McIntosh is the most important apple cultivar in the Northeastern USA. It is a high ethylene producing variety which has excessive pre-harvest drop. Aminoethoxyvinylglycine (Retain) which blocks ethylene synthesis limits pre-harvest drop and has become an essential tool in managing harvest of McIntosh in the Northeastern USA (Greene and Schupp, 2004; Robinson et al., 2006). In addition to reducing preharvest fruit drop, Retain also delays fruit color development which is a negative side effect for many growers (Robinson et al., 2006). Thus, many fruit growers apply less than the labeled rate or apply the spray close to the normal harvest date to minimize the negative impact on fruit color but often this results in less than satisfactory pre-harvest drop control.
The traditional use of NAA to control preharvest drop of McIntosh is often ineffective. This is likely due to the high ethylene production of this variety which stimulates ripening. Retain has proven to be a much better drop control material but the two limitations of Retain are that color development is delayed when a full rate is used and in hot years pre-harvest drop is inadequately controlled. This is likely caused by the incomplete control of abscission zone genes resulting in significant fruit drop. However, the combinations of Retain and NAA applied 2 weeks before normal harvest work synergistically and can control drop better than either chemical alone. Further a half rate of Retain (167 g/ acre) combined with 10-20ppm NAA was very effective in preventing drop and was very similar to the full rate of Retain but the drop control and did not last as long as a full rate. The best Retain+NAA treatment gave similar drop control as sprayable 1-methycyclopropene (Harvista).