Pre- and post-anthesis activity of fenbuconazole and triforine against blueberry flower infection by Monilinia vaccinii-corymbosi

The fungus Monilinia vaccinii-corymbosi infects open blueberry flowers via stigma and style, causing mummy berry disease. Management of flower infection in this pathosystem relies on multiple fungicide applications during bloom. However, flowers that open between spray applications may remain prone to infection because the stigmatic surface (infection court) is not exposed to active ingredient, unless there is systemic movement in the pistil and/or eradicant activity in the ovary. To quantify and compare pre- and post-infection fungicidal activity in flowers, we examine here the ability of two systemic fungicides to prevent flower infection and subsequent fruit colonization by M. vaccinii-corymbosi. The fungicides were applied before or after anthesis (first day of open bloom) in an experimental system where flower development stages and inoculation timing were monitored and controlled precisely. In greenhouse experiments with potted rabbiteye blueberry plants, treatment with fenbuconazole, triforine, or water was made when sufficient numbers of flower clusters at anthesis and earlier pre-anthesis stages were present simultaneously. Treated flowers (n = 10,992) were tagged, observed daily, and individual flowers were pollinated and inoculated 1 day after opening. Fruit from treated flowers were assessed for incidence and severity of mummification. Both fungicides provided excellent protection to newly opened flowers that were at the anthesis stage during fungicide application, but differed in their activity in unopened flowers at pre-anthesis stages during application. Fenbuconazole suppressed infection in unopened flowers that were on average 2.5 days pre-anthesis at the time of application, whereas triforine provided substantial protection to earlier flower stages up to 6.5 days pre-anthesis. In a separate experiment to quantify post-infection activity (n = 13,068 flowers), fenbuconazole and triforine significantly suppressed fruit mummification when applied within 2 and 5 days after anthesis, respectively (with inoculation and pollination 1 day after anthesis). Thus, the total window of activity of fenbuconazole in blueberry flowers was 4.5 days, whereas that of triforine was at least 11.5 days. Prevention of ovary colonization by eradicant activity appeared to be the predominant mode for superior activity of triforine against M. vaccinii-corymbosi in flowers. To better manage flower infection by this pathogen, active ingredients such as triforine with superior activity in floral tissues need to be identified and characterized.