European and USA researchers devoted much R&D effort in the 1990s to testing the efficacy of endothallic acid (TD 2337-2 or ‘Endothall’) as a blossom thinner for apples.
- The chemical is a 5% (w/w) active ingredient formulation of dipotassium 7-oxobicyclo (2,2,1) heptane-2,3,-dicarboxylate and it is approved for use in some countries as an aquatic weedkiller.
In trials at East Malling, it was tested as a blossom thinner for Royal Gala and Queen Cox at rates of 500, 1000 and 2000 ppm in high volume (1000//ha) sprays (Webster and Spencer, 1999; Webster and Spencer, 2000).
- Most of the treatments had some thinning effect and the thinning severity was related to spray concentration.
- In the first year of treatment, the sprays increased grade-outs of large size fruits.
- However, although treatment of the same trees in the second year also reduced fruit set, no improvements in final fruit sizes were achieved.
- This was explained by the large differences in flower abundance on the trees at the start of the second year.
- Treatments in the first year increased flower density in the second year very significantly, whilst overcropping of the unthinned controls in the first year reduced flower density in the subsequent season.
- Although the sprays reduced percentage fruit set in both years, this was insufficient in the second year to compensate for the large increase in flowering on the previously treated trees.
- In these situations two or more sprays of a chemical thinner may prove necessary in seasons or situations of high flower abundance.
No phytotoxicity to leaves or fruits was observed in UK trials. However, phytotoxicity and variable responses were recorded in USA trials with this product (Andrews and Collier, 1995).
Although proven quite effective as a blossom thinner in European trials, work on endothallic acid has now ceased.
It was believed that gaining clearance for its use as a blossom thinner would prove too difficult.
Endothallic acid (Endothall) is not approved currently for use as a flower thinner in UK orchards.