2019 ASHS Annual Conference

Metamitron is a relatively new postbloom thinning compound for pome fruit. Metamitron interferes with the Photosystem II (PS II) pathway of photosynthesis and, consequently, creates a carbohydrate deficit that leads to fruit abscission. Metamitron is commercially available in several countries as a pome fruit thinner under the trade-name Brevis but the compound is not registered in the US. We have evaluated the thinning efficacy of metamitron for ‘Bartlett’ pear over three consecutive years (2015-2017) in Oregon and for ‘Honeycrisp’ and ‘Gala’ apple (2017 and 2018) in Michigan. Irrespective of the species (or cultivar in the case of apple), metamitron markedly reduced photosynthesis (PN) compared to non-treated controls, typically for a duration of two weeks, though in some cases the effect persisted for 25 days after application. Photosynthesis decreased linearly with increasing metamitron rate for pear but not apple. Pear fruit set was negatively, linearly related to metamitron rate. Apple fruit set was inconsistent between years for a given rate. High temperatures the preceding and subsequent days from application were associated with severe apple thinning in 2018. Application timings ranged between 6 and 16 mm fruit diameter depending on the year and species. For apple, timing had only a mild effect on thinning, with a slightly stronger response at delayed timings. For pear, early timings had little effect on thinning. The most efficacious rate for ‘Bartlett’ pear was between 200 and 300 ppm. More work is required to optimize rates for apple. In both crops, fruit size and quality was related to crop load and was significantly improved for metamitron treatments. Phytotoxicity (leaf burning and yellowing) was observed for apple when rates exceeded 400 ppm. Pear leaves displayed insignificant levels of phytotoxicity despite rates of 600 ppm. Future work is needed to evaluate the effect of surfactants, humidity and temperature on the absorption characteristics of apple leaves.