Influence of light on the Monilinia laxa–stone fruit interaction

Light represents a signal for the regulation of virulence in many microbial pathogens. Two stone fruits, nectarines and cherries, were used to investigate the influence of light on brown rot caused by Monilinia laxa. Three single-spore strains were inoculated on nectarines and incubated under different white lights, red light, blue light, greenight, and black light with two photoperiods. In addition, to understand the effect of daylight irradiance on brown rot, M. laxa was inoculated on different cherry cultivars and incubated under two simulated solar irradiations. Significantly higher disease severity and sporulation were reported on inoculated nectarines incubated under 58 W white light for 12 hr light/12 hr darkness than on nectarines incubated in continuous darkness. Only red light caused a significant increase in the incidence and severity of the disease in nectarines inoculated with the three M. laxa isolates, compared to fruit incubated under white light. High light intensity (185.45 W/m2), caused greater brown rot severity in all cherry cultivars, both early and late varieties, than low irradiance (145.85 W/m2). Significant up-regulation of the pathogenicity-related MlPNL2 gene was observed as an early response after cherry inoculation under high-intensity light, especially in late cherry cultivars, while MlPG1 expression did not show any changes under different light irradiances. M. laxa was shown to be a light-responsive fungal pathogen, and light seemed to play an active role in brown rot development.