Integration of laboratory sensitivity analysis into Monilinia blight management technologies for wild blueberry production

Demethylation inhibitors (DMIs) are systemic and site-specific fungicides that have medium risk of developing resistance by a target fungal pathogen after prolonged, repeated applications. Propiconazole (the active ingredient in DMI fungicides such as Tilt^® and Mission^®) has been extensively used throughout the wild blueberry industry for Monilinia blight control over the past 20 years with multiple fungicide applications typically having to be made during the cropping year of production. Given the apparent loss of Monilinia blight control observed in field R&D trials and also in commercial fields, concerns were raised that the natural population of Monilinia vaccinii-corymbosi (causal fungal agent of Monilinia blight) was becoming less sensitive to propiconazole. Combined with this was uncertainty of the sensitivity of the M. vaccinii-corymbosi population found in wild blueberry fields to other fungicides applied prior to bloom in the cropping phase of production. Given this, laboratory sensitivity analysis of M. vaccinii-corymbosi single-spore isolates to propiconazole, difenoconazole, prothioconazole-desthio, boscalid, penthiopyrad, and cyprodinil was completed through the use of a mycelium growth assay. A significant shift toward reduced sensitivity to propiconazole in the population of M. vaccinii-corymbosi was not detected. EC₅₀ values (effective concentration that inhibit the growth of mycelium by 50%) ranged from 0.007-0.013 μg mL^-1 with mean value of 0.010 μg mL^-1. Prothioconazole-desthio was the most effective active ingredient examined with respect to its ability to inhibit mycelium growth of M. vaccinii-corymbosi on agar medium. By contrast, cyprodinil was the least effective active ingredient. Therefore, results from this study have confirmed that propiconazole is still an effective fungicide for Monilinia blight control, new fungicides including prothioconazole-desthio and difenoconazole are also effective, and the use of laboratory growth medium sensitivity analysis is an important tool in determining effective disease control.