Evaluation of apple thinning programmes

The main aim of this project was to evaluate the chemical thinning agents metamitron (Nevis^TM) and 6-benzyl adenine (6-BA; MaxCel^TM) on their own or as part of thinning programmes over a number of seasons to develop an industry reference base on thinning results. One of the project aims was to find potential replacements for carbaryl (Sevin^TM).

Chemical thinning treatments were compared over seven seasons (2009/10-2015/16) in both EGVV and Ceres, except for 2009/10 in Ceres and 2014/15 in EGVV. Cultivars used included Fuji (all seasons except 2009/10), Golden Delicious (all seasons except 2011/12), Cripps’ Pink, Cripps’ Red and Royal Gala (only 2009/10 – 2011/12). Until 2011/12, the research focused on evaluating the use of 6-BA in combination with other chemicals such as naphtelene acetic acid (NAA; Planofix^TM or Pomaxa^TM) or on its own compared to industry standard thinning programmes.  From 2012/13, the project scope was broadened to also evaluate metamitron. Treatment were applied by spray cart until 2010/11 and whole orchard rows served as plots with every second row serving as guard row.  Treatments were applied by motorised backpack sprayer from 2011/12 onwards with single tree plots.

2009/2010: ‘Cripps’ Red’ benefitted from chemical thinning with either carbaryl or 6-BA. 2010/11: 6-BA could be applied as an alternative to carbaryl. The weather in the days following the first application facilitated good thinning, but inclement weather after the second application resulted in poor thinning. 2011/12: Most thinning treatments decreased fruit numbers and increased fruit size in ‘Cripp’s Red’ and ‘Golden Delicious’. 2012/13: Metamitron showed promise as a potential chemical thinner on ‘Golden Delicious’. However, cloudy weather before, and sunny conditions after applications at an early stage resulted in over thinning. 2013 /14: Metamitron reduced fruit set and the hand thinning requirement in both ‘Fuji’ and ‘Cripp’s Red’. Two consecutive metamitron applications thinned more aggressively than NAA followed by metamitron. 2014/15: All three chemical thinning treatments were very effective and possibly over-thinned ‘Golden Delicious’ trees.  Conditions were very good for chemical thinning. In ‘Fuji’, metamitron at 4 mm fruit size and 6-BA (750 ml) 7 days later was a very good thinning treatment. 2015/16:  Metamitron at 4 mm fruitlet diameter increased fruit size in ‘Golden Delicious’ without affecting yield in EGVV. In Ceres, metamitron at 4 mm and 6-BA at 12 mm fruitlet size seemed to overthin ‘Golden Delicious’. In ‘Fuji’, none of the treatments were effective in EGVV. In Ceres, all treatments were effective in decreasing the hand thinning requirement.  Two metamitron applications, metamitron followed by 6-BA as well as NAA followed by NAA and 6-BA followed by carbaryl thinned very aggressively.

Metamitron and/or 6-BA (750 ml) at 4 mm fruit size appeared to be effective apple chemical thinners. Double metamitron applications or metamitron followed by 6-BA may result in excessive thinning. More research is required to evaluate combinational use of the different thinning agents