Influence of prohexadione calcium (Apogee®) on shoot growth of non-bearing mature apple trees in two different growing regions

Orchard experiments were conducted on mature non-bearing apple ( Malus × domesticaBorkh.) trees to determine the efficacy of prohexadione-calcium (PC), formulated as Apogee ® [27.5% PC + 56.1% (NH 4 ) 2 SO 4 +16.4% ot her proprietary additives] for shoot growth control on six cultivars grown in Ontario (ON) and one grown in New Brunswick (NB), Canada. Seasonal patterns of extension shoot growth among cultivars in both locations were also compared. Results indicate that PC applications are most effective at the beginning of the season, when relative growth rates were greatest in eastern Canada. Four applications of PC failed to significantly reduce shoot growth more than two applications in either location. However, the level of control might have been more effective in ON if treatment applications were initiated earlier in the season. Although tree vigor and shoot growth differed between cultivars and locations, PC significantly and consistently reduced shoot growth and relative shoot growth rates for all cultivars at both locations. Empire shoots treated with PC were approximately 33 and 37% shorter at the end of the season in NB and ON, respectively. Key words: Calcium 3-oxido-5-oxo-4-propionylcyclohex-3-enecarboxylate, anti-giberellin, plant growth regulator, relative growth rate, Malus