Comparative in vivo and in vitro study on Monilia fructicola causing brown rot of stone fruit in Brazil and California

Stone fruit production plays an important economical role in southern Brazil as well as in California, United States. In both countries, brown rot caused by Monilinia fructicola is the most important stone fruit disease, responsible for significant pre- and post-harvest fruit losses. The United States is among the main importers of stone fruit to Brazil. Imported fruit carrying the pathogen may introduce isolates with greater ability to infect stone fruit orchards in Brazil. In this study, 26 M. fructicola isolates from Brazil and California, recovered from commercial stone fruit orchards, were randomly selected and compared based on in vitro (optimum growth temperature, mycelial growth rate, germination, spore production and germ tube rate) and in vivo (sporulation, area under disease progress curve, incubation and latent period) studies on brown rot infection components. The isolates behaved similarly for most of the studied variables, except for germ tube length and sporulation on fruit, which were higher in isolates from California and Brazil, respectively. The negative correlation between these variables may be disadvantage for isolates from California to cause infections in the main peach cultivar grown in South Brazil. Significant differences in incubation and latent period and the area under disease progress curve were found among isolates from a same country. Such differences may be due to intraspecific diversity associated with the occurrence of the perfect stage in California, and by the broad sampling area from where isolates from Brazil were obtained. It is still difficult to determine the effect of isolates arising from imported California fruits in Brazilian stone fruit orchards. Further genetic variability studies and comparative epidemiology in the field may help to understant the real threat that foreign isolates represent.