Tree Fruit Leader, Vol. 5(1) April 1996

1995 Spray Thinning Research

by Mike Sanders, Tree Fruit Specialist BCMAFF, Kelowna

The following is a summary of 4 different spray thinning trials done in 1995. 

Fuji 

Quite a bit of effort was spent on two Fuji projects because of the biennial problem with this cultivar. 

In the first Fuji trial, 12 treatments using various combinations of single application ammonium thiosulphate (ATS), double ATS applications, Sevin and NAA were tried. The only treatment resulting in significant thinning was 2 applications of ATS at 1.6 litres/100 litres applied 4 days apart. 

The second Fuji trial tested 8 treatments using various combinations of ATS, Ethrel and Sevin. The most thinning was caused by ATS alone, Sevin alone and Ethrel plus Sevin. 

Evaluation of return bloom has not been done by time of writing. 

Based on these results and the timing discussed by Mr. Komatsu, I would suggest 1 or 2 applications of ATS directed at the lateral flowers on 1 year old wood and Sevin directed at the side blossoms of the terminal fruit buds. You might even decide to try 2 Sevin applications - one near petal fall and the other when fruit diameter is 10 to 12 mm. Remember the temperature requirements for Sevin. 

Early and Late Sevin Applications 

On Spartan, Sevin applied at petal fall was compared to Sevin applied 16 days from full bloom and a check treatment. 

The Sevin treatments were almost identical in effect and showed significant thinning over the check treatment. The petal fall application resulted in 121 fruit per 100 fruit buds, the later application 115 fruit per 100 fruit buds whereas the check treatment resulted in 177 fruit per 100 fruit buds. Neither Sevin treatment removed enough fruit, however . This points out that Sevin should not normally be used as a single treatment but rather as part of a thinning program. Hindsight is wonderful, but I should have included another treatment combining both the petal fall and later Sevin treatments. 

Ammonium Thiosulphate (ATS) 

Various rates and volumes of ATS were tested on Mclntosh, Spartan, Golden Delicious and Red Delicious. For the fourth year running I have failed to show that concentrate applications (at 18 litre and 24 litre per acre) are as effective as dilute applications. My general recommendation remains the same, 1.6 litres per 100 litres water applied to the point of run-off for most varieties and 1.2 litres per 100 litres water for easy to thin varieties such as Gala and Jonagold. Aggressive thinners may wish to try 2.0 litres per 100 litres water on blocks that have shown inadequate thinning with the 1.6 litres per 100 litres rate. 

The 1995 trials offered the opportunity of seeing what happens when ATS is applied to wet foliage. The result is leaf burn and excessive thinning. Do not apply ATS to wet leaves or under slow drying conditions.

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