Polar — a potent Polyoxin B compound for controlling powdery mildews in apple and nectarine trees, and grapevines

Abstract

The efficacy of Polar, a polyoxin B compound, in controlling powdery mildews was evaluated in commercial orchards and a vineyard in 1997–1999 in the Golan region of Israel. Foliar sprays of 0.0125–0.05% (v/v) suspensions of Polar [1-{5-N-(5-O-carbomonyl-2-amino-2-deoxy- l-xylonyl)-5-amino-5-deoxy-β- d-allofur anosyl uronic acid}-5-hydroxymethyluracil], sulfur, the new strobilurin fungicide Kresoxim-methyl, and sterol inhibitor (SI) fungicides inhibited the development of the powdery mildew fungi on leaves, fruits and leaves, and fruit clusters of field-grown apple trees, nectarine trees, and grapevines, respectively. The effectiveness of polyoxin B in controlling powdery mildews on apple trees and grapevines was similar to that of the standard treatment with the SI fungicides or sulfur. Applications of polyoxin B on nectarine trees provided better control of Sphaerotheca pannosa (Wallr.) Lev. than did the SI fungicides. Alternating treatment of polyoxin B with the SI fungicide, myclobutanil enhanced the inhibitory effect against Podosphaera leucotricha on apples and provided the best control of powdery mildew. These results indicate that the use of polyoxin B enables the number of SI fungicide treatments against powdery mildew to be reduced by up to 50%. Application of polyoxin B was not phytotoxic to plant tissue, and was more effective against mycelial growth and sporulation of powdery mildew fungi than against germination of conidia. At concentrations of 1, 10 and 100 mg/l polyoxin B inhibited germination of Uncinula necator conidia, by 52.5, 76.6 and 100%, respectively. A foliar spray (100 mg/l) suspension of Polar on greenhouse-grown grapevine leaves before inoculation with U. necator provided >90% protection against powdery mildew development, while spraying on mildewed leaves bearing sporulating colonies of U. necator suppressed the fungus. The inhibitory effectiveness of polyoxin B makes it a good candidate for the control of powdery mildews on grapevines, apple, and especially on nectarine trees.