Effectiveness of the resistance inducer prohexadione‐Ca against fireblight in shoots of apple trees inoculated with Erwinia amylovora¹

Abstract

Due to the lack of effective, non‐phytotoxic and publicly acceptable materials for controlling fireblight in pome fruit trees, novel strategies against Erwinia amylovora are being sought. Resistance‐inducing compounds, such as prohexadione‐Ca, represent promising alternatives. Prohexadione‐Ca is the active substance of the bioregulator Regalis, currently being introduced in several European countries and overseas. Prohexadione‐Ca reduces shoot elongation due to inhibition of gibberellin biosynthesis. Furthermore, it leads to significant changes in the spectrum of flavonoids and their phenolic precursors in pome fruits, which causes reduced susceptibility to fireblight and other pathogens. In 2002 and 2003, container‐grown apple trees of the cultivars ‘Idared’ and ‘Freedom’ were treated with different dosages of prohexadione‐Ca two weeks before inoculation with E. amylovora. The effect of prohexadione‐Ca against shoot blight was determined by measuring the lengths of necrotic lesions and symptoms on vascular bundles caused by the pathogen. Treatments with prohexadione‐Ca turned out to be much superior to the ones with streptomycin, kasugamycin and a bacterial antagonist, which were used for comparison. Acibenzolar‐S‐methyl (Bion), another resistance‐inducing compound, was included in some of the experiments and gave intermediate results. The simultaneous control of excessive shoot growth and shoot infections by fireblight is seen as a major advantage of using prohexadione‐Ca in pome fruit trees.

Using Molecular Tools To Decipher the Complex World of Plant Resistance Inducers: An Apple Case Study

“…It is noteworthy that the most efficient products (C, G, and H) displaying protective effects together with defense elicitation are synthetic inputs dealing with phytohormone signaling pathway. Although the effects of product C (acibenzolar-S-methyl) and G (prohexadione-Ca) were expected, 13,36 the activity of product H gave interesting prospects. This preparation is an auxin analogue (naphthaleneacetic acid) used as a thinning agent in apple orchards.…”

The Effect of Repeat Annual Applications of Prohexadione–calcium on Fruit Set, Return Bloom, and Fruit Size of Apples

“…Prohexadione-calcium (ProCa) has emerged as the primary plant growth regulator used to control vegetative growth on apples (Basak, 2004;Byers et al, 2004;Greene, 1999;Medjdoub et al, 2005;Miller, 2002;Rademacher and Kober, 2003;Unrath, 1999). It is also very effective in the control of fire blight in the shoot phase of this disease (Buban et al, 2004;Norelli and Miller, 2004;Rademacher, 2004;Rademacher and Kober, 2003;Yoder et al, 1999). The timings for the first application of ProCa to retard vegetative growth and to control fire blight are the same.…”

Prohexadione‐Ca induces resistance to fireblight and other diseases1

“…It was also found that trees treated with prohexadione‐Ca were less infested with insect pests, such as aphids (Krawczyk & Greene, 2002), Empoasca fabae (Leahy et al. , 2002) and Cacopsylla pyri (Bubán et al. , 2004; D. Bylemans, Gorsem (BE), P. François, BASF (FR), pers.…”

“…Additional trials revealed that similar effects could also be induced in other, but not all, host-pathogen-combinations (Bazzi et al , 2003a,b; Table 1). It was also found that trees treated with prohexadione-Ca were less infested with insect pests, such as aphids (Krawczyk & Greene, 2002), Empoasca fabae (Leahy et al , 2002) and Cacopsylla pyri (Bubán et al , 2004; Bylemans, Gorsem (BE), P. François, BASF (FR), pers. comm.…”