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

The efficacy of Polar, a polyoxin B compound, in controlling powdery mildew s was evaluated in commercial orchards and a vineyard in 1997-1999 in the G olan region of Israel. Foliar sprays of 0.0125-0.05% (v/v) suspensions of P olar [1-{5-N-(5-O-carbomonyl-2-amino-2-deoxy-L-xylonyl)-5-amino-5-deoxy-bet a-D-allofur anosyl uronic acid}-5-hydroxymethyluracil], sulfur, the new str obilurin fungicide Kresoxim-methyl, and sterol inhibitor (SI) fungicides in hibited the development of the powdery mildew fungi on leaves, fruits and l eaves,and fruit clusters of field-grown apple trees, nectarine trees, and g rapevines, respectively. The effectiveness of polyoxin B in controlling pow dery mildews on apple trees and grapevines was similar to that of the stand ard 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 Podos phaera leucotricha on apples and provided the best control of powdery milde w. 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 t han 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 provid ed > 90% protection against powdery mildew development, while spraying on m ildewed leaves bearing sporulating colonies of U. necator suppressed the fu ngus. 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. (C) 2000 Elsevier Science Ltd. All rights reserved.