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Influence of rootstock on response of 'Golden Delicious' apple trees to applications of prohexadione calcium

Crassweller, R. M., Smith, D. E.
Acta horticulturae 2017 no.1177 pp. 107-110
Malus domestica, chlorophyll, crop losses, frost, growing season, leaves, prohexadione calcium, rootstocks, trees
The 2012 growing season was characterized by an early bloom followed by two severe frost events that resulted in a total crop loss. Prohexadione calcium (PC) was applied when average shoot growth was between 17 and 23 cm in length. The trees in this study consisted of 10th leaf 'Golden Delicious' trees grafted onto M.9 T337, M.26 EMLA and G.16. Applications of PC were made on May 30, June 13 and June 26 at rates of 0, 125 and 188 ppm with a handgun to drip. Shoot length was not significantly reduced until after two applications of PC were made compared to the control. After the subsequent applications (May 13 and June 26) there was a significant rootstock × treatment interaction. G.16 trees receiving the 125 ppm treatment had significantly less growth than the control while those receiving the 188 ppm were not significantly different from the control. For the other two rootstocks the increase in growth was inversely related to the PC rate. Chlorophyll level as measured by SPAD meter was significantly lower for trees on G.16 rootstock. Shoot growth among rootstock was significantly lower for trees on G.16 than on M.9 or M.26 at the end of the season.