Tree Fruit Leader, Vol. 2(1) Feb. 1993

Ammonium thiosulphate (ATS) - a fertilizer that thins

Mike Sanders, P.Ag., Tree Fruit Specialist BCMAFF, Kelowna and Norm Looney, Pomologist Agriculture Canada Research Station, Summerland

ATS (Amthio; Allied Chemical Company, Houston, TX) is a liquid fertilizer containing 12% nitrogen and 26% available sulphur in a commercial product containing between 58 and 60% ammonium thiosulphate.

When applied as a spray to apples at full bloom it damages flower parts in much the same manner as Elgetol. Thus, it has clear potential use as a chemical thinner even though it is not specifically registered for that purpose.

Byers (Virginia) and Looney (B.C.) previously examined the potential of ATS as a blossom thinner for peaches. The results were not spectacular. Looney had better success with apricot in tests starting in 1987, and based upon this experience he distributed product to several packinghouse fieldmen to test on apples in 1991. The results of these preliminary trials were encouraging and additional trials were conducted in 1992.

Various rates and application methods have been examined, but the most suitable seems to be a 1% solution (1.6 l of product per 100 l of water) applied as a dilute spray. However, proper timing may prove to be as important as rate in achieving selective blossom removal with ATS. The theory is the same as for Elgetol. Applications made after full bloom will under-thin if pollination conditions have been fully satisfactory. Sprays ahead of full bloom will lead to more of the crop on side bloom and the total amount of thinning will likely be inadequate. While as yet untested, double spraying may prove useful in years where the blossom period is longer than normal.

The following is a summary of the results of trials involving ATS blossom-time sprays.

In 1991 Duane Holder of B.C. Fruit Packers conducted a replicated trial involving five cultivars and three orchards. He showed that 1% ATS applied as a dilute spray at full bloom and shortly afterwards substantially reduced initial fruit set (counts made before the June drop), especially on McIntosh. The cultivars treated at full bloom were McIntosh and Fuji, and Gala and Braeburn were treated within two days of full bloom.

ATS was virtually without effect when applied ahead of full bloom to Jonagold. Close observation of the treated trees later in the summer revealed no adverse effects on fruit or foliage.

Bill McPhee of the Okanagan-Similkameen Coop used 1% ATS dilute sprays on Delicious and Golden Delicious and observed flower thinning results virtually indistinguishable from those obtained with Elgetol. Some foliar injury was observed when spray concentrations of ATS exceeded 1%.

A 1992 trial by Looney and Beulah compared ATS with Elgetol and Wilthin (monocarbamide dihydrogen sulphate; D-88; Unocal Corporation, Los Angeles, CA) for blossom thinning McIntosh apple. They applied all chemicals just before full bloom and handgun (dilute) and airblast (concentrate) spray equipment was compared. Handgun and airblast sprays with water served as the controls. Field notes taken shortly after spraying indicated that ATS and Elgetol reduced initial set more effectively than Wilthin. However, actual fruit set counts were made after completion of the June drop, at which time no significant differences were found. The heavy June drop on the Wilthin and water control trees evened things out.

Measurements of average fruit weight at the time of harvest support the contention that ATS and Elgetol reduced early crop load more effectively than Wilthin or water. Also, based on fruit weight at harvest, the method of application (dilute vs. concentrate) influenced fruit thinning effectiveness only in the case of Wilthin. Wilthin applied by handgun resulted in larger fruit size at harvest than when applied by airblast sprayer.

The effects of these treatments on return flowering will be recorded in 1993 and should provide further indirect evidence about which treatments most effectively reduced early fruit numbers.

The largest fruit came from ATS-treated trees and it could be argued that the nitrogen fertilization component of ATS contributed to this result. The airblast application added nearly 20 g of N per tree whereas the handgun spray added about 7.5 g of N. This question will be addressed in future experiments. It will be especially important to know if fruit colour and firmness can be adversely affected by ATS foliar sprays.

Two other 1992 trials were conducted at the BCFG Test Orchard by Mike Sanders. In the first, involving 10-year-old Gala trees, all chemical solutions were applied to run-off with a handgun sprayer. Sprays were applied at full bloom. Both ATS and Wilthin reduced final fruit set and increased fruit diameter at harvest. A rain shower within an hour of treatment did not appear to reduce effectiveness. Indeed, it may have increased the amount of thinning that occurred since final fruit set was below optimal.

The second trial at the Test Orchard involved an 8-year-old Spartan/M4 block with severe frost damage to blossoms in 1992. The aim of the test was to assess foliar damage from concentrate airblast sprays of ATS applied during the bloom period. The concentrations of ATS compared were: 1% ATS applied to run-off and 2%, 4%, 6% and 8% applied in 1135 l of spray solution per ha (100 gal/acre). Elgetol at 15 l per ha in 1135 l of water (6 l per acre with 100 gallons of water) was another internal control. Rates were adjusted for tree row volume. Conditions were dry following all sprays.

Based on visual assessments of phytotoxicity, the dilute (1%) ATS treatment resulted in more leaf injury than the 2% and 4% ATS airblast sprays and was about equal to that observed from the Elgetol and the 6% ATS treatments. None of this damage was considered significant. The highest concentration of ATS (8%) resulted in an unacceptable level of leaf damage.

All in all, it appears that Amthio fertilizer applied to run-off in a spray containing 1% ATS results in flower thinning and larger fruit size at harvest. When applied as a concentrate spray it appears to require about the same degree of concentration as has been historically used for Elgetol, in the range of 6X to 12X, depending on the cultivar and the amount of chemical applied per acre or hectare will be higher than for a dilute spray. Since even a dilute spray will add a meaningful amount of nitrogen to the orchard environment this fertilization effect must be considered when planning the annual orchard nutrition program.

As with any new technology it must be stressed that there are still many un-answered questions. As we gain experience with various cultivars and various orchard conditions, growers may find that ATS will play an increasingly important role in British Columbia apple production. It has the potential to address important fruit tree nutrition and crop regulation objectives with a single blossom time spray.

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