Craig Kallsen

UCCE Farm Advisor, Kern County

Citrus, Subtropical Horticulture, Pistachios

July 29, 2002

The fruit of the pistachio tree, what we normally call the nut, is composed of a hull (made up of the epicarp and mesocarp of the fruit), a shell (botanically called the endocarp) and a kernel (also called the embryo). Although the harvest typically begins in early September, the relative success of that harvest is dependent on botanically important events that occur beginning in mid-June through early July.

The shell of pistachio nuts tends to get ahead of what is happening in the rest of the nut. The shell and its protective hull grow first. Cut a pistachio nut open in early-June and what will be seen is a green hull stuck tightly to an empty shell. A white structure will be visible within the shell but almost nothing of what will become the kernel is visible to the unaided eye. At this time, the shell will cut easily and can actually be flattened with the pressure of thumb and forefinger. Beginning in mid June, two things happen to many of the nuts on the tree. The kernel within begins to grow and the shell begins to harden. In June the tree is still adjusting its crop load. The tree normally produces many more nuts in the spring than can be expected to reach maturity. The tree normally drops a percentage of the small developing nuts during May and June, damaged or not, in response to the amount of carbohydrate stored in the tree that is available to produce nuts or in response to weather, irrigation and other variables. Nuts that are damaged by insects or disease before mid June will generally fall from the tree and not adversely affect crop yield at harvest. There are many other nuts on the tree to compensate for this loss.

Cutting into nuts before July 1 is not going to provide much information on how the pistachio crop is developing. Many nuts on the tree will not necessarily mean that a bumper crop is on the way. Pistachio fruit can develop parthenocarpically, which means fruits will stay on the tree even though they may not contain a seed (i.e. an embryo or kernel). These parthenocarpic fruit are called blanks. For a nut to exist (either blank of filled) the female flower must come into contact with pollen from a male tree. However, fertilization does not necessarily need to take place to keep a blank on the tree because the simple stimulus of the pollen is sufficient for parthenocarpy to occur. Even if fertilization does take place a kernel is not guaranteed. The embryo can also abort at some point during its later development, which will also result in a blank. Again, weather variables and insufficient irrigation can cause a high degree of blanking, and many of the blanks that are on the tree in September can trace their failure to produce a viable kernel to events in May or June.

Through July and into August those empty shells should be filling with developing kernels and cutting nuts during this time period will be more meaningful. Although the feeding of small plant bugs like lygus or phytocoris prior to shell hardening may not adversely affect harvest yield before the tree has adjusted its crop load, big plant bugs, like stink bugs and leaf-footed bugs, can severely impact harvest by their activities after shell hardening in July and August. These bugs have the ability to penetrate hardened shells resulting in destruction of the kernel inside.

Sufficient irrigation in mid-June, July and most of August will ensure that those kernels keep growing and exert sufficient force on the shell to split it. If the shell is split and retains the kernel, growers are paid for both the weight of the shell and the kernel. The grower is paid only for the kernel if the shell remains closed and receives nothing for a blank.

In early July the harvest is still two months away but they are important months. Pistachios growers are fortunate in that the nut provides them with the clear signal of shell hardening for when to get serious about maximizing irrigation and pest control strategies.