Multiple Stresses Combine and Decrease Pecan Crop
Can you spell D-R-O-U-G-H-T? Unfortunately, much of Oklahoma and Texas has been suffering from drought conditions most of the season. Looking at the U.S. Drought Monitor, in mid-June, when the industry made the first crop estimate, about 41% of Oklahoma was being affected by some level of drought. But by the time we arrived at mid-September when the last crop estimate was made, 99.97% of Oklahoma was in some level of drought, with 48.8% being in extreme/exceptional drought (this increased to 66.8% by Nov. 1).
But before we discuss the drought, we have to go back even earlier to March and April to see the complete picture. As you know, fruit and nut trees cease active growth in the fall so they can survive the cold winter months. This physiological rest period, also called dormancy, helps to regulate the timing of budbreak in the spring. Bud activity depends on a number of factors, including temperature, hormone activity, light intensity, and day length. Before initiating growth in the spring, the vegetative and reproductive buds must receive a minimum exposure to chilling temperatures during the winter months. So what effect does insufficient chilling have on pecan tree growth? In regard to budbreak, the trees will have a delayed, protracted bud initiation period. Non-uniform budbreak results in fruit of different maturities on the tree. Because each pecan variety has different chill hour requirements, pollen release and pistillate receptivity can be shifted so that varieties that normally provide good cross-pollination may not have any overlap at all. This shift can lead to poor pollination and increased selfing, or self-pollination, in a variety which can lead to a greater nut drop during the growing season. Due to a late budbreak and slow growth in the spring, this year’s crop has been running two to three weeks later than normal all season.
Now, getting back to the drought discussion, initially, water stress will cause marginal scorching (browning) of the pecan leaflets. As the drought intensifies and water becomes extremely limited, pecan trees will shed leaves to minimize water loss. Continuation of the drought can lead to complete tree defoliation. Drought reduces photosynthesis not only by premature leaf loss but also by a decline in leaf expansion, photosynthetic machinery reduction, and by damaging associated enzyme systems. This decline in photosynthesis leads to a reduction in new carbohydrate production which causes the tree to use stored food reserves for plant maintenance. In addition to less carbohydrate production, drought stress inhibits the translocation of proteins, enzymes, growth regulators, mineral nutrients, and other essential materials needed for normal plant growth. These factors further contribute to reduced growth and development.
It takes almost three months after pollination for a pecan nut to enlarge to its final size. This period is referred to as Phase I, and for most varieties, it is completed by August. But the slow start caused by insufficient chilling extended this phase well into September this year. Towards the end of Phase I, it is not uncommon for a pecan tree to shed a few nuts (often referred to as a stress drop) due to embryo abortion. At this time, the embryo has grown to full size and the nut is on the brink of initiating shell hardening. If the embryo aborts after shell hardening, the nut will mature but is usually a pop or hollow nut. Drought or water stress is known to accentuate the nuts aborted at this phase, and we saw a large stress drop in some orchards in the state.
Drought predisposes trees to pests because of lower food reserves, poorer response to pest attacks, and poorer adjustment to pest damage. A common cause of early nut drop is damage resulting from nut feeding insects. Puncturing of the ovary wall before shell hardening by feeding stink bugs and leaf-footed bugs will usually result in nut abortion in three to four days. Hickory shuckworm and pecan nut curculio can also cause nut drops in June and July. A puncture can usually be seen on nuts damaged by insects. A white blotch can frequently be seen around the puncture made by shuckworm. Prior to shell hardening, pecan weevil and curculio damage appear as a dark stain fluid on the surface of the shuck. The variable nut maturity caused by insufficient chilling and the delay in nut maturity caused by the drought probably resulted in a higher nut loss to probing insects due to the nuts still being in water stage well into September, much later than usual.
During late summer/early autumn, it is fairly common for pecan aphid (black margined aphid; yellow pecan aphid) populations to explode in size and deposit large quantities of honeydew onto the upper leaf surface. This year I started receiving calls and texts in late August about trees dripping with excess honeydew. This excess is especially common during drought periods, as there are no rain showers to wash the honeydew off the leaves. Honeydew is rich in sugars and nitrogenous compounds and leads to the growth of Capnodium sp., a saprophytic sooty mold fungi. The resulting black mold layer growing on the upper leaflet surface can reduce the penetration of sunlight and suppress photosynthesis. Even without the growth of sooty mold, just the act of pecan aphids feeding on the phloem of pecan leaves can induce drastic reductions in inshell nut production and nut quality. This loss of tree productivity is due, at least in part, to the direct removal of energy from the tree and suppression of photosynthesis. The combination of the removal of carbohydrates by yellow aphids and prevention of photosynthesis from sooty mold accumulation often sets up a tree for a short crop the following year because it can’t accumulate sufficient reserves to drive strong flowering in the spring.
Inadequate soil moisture a few weeks ahead of shuck dehiscence can also increase the incidence of shuck decline. Excessive stress can result in incomplete kernel development that has been associated with shuck deterioration. The first symptom of shuck decline is a thin, dark, necrotic line on the inner shuck that then spreads to the outer surface of the shuck. The shuck looks water-soaked in appearance before turning black. Affected pecans will often stay in the cluster but have premature shuck opening. While the condition is often associated with excessive fruiting, it can also occur in trees with a moderate crop when coupled with inadequate soil moisture. I observed some shuck decline on ‘Pawnee’ at our Noble Pecan Orchard.
While the lack of rainfall and high temperatures has been beneficial in reducing scab infection, low soil moisture often causes a delay in shuck dehiscence or opening. Delayed or improper shuck opening because of inadequate soil moisture is typically isolated to hot, dry areas in southern Texas and parts of Arizona, California, and Mexico. But it has been a very common occurrence in Oklahoma this year. Our ‘Choctaw’ pecan nuts still have not initiated shucksplit in what is now the first week of November. Many of the native trees had not started shucksplit in mid-October. This delay leads us to the last problem I want to discuss: the loss of this year’s pecan crop. In northeast Oklahoma, temperatures dropped into the upper teens on October 19, 2022. It wasn’t as early as the 2000 freeze, but it still damaged the pecan crop. For improved varieties and natives that had not initiated shucksplit, the shucks were frozen still tight around the pecan nuts. Dehullers may be able to salvage some of the nuts, but all in all, the crop loss in some orchards will approach 50 percent.
In summary, moisture stress from drought periodically affects pecan orchards. The impact of drought on tree growth and production varies with the severity and duration of the drought, as well as other factors, including tree age, soil conditions, insects, diseases, and other stresses that may affect the plant. Stresses on a tree are cumulative, so you must manage the stress on a pecan tree in late season this year to have a good crop next spring. An accumulation of stresses the year before above a specific level is certain to set the tree up for a short crop. This year we have had to deal with insufficient chilling, drought, high hickory shuckworm and aphid populations, and finally, an early freeze. All of these stresses have taken a toll on the 2022 pecan crop and will probably impact next year’s crop as well.