Walnuts and Almonds

walnuts and almondOne of the biggest problems walnut growers struggle with is Pistillate Flower Abscission or Abortion (PFA). Occurring during bloom, PFA is the loss of nut-producing pistillate (female) flowers due to an excessive pollen load on them. That pollen load stimulates production of ethylene, causing flower drop. With PFA, flowers stop growing at the 2-3 mm size and fall about 10 days after bloom, leading to reduced fruit set and low yields. A naturally occurring compound isolated from a soil microorganism, called aminoethoxyvinylglycine (AVG — the active ingredient in ReTain® plant growth regulator), will inhibit the production of ethylene that causes PFA during bloom.

ReTain is the first commercial tool to combat PFA with AVG. It’s particularly effective on the Serr variety of walnuts, where yields can be reduced by as much as 90% by PFA. PFA can reduce yields in several other popular varieties as well, including Chandler, Hartley and Tulare.

The use of ReTain on walnut orchards with PFA problems offers significant return on investment because it delivers reduced PFA pressure, increased fruit set, increased yield, improved balance between nut production and tree canopy management, and increased net returns. In fact, it may be appropriate for growers to adjust their fertilizer and nutrition programs and to prune trees appropriately in order to support the higher crop load that will result when PFA is reduced.

Growers should follow a number of application guidelines when using ReTain.

Keeping Pollen In Check

Researchers have conducted lab and field studies to see if removing pollen sources will reduce pollen “overload” – and stop PFA. They found the approach wasn’t viable for a number of reasons, including the limited window of time available to shake all trees; the fact that not all growers have shakers; multiple shakes are needed to remove all catkins, and pollen from neighboring orchards is still present.


They should time first application as soon as 5-30%, with a second at 40-60%, of the female flowers bloom (reach peak receptivity). A calibrated sprayer should travel at 2.4 to 3.2 km/hr (1.5 to 2.0 mph) and make sure to adequately cover all flowers and foliage. Spray volume should be at 1,000-2,000 L / Ha (100-200 gal/acre) depending on tree size and historical PFA incidence. Adjuvants are not necessary, and ReTain should not be used in any tank mixes. It has a non-toxic mode of action and will not alter the ecological balance in orchards.