Germination of dry, airborne conidia of Monilinia laxa and disease expression on nectarine fruit
The growth and infection from dry Monilinia laxa conidia were studied on nectarine fruit (cultivar Flamekist) at different phenological stages. Fruit were dusted with dry conidia in a settling tower giving an average of five solitary conidia/mm2 and incubated for 3 to 48 h at high relative humidity (≥ 93%, 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. Unsterile or surface-sterilised fruit were selected for skin isolations onto potato-dextrose agar, paraquat treatment or dry incubation (≤ 56% 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. Microscopical observations and findings of the fruit tests clearly showed that fruit at the pit hardening stage were resistant to penetration and disease expression. This indicates that infection at this stage should not contribute to a gradual build-up of secondary inoculum in nectarine orchards or to disease expression shortly before or after harvest. Results also showed that resistance to penetration and disease expression decreased with fruit maturity and with cold storage. Furthermore, increased wetness and wetness duration markedly increased penetration and disease expression on ripening fruit and should therefore be decisive in M. laxa epiphytotics in nectarine orchards.