AVG Combined with NAA Control Preharvest Drop of ‘Mcintosh' Apples Better than Either Chemical Alone

In some years aminoethoxyvinylglycine (AVG, trade name Retain) imperfectly controls pre-harvest drop of ‘McIntosh’ apple. These years are warm with daytime temperatures over 35 °C (95 °F) in August.  In 2008–2011, we evaluated whether the combination of AVG and napthaleneacetic acid (NAA) would give better preharvest drop control of ‘McIntosh’ apple than AVG or NAA alone.  AVG (832g Retain/ha or 416g Retain/ha) was applied 3, 2 or 1 week before normal harvest.  At either the 2 or 1 week before normal harvest, a combination of AVG and 20 mg·L^-1 NAA was also applied. Pre-harvest fruit drop from untreated control trees exceeded 20% by September 21 and by the end of September had reached 60% drop.  The traditional drop control treatment of 20 mg·L^-1 NAA applied alone on September 7 did not reduce drop at any date.  AVG applied alone reduced fruit drop if applied 3 or 2 weeks before normal harvest but not when applied 1 week before harvest.  The addition of 20 mg·L^-1 NAA to the AVG sprays, either 2 or 1 week before harvest, improved pre-harvest drop control compared to AVG alone.  The combination spray applied 2 weeks before harvest resulted in the lowest pre-harvest drop of any treatment.  However, if AVG+NAA was applied 1 week before harvest then its efficacy was reduced but was still similar to AVG alone applied 3 or 2 weeks before harvest. When a reduced rate of AVG (416g/ha) was combined with NAA, the efficacy in preventing drop was very similar to the full rate of AVG. A hypothesis to explain the improved drop control of McIntosh apples from combining NAA and Retain is: when NAA is used alone to control pre-harvest drop, it controls the genes associated with abscission zone formation but as a negative side effect it stimulates ethylene production which advances ripening including color formation and softening.  With McIntosh, the high production of ethylene often overwhelms the stop drop effect of NAA. In contrast, Retain acts by controlling ethylene biosynthesis but in hot years, pre-harvest drop of McIntosh is not adequately controlled indicating that abscission zone genes are not totally under the control of ethylene and that Retain does not control these genes adequately under stress conditions. When NAA and Retain are sprayed together, NAA controls the genes associated with abscission better than Retain while Retain blocks the production of ethylene caused by NAA.