|Norelli, John (jay)|
Submitted to: Acta Horticulturae
Publication Type: Proceedings
Publication Acceptance Date: July 1, 2004
Publication Date: February 10, 2006
Citation: Norelli, J.L., Miller, S.S. 2006. Using prohexadione-calcium to control fire blight in young apple trees. Acta Horticulturae. 704:217-223. Technical Abstract: Prohexadione-calcium (Phd-Ca) is a plant growth regulator that suppresses shoot growth and fire blight in apple. In mature orchards, Phd-Ca is effective in managing fire blight. However, in young apple orchards where there is a need to control fire blight and allow sufficient tree growth for tree establishment, the utility of Phd-Ca was unclear. When Phd-Ca (Apogee) was applied to orchard-grown apple trees ranging in age from newly planted to fifth season of growth (4-year-old orchards), it was found that two applications of 125 mg liter-1 Phd-Ca provided a better balance between fire blight control and growth in young orchards than three or more applications of 63 or 30 mg liter-1. Although the high rate of Phd-Ca suppressed early season shoot growth more than the lower rates, trees that received the high rate of Phd-Ca tended to grow more in the latter part of the season, resulting in little or no difference in total seasonal growth between trees that received two high or three low rate applications of Phd-Ca. Fire blight control with Phd-Ca required shoot growth suppression early in the growing season, and 125 mg liter-1 Phd-Ca often provided significantly better fire blight control than treatment at lower rates. Poor fire blight control occurred when the rate of Phd-Ca was lowered sufficiently to allow greater early season growth. The results indicate that one to two Phd-Ca applications at 125 mg liter-1 can be used to manage fire blight in the fourth to sixth season of growth when there is a high risk of shoot blight.