Evaluation of Retain for Reducing Pistillate Flower Abortion and Increasing Yield in ‘serr’ Walnuts

Pistillate flower abortion (PFA) is the loss of nut-producing pistillate flowers early in the season, typically 2 to 3 weeks after bloom. PFA is the principal factor limiting productivity in ‘Serr’ walnut while affecting other cultivars to lesser and varying degrees. Research by Vito Polito at UC Davis showed that excessive pollen tubes growing down the style of the walnut pistillate flower produce a surplus of ethylene that triggers flower abscission. Valent BioSciences developed ReTain, a naturally occurring plant growth regulator product produced by fermentation. The active ingredient, AVG (aminoethoxyvinylglycine hydrochloride), temporarily reduces ethylene production by blocking a key enzyme (ACC synthase) in the biosynthesis pathway. Preliminary single limb spray trials conducted by UC Farm Advisor Bob Beede in 2003 showed a substantial reduction in PFA from 79% to 11% with a single early bloom application. An in vitro study conducted by Polito in 2004 (2005 Walnut Research Reports) showed that ReTain reduced ethylene levels in pollinated walnut pistillate flowers. Further whole tree spray trials by Beede and Grant in 2004 and 2005 showed reductions in PFA between 19% and 52% with attendant yield increases ranging from 0.4 to 1.2 tons per acre. These results prompted the addition of walnuts to the ReTain use label in 2006 for increasing nut set.