Germination of dry, airborne conidia of Monilinia laxa and disease expression on plum fruit

The growth and infection from dry Monilinia laxa conidia were studied on plum fruit (cultivar Laetitia) at different phenological stages. Fruit were dusted with dry conidia in a settling tower giving an average of five solitary conidia/mm(2) and incubated for 3 to 48 h at high relative humidity (greater than or equal to93%, humid fruit) or covered with a film of water (wet fruit). Germination of solitary conidia, appressorium formation and germling viability were examined by fluorescence microscopy after each incubation period. Inoculated fruit were left unsterile to detect the pathogen on the fruit surface or surface-sterilised to detect mycelia in the skin tissue. From each group, fruit were selected for skin isolations onto potato-dextrose agar, paraquat treatment or dry incubation (less than or equal to56% RH). The tests were conducted to determine surface colonisation, penetration and symptom expression. Paraquat was used to terminate host resistance and to promote development and sporulation of M. laxa. Results clearly showed that fruit at the pit hardening stage were resistant to penetration and disease expression. Humid fruit at the 2 weeks before harvest stage, harvest stage and after cold storage remained asymptomatic. Fruit at these stages developed disease only after prolonged periods (greater than or equal to12 h) of wet incubation. Fruit became more susceptible to latent infection as they ripened. According to these findings, wounding of fruit approaching maturity or during harvest and handling practices should be decisive in the infection of plum fruit by M. laxa.