-resistant cultivars.