Prohexadione-calcium suppression of apple tree shoot growth as affected by spray additives

Technical grade probexadione-calcium (93.2% a.i. P-Ca) applied to 'Fuji'/M.9 trees in three applications in deionized water reduced shoot growth by 25%, but the addition of (NH4)(2)SO4 to P-Ca suppressed shoot growth by 47%. If P-Ca was mixed in well water (high in calcium salts), P-Ca did not suppress shoot growth at all. The commercially formulated prohexadione-calcium [Apogee: 27.5% P-Ca + 56.1% (NH4)(2)SO4 + 16.4% other proprietary additives] + Regulaid in well water (high calcium) was not as effective (reduced growth by 30%) as when additional (NH4)(2)SO4 was added (reduced growth by 53%), and if CaCl2 (used to control corking) was tank mixed with Apogee + Regulaid, the Ca++ interfered with the growth suppression of P-Ca. If (NH4)(2)SO4 was added at the same rate as CaCl2 (w/w), the Apogee growth suppression was completely restored (reduced growth by 50%). Choice (a commercial water conditioner that has (NH4)(2)SO4 in the formulation, among other ingredients) + Li-700, or (NH4)(2)SO4 + Silwet L-77, or (NH4)(2)SO4 + Silwet L-77 + Oil were among the most effective adjuvant combinations with Apogee. The addition of ethephon at 270 mg(.)L(-1) improved the growth suppression of Apogee + (NH4)(2)SO4 + Regulaid. Solubor compromised the effectiveness of Apogee + Regulaid. Adjusting the pH of the Apogee + (NH4)(2)SO4 + Regulaid spray to either pH = 4 or pH = 9 did not affect efficacy. The combination of Apogee + (NH4)(2)SO4 + Regulaid caused increased fruit cracking of 'Empire' fruit as compared to the control (7%), presumably due to increased absorption of P-Ca. Chemical names used: Prohexadione-calcium (P-Ca, 3-oxido-4-propionyl-5-oxo-3cyclohexenecarboxylate) formulated as BAS-125 (10% P-Ca); Apogee (27.5% P-Ca), or Technical 93.5% P-Ca); Regulaid (polyoxyethylenepolypropoxypropanol, alkyl 2-ethoxethanol, and dihydroxy propane); Silwet L-77 (polyalkyleneoxide modified heptametyltrisiloxane, silicon surfactant), LI-700 (80%, phosphatidylcholine, methylacetic acid and alkyl polyoxyethylene ether); Superior Oil (Drexel Damoil 70-second delayed dormant spray oil); ethephon (2-chloroethyl phosphonic acid); Solubor (20.5%, Boron equivalent); captan (N-Trichloromethylthio-4-cyclohenene-1,2-dicarboximide).