How Nut Trees Set Fruit
Most nut-bearing trees have male pollinators and female flowers, both called catkins. Female flowers produce nuts during the current season's growth and have to survive through that year's growth before the crop of nuts is set. It is important to know this bit of reproductive botany to fully understand how nuts can drop before maturity. There are several natural nut drops between mid-May's pollination through the end of August's fruit set and can be minimized by proper tree fertilization.
A Tree's Physical Condition
Poor tree health can cause premature dropping of nuts. A tree's health is often compromised because of inadequate nutrient uptake, is noticed and most evident during drought. Insect and disease infestations increase during these times of tree stress and can further degrade a tree's condition, especially if trees are growing in poor soil. Any early defoliation will cause nut drop and low quality fruit.
Weather Influence on a Tree's Nut Crop
Excessive rain during late spring - early summer pollination will cause inadequate pollination of female flowers. Those poorly pollinated flowers may produce a nut that will drop early or no nut at all. Sometimes, male pollen can mature either before or after the female flower is receptive and this condition is usually weather related. An extended drought during nut growth may also result in the dropping of tree nuts.
Remember, too, that mechanical injury to leaves and nuts from hail and wind may cause premature dropping.
Insects and Diseases of Nut Trees
Early pecan scab infection of young nuts will cause nuts to drop and is a major cause of pecan crop failure. Black Walnut is highly vulnerable to anthracnose and the disease is of much concern in commercial orchards. Leaf diseases in nut trees such as scab, scorch, mildew, blotch, brown spot, and downy or vein spot also may cause nut shedding.
The pecan nut casebearer probably causes more nut shedding than all other insects combined in pecan orchards. Codling moth causes significant premature nut drop in black walnut groves. Other insects such as black aphids, walnut caterpillar, shuckworms, stink bugs, and pecan weevils may cause early nut dropping.