While the so-called “June drop” of nuts in almond orchards may be the most visible to growers, the losses then are likely to be the lowest of three periods of crop loss that regularly occur in orchards.

David Doll, University of California farm advisor in Merced County, says since “the nuts have size” at that point, the growers spot the losses.

But Doll says there are greater losses at another stage during the second drop, caused by the loss of flowers that are not pollinated or fertilized. The June drop is triggered, he said, by resource competition.

In a blog, Doll explained the three periods of nut drop. He noted that not all flowers on a tree set a nut. The amount that does can range between 15-40 percent, with the average around 25 percent.

“The percentage varies year to year and is dependent on flower density, temperature at bloom and post-bloom, and tree health,” Doll said, adding that drought conditions lead to stress and malformed flowers and an increase in dropped flowers.

The first period of nut drop occurs shortly after bloom when defective flowers drop from the tree. The second occurs within a month or so after bloom which involves dropping pea-sized flowers which have not shed their jackets.

Larger developed nuts that may have been fertilized may also drop during the second period.

Almond pollination and fertilization can occur over a wide range of temperature. Doll said the ideal for pollen tube germination and growth is from 50-70 degrees Fahrenheit.

“Temperatures below or above (that) can slow or prevent development,” the farm advisor said.

“Of even more importance is that the flower only remains receptive for three or four days. Extreme temperatures, rain, or wind can impact flower receptivity, decreasing the nut set. Rain or excessive free moisture can also cause pollen grains to burst, preventing pollination.”

Doll said spur dynamics play a key role in fruit bud density and the ability for a flower to set. He pointed to work by researchers Sergio Tombesi, Bruce Lampinen, Samuel Metcalf, and Ted DeJong which indicated that a fruiting spur, if maintained in a position with ample light for photosynthesis, tends to alternate bear.

The researchers tagged hundreds of spurs and found that spurs may flower the year after cropping but rarely set a nut due to carbohydrate and nutrient depletion within the spur.

“Surprisingly,” Doll said, “tagged spurs that double or triple crop die, regardless of light position.”

Therefore, orchards with a high set percentage deplete the spur pool, leading to a reduced set in the following year. But most orchards are able to re-develop spur positions which lead to sustained yields.

Doll said grower practices come into play in developing and maintaining spurs.

“They include proper irrigation and nutrition as well as adequate potassium levels to reduce spur mortality.”

Doll said the tagging studies also found that the set percentage is inversely related to flower density. This means that trees with fewer fruit buds and flowers will set at a higher percentage than trees with high fruit bud and flower counts.

“This most likely is due to a greater amount of resources able to be allocated to a fewer number of buds,” he said. “But this compensation for the lower bud does not typically lead to a higher yield.”

Diseases and “any true bug that would feed” also play roles in nut drop, Doll said. For the past two years, one of those villains has been the leaffooted plant bug, the subject of another blog by Doll.

He said damage by the pest is evident “as a small pinhole through the hull and shell and into the kernel...Knowing the cause of the drop can provide information relevant to treatment decisions,” Doll said.

“If the drop is due to leaffooted plant bug, a treatment may be warranted. It is important however to determine if the bugs are still within the orchard since the nut drop is visible several weeks post feeding.”

If the bugs have moved out of the orchard, he said spraying may not provide the wanted control.

Kris Tollerup, University of California Cooperative Extension area-wide IPM advisor, said this could be another year for large populations of the leaffooted plant bug.

Although cold winter temperatures have the potential to significantly reduce overwintering populations, it appears that cold temperatures in the fall and winter of 2015–2016 had no significant negative impact on the pest, Tollerup said.

This year, very large overwintering populations on pomegranate have been reported to UC farm advisors.

In another blog, professional forester and natural resource consultant Steve Nix wrote that other nut bearing trees, including walnuts and pecans, drop their fruit before full maturity.

“Sometimes it can be a natural shedding of a portion of the nut crop,” Nix said. “Other causes can be more problematic, including adverse weather conditions, tree health, poor pollination, insects, and disease.”

He said the pecan nut casebearer pest “probably causes more nut shedding than all other insects combined in pecan orchards.”

Other insects that pose a problem include black aphids, walnut caterpillar, shuckworms, stink bugs, and pecan weevils.

Nix said insect and disease infestations increase during times of drought stress, “especially if trees are growing in poor soil.”