Chemical thinning of pears

Chemical thinning trials have previously been conducted on ‘Early Bon Chretien’, ‘Forelle’ and ‘Rosemarie’.  The results on ‘Forelle’ have not been very promising, but very good results have been achieved with the other two cultivars (the MSc thesis of Tinashe Chibikwa is available online).  ‘Abate Fetel’ was not included in the research since attaining good fruit set has proved to be major problem.  It is only in recent years that consistently heavy set has been achieved.  Hence, the original objective of this project was to develop a chemical thinning programme for ‘Abate Fetel’ that would minimize hand thinning and result in regular, good quality yields. The scope of the project was subsequently expanded upon request from the Crop Production TAC to include ‘Forelle’

Chemical thinning agents such as naphtalene acetic acid (NAA; Planofix^TM or Pomaxa^TM), 6-benzyl adenine (6-BA; MaxCel^TM), metamitron (Nevis^TM) or a combination of these chemicals were compared from the 2011/12 to 2015/16 seasons on ‘Abate Fetel’ (all 5 seasons) and ‘Forelle’ (from 2014/15) pear trees. For the 2014/15 season, the Crop Production TAC advised the evaluation of multiple Nevis applications or application of Nevis in combination with other thinning agents to try to increase the thinning effect that in previous seasons varied from no to slight thinning. Treaments were applied using a motorised backpack sprayer.

The chemical thinning treatments were generally ineffective in thinning ‘Abate Fetel’ and ‘Forelle’ pears. In both 2011/12 and 2012/13, the timing of the spray treatments was not optimal due to weather conditions. During the 2013/14 season, 11 ml NAA at petal drop and 750 ml MaxCel 7 days later as well as Nevis (600 ml/1000L/ha) at 14 mm fruitlet diameter had a slight chemical thinning effect in ‘Abate Fetel’, but there was still a significant hand thinning requirement and fruit quality was not affected.  Chemical thinning with Nevis (600 ml/1000L/ha) at 4 mm as well as with Nevis at 4 mm and 12 mm fruitlet diameter increased fruit size in ‘Forelle’ in 2014/15 without a discernible thinning effect. Return bloom was not affected. In 2015/16, all the chemical thinning treatments decreased fruit size of ‘Abate Fetel’ despite not affecting yield compared to the handthinned control. Treatments did not affect return bloom.  Chemical thinning treatments did not affect the hand thinning requirement, yield, fruit size or return bloom compared to the control in ‘Forelle’ in 2015/16.

In agreement with the earlier research of T Chibikwa and K Theron, chemical thinning was generally ineffective in ‘Forelle’. Chemical thinning was only (slightly) effective in one season out of five in ‘Abate Fetel’.  Climatic conditions preceding, during and after treatment application may have affected results in two seasons.  However, seen holistically, our research does not suggest that the thinning treatments applied, i.e. NAA, 6-BA, metamitron and combinations of these chemicals) are effective in thinning ‘Forelle’ and ‘Abate Fetel’ pears. Nevis application did increase fruit size in one season in ‘Forelle’, but the risk of phytotoxicity warrants caution and necessitates further research.