Why does apple fruit drop prematurely?
There’s more going on than simply gravity when apples fall prematurely.
Unexpected apple drop just prior to harvest is a serious threat for some varieties grown in Michigan. We have some tools to help prevent premature apple drop such as NAA and Retain, but Michigan State University Extension will go over some of the reasons for premature drop. All apple cultivars have some fruit drop as they move through the ripening process. Some varieties, such as McIntosh, are very prone to pre-harvest fruit drop. This problem is exasperated when fruits are left to hang for better red color to meet market demands and fruit drop often occurs when waiting for red color to develop.
As apples begin to ripen they produce large amounts of ethylene, the ripening hormone. Ethylene stimulates softening of fruits and the formation of an abscission layer in the stem. Ethylene enhances the production of enzymes that break down the cell walls and the complex sugars that hold cell walls together in the abscission zone of the stem. As these glue-like substances break down, they leave the fruit connected only by the vascular strands, which are easily broken.
The role of ethylene is well understood by commercial apple growers. There are other stress factors that might come into play with pre-harvest apple drop and can be related to the severity of drop from one year to the next. These include orchard and climatic factors such as fruit load, nutrition imbalance, summer pruning, insect or disease issues, water and weather extremes during the growing season.
Fruit load. A large crop of a short-stemmed apple variety, particularly those that set in clusters, will “push off” each other close to harvest. Good, early season thinning, especially reducing clustered fruits, will help prevent this type of drop. When fruit are pushed off, it stimulates ethylene which can cause even more pre-mature drop in fruits remaining on the tree.
Tree nutrition and soil type. Drop is often worse in orchards where soils have incorrect nutrient levels, in particular low magnesium (Mg), high potassium (K) and high boron (B). Also, the variations in soil type can play a part. For instance, sandy areas will ripen early and drop ahead of heavier soil types.
Summer pruning. Pre-harvest drop can be more severe in orchards that are heavily summer-pruned. It is thought that this problem is likely associated with a limitation or deficit of carbohydrate supply from too many leaves being removed, especially younger, more functional leaves. Drop will be increased if pruning reduces the leaf to fruit ratio below 20:1.
Insects and mites. When leaf-infecting insects are high in number, they can reduce the photosynthate produced by leaves. This limits carbohydrate availability and can lead to pre-mature fruit drop.
Water availability. Pre-harvest drop is more severe in dry seasons, where irrigation is not available.
Growing season temperatures. Some apple varieties are affected by hot temperatures more than others, particularly in the early formation of ethylene which promotes early drop.
Harvest season weather and cultivar characteristics. Windy weather close to harvest also impacts fruit drop and can be worse in some varieties, especially those naturally prone to drop. Below is a summary of some varietal characteristic when it comes to drop.
- Arlet (Swiss Gourmet)
- Autumn Gold
- Early Golden
- Golden Delicious
- Golden Supreme
- Red Delicious
Alone, each of these factors can influence pre-mature drop to some degree, however, when they occur in combinations, severe drop can be the result. This is especially true in very drop-prone varieties such as McIntosh. Every grower knows his or her own blocks best including those that tend to have a history with early drop. Perhaps looking a little more closely at some of the other factors mentioned above can also help prevent early apple drop from occurring.
For more detailed reading, read the following sources:
- Factors Affecting Preharvest Fruit Drop of Apples, Daniel Lee Ward, Virginia Polytechnic Institute and State University
- The Physiology of Apple Pre-harvest Fruit Drop, Terence Robinson, Cornell University