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Field experiment was carried out during two successive seasons of 2012 and 2013 on Washington Navel sweet orange cultivar (Citrus sinemsis, L) onto sour orange rootstock (C. aurantium , L) grown in well drainage clay loam soil of a commercial orchard located in Shiwah Valley (30°51'55.78"N, 31°16'23.14"E), near Aga city, Dakahleia Governorate, Egypt. The investigation was designed to throw some light on the effect of two natural biostimulants, i.e., Milagrow at 10, 15 and 20 ppm (extracted from pollen grains of rape seed (Brassica napus) as a source of brassinolide phytohormone) and yeast extract at 1000, 1500 and 2000 ppm on fruit set, fruit drop, yield and fruit quality of Washington Navel orange trees. The natural biostimulants were tested as foliar sprays at full bloom stage and one month later. Data indicated that, all biostimulants treatments increased fruit set, yield and fruit quality and decreased fruit drop as compared with control treatment. Data also revealed that, foliar sprays of trees by Milagrow at 15 and 20 ppm were superior for inducing the highest increase of fruit set and yield, in addition yeast extract at 2000 ppm comparing with rest concentrations of both tested biostimulants. On the other hand, no significant differences were observed among all tested concentrations of two natural biostimulants with respect to fruit weight in two seasons. The foliar applications of Milagrow and/or yeast extract increased fruit SSC and reduced acidity contents in fruit juice as compared with control treatment. Moreover, it was noticed that trees sprayed with two tested biostimulants gave the best results for reducing fruit drop. It could be recommended that best treatment for increasing yield and improving fruit quality was spraying Washington Navel sweet orange cultivar with Milagrow at 20 ppm and yeast extract at 2000 ppm during full bloom stage and one month later especially in the same conditions to experimental area.

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