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Foliar Boron Application to Decrease PFA, Increase Fruit Set and Yield in Walnut

In 1996-7, foliar B application dramatically reduced pistillate flower abortion (PFA) and significantly increased fruit set in several walnut cultivars and sites in California. In 1998 foliar application of B to walnut trees had little to no effect on reducing PFA. This year boron was applied to large uniform Serr orchards at four field sites (Stockton, Visalia, and two sites near Modesto) at either (1) early catkin stage ; (2) two weeks after nut set ; (3) early catkin and two weeks after nut set. Extensive tagging and counting of flowers and nuts were conducted at each experimental site. Contrary to the previous years, this year results indicate that foliar application of B to walnut trees had no effect on reducing the incidence of PFA. The trees at all experimental sites exhibited PFA from around 10 to 25 %, and nut set more that 70%. These results suggest that in cases where PFA is low the application of B has no effect on reducing PFA. Considering the results from 1996, 1997 and 1998, it appears that walnut response to foliar boron application is proportional to the PFA incidence, while PFA is variable and strongly dependent on prevailing weather conditions.

Boron; foliar application; walnut

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