|NORELLI, JOHN (JAY)|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 24, 2004
Publication Date: October 1, 2004
Citation: Norelli, J.L., Miller, S.S. 2004. Effect of prohexadione-calcium dose level on shoot growth and fire blight in young apple trees. Plant Disease. 87(7):756-765. Interpretive Summary: The purpose of this research was to develop a management strategy to control fire blight in young apple orchards using prohexadione-calcium (Phd-Ca). 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 there are conflicting requirements to control fire blight and allow sufficient tree growth for tree establishment and the utility of Phd-Ca in young orchards 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 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.
Technical Abstract: Prohexadione-calcium suppresses both shoot growth and fire blight in apple. In young apple orchards there are conflicting requirements to control fire blight and allow sufficient tree growth for tree establishment. Application of prohexadione-calcium to various cultivars of orchard-grown apple trees ranging in age from newly planted to fifth leaf trees indicated that fewer high-dose (125 or 250 mg·liter-1) applications of prohexadione-calcium provided a better balance between fire blight control and growth in young orchards than multiple low-dose (30 or 63 mg·liter-1) applications. The response of early season shoot growth to prohexadione-calcium treatment dose was linear. However, trees that received high doses of prohexadione-calcium 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 a few high or multiple low doses of prohexadione-calcium. Enhancement of fire blight resistance by prohexadione-calcium was correlated with shoot growth suppression at the time of inoculation and the resistance response to prohexadione-calcium treatment dose was linear. Fire blight management strategies that use prohexadione-calcium in young apple orchards are discussed.