Apple Rootstock Response to Prohexadion Calcium and SPAD Readings in a Non-cropping Year

Application of prohexacione calcium (PC) beginning shortly after petal fall can prevent excessive vegetative growth. Recommended application timing is 2.5 to 7.5 cm shoot length.  There is little information about application at later stages The study block consisted of 10^th leaf ‘Golden Delicious’ trees grafted onto M.9 T337, M.26 EMLA, and G.16. Applications of PC were made on 30 May, 13 June, and 26 June at rates of 125 ppm and 188 ppm with a handgun to drip. At the time of first application average shoot growth was 21, 20, and 18 cm for M.26, M.9, and G.16, respectively. Two weeks after the first application shoot growth was significantly reduced for all rootstocks for both rates of PC compared to the control. Shoot growth by rootstock was significantly lower for trees on G.16 than on M.9 or M.26. After the subsequent applications (13 May and 26 June) there was a significant rootstock x 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. Rootstock also influenced total shoot growth at the end of the season.