In vitro and in planta screening of compounds for the control of Pseudomonas syringae pv. actinidiae in Actinidia chinensis var. chinensis

Pseudomonas syringae pv. actinidiae (Psa) has caused severe damage to kiwifruit plantations worldwide. It is still crucial to find compounds that are able to control the disease. In this work we developed a time and space-saving leaf disk assay using highly susceptible yellow-fleshed cultivars. The assay enabled 29 molecules/compounds to be screened in relation to their capacity to condition the development of necrosis on leaf disks by treating them before the inoculation. A necrosis index was thereby created. Fosetyl-aluminum, DL-β-Amino-n-butyric acid (BABA) and saccharin (BIT) reduced symptoms significantly, and Fosetyl-aluminum was the most effective. Maxim®, triclopyr and Methyl jasmonate enhanced the necrosis index significantly. S -Methyl 1,2,3-benzothiadiazole-7-carbothioate (BTH) also enhanced symptom severity, though there was no statistical support confirming this. Salicylic acid did not condition symptom development. In vitro investigations into the direct effect of the compounds on Psa highlighted that Fosetyl-aluminum and the polyamines, spermidine and cadaverine, inhibited growth and also had a biocidal effect in a dose-dependent fashion. Salicylic acid and spermine also strongly reduced bacterial growth. Long-term trials with potted plants in the greenhouse confirmed the protective effect of Fosetyl-aluminum and clearly showed that BTH enhances the susceptibility of kiwifruit to Psa. Fosetyl-aluminum significantly decreased symptom expression in kiwifruit plantations (cv. Hayward) subjected to natural Psa infection. Monitoring Psa growth dynamics in the leaf disks by time-point-based Real-Time PCR, showed that treatment with Fosetyl-aluminum restricts bacterial multiplication within the host plant.