Factors affecting latent infection of prune fruit by Monilinia fructicola

Experiments were conducted in three prune orchards in California. In each o rchard, inoculations with Monilinia fructicola, the causal agent of brown r ot of stone fruits, were performed on branches of trees at bloom and fruit developmental stages. Five inoculum concentrations were used in each inocul ation. Six and four wetness durations were created for each inoculum. conce ntration at bloom and fruit developmental stages, respectively. Fruit were harvested 3 weeks before commercial harvest. The overnight freezing incubat ion technique was used to promote sporulation and to determine incidence of latent infection (ILI) of fruit brown rot. No differences in ILI among loc ations were found. A seasonal pattern of bloom and fruit susceptibility to latent infection was determined. Susceptibility to latent infection at bloo m stage was at a moderate level and increased to reach the highest level at pit hardening stage. Subsequently, fruit susceptibility to latent infectio n decreased, reaching the lowest level in early June at embryo growth stage . Thereafter, the susceptibility increased again with fruit development and maturity until harvest. Linear relationships between ILI and inoculum conc entration were obtained for most combinations of growth stage and wetness d uration. Incidence of latent infection increased linearly with increased we tness duration at bloom stage and increased exponentially with increased we tness duration at early and late fruit developmental stages. The optimum te mperatures for latent infection at pit hardening stage ranged from 14 to 18 degreesC, but the effect of temperature on latent infection was reduced at resistant stages. The temperature range favorable to latent infection vari ed for different wetness durations.