A fast and reliable method for Diplodia seriata inoculation of trunks and assessment of fungicide efficacy on potted apple plants under greenhouse conditions

Diplodia seriata is a polyphagous and widespread pathogen that infects trunks, shoots, fruit and leaves of apple plants. Fungicides applied in integrated pest management programmes can act against D. seriata. However, introduction of scab-resistant apple cultivars and the consequent reduction in fungicide applications under low input disease management may increase the incidence of disease caused by D. seriata. Despite potential outbreaks of trunk canker, no fast and reproducible protocols for artificial inoculation of D. seriata and fungicide efficacy tests are available for apple plants. In this study, protocols for mycelium plug- and conidium suspension-inoculation of apple stems and shoots were optimised; canker disease assessments were carried out on potted apple plants under greenhouse conditions and coupled with D. seriata DNA quantification with quantitative PCR (qPCR). Efficacy tests of commercial fungicides showed that captan, dithianon and fluazinam inhibited D. seriata mycelium growth and conidium viability in vitro, while penconazole and ziram did not. However, dithianon spray applications did not reduce trunk canker severity and amount of D. seriata DNA in artificially inoculated plants under greenhouse conditions. This optimised protocol for fast and precise assessment of fungicide efficacy is suitable for further investigating the effects of other fungicides against D. seriata.