By Dr. Thomas T. Yamashita

Apples, pistachios and pecans are but a few notorious agronomic plants which experience alternate bearing. They will produce large crops one year, then alternate the following season with a dramatically reduced yield, before returning the next for another large crop. This alternate bearing characteristic not only costs growers, but also places undue physiological stress on the plant. Many in the agricultural industry believe that this phenomenon is built into the genetics of the plant and that off- and on-years are synchronized within plant species. This is not the case.

This article explores and characterizes alternate bearing, defining key principles underlying its development and concomitant means of combating this costly, physiological malady.

To understand alternate bearing, it’s necessary to examine cultural growing practices.

Many years ago, we conducted studies on pistachio trees to help elucidate alternate bearing. Eighteen-year-old blocks were selected which had a long history of alternate bearing. The orchards also had several replants, in response to a serious case of Verticillium wilt. In many of these blocks, for example, 90% to 100% of the orchards represented replanted trees.

Having conducted several studies on noninfectious bud failure (NIBF) of almonds, I felt that this malady originated along the same lines as imbalanced energy dynamics. As discussed elsewhere, we found that NIBF was traced to heat-induced deficiencies in photosynthates, stored energy, and tissue integrity. These deficiencies led to heat inactivation of key enzymes located in vegetative buds of the almond shoots, eventually leading to a blighting of these buds and the manifestation of the bald shoots characteristic of NIBF.

l began my studies by first examining the common cultural practices for pistachio orchards. One practice that caught my attention was the minimal pruning techniques. Most pistachio farms maintained a low maintenance pruning system in which every 5 years the trees received a light clipping of the shoot tips. There were no practices entailing structural opening to maximize sunlight, nor efforts to balance yearly crop loads through annual shoot thinning. Upon examination of the irrigation schemes, there was even a question raised in the industry as to whether or not pistachio trees needed much water.

The great majority of fertilization schemes involved nitrogen with occasional adjustments with triple mixes such as 8-8-8 (liquid) or 16- 16-16 (prilled). The primary pests combated through sprays were stinkbugs (e.g. leaf-footed plant bug) and citrus flat mite. Use of foliar nutrients appeared to be rare, beyond an occasional addressing of zinc deficiency.

Soils planted to pistachios commonly had a history of many years of cotton plantings, a crop well-noted for its support of various plant-parasitic nematodes and Verticillium wilt. These soils were characteristically high in pH, salts, and alkaline-induced tie-up of various minerals, and low in soil microbial activity. Furthermore, the average summer temperatures hovered around 96 to 97 degrees Fahrenheit. In summary, the pistachio plantings were placed into an environment and exposed to cultural practices that induced several forms of stress.

Crop production robs energy from surrounding plant tissues.

For a pistachio tree to produce 90 pounds of pistachios, it needs about 180 sunny days to harvest the necessary radiant energy. But this is strictly the energy needed to produce the pistachios themselves—additional energy is needed to sustain the leaves, branches, roots, and so on.

This introduces a problem. In a given growing season, the pistachio fruit buds for the following year’s crop are borne on the current season’s shoot growth, which extends beyond the clusters of nuts produced in the current season.

Thus, to maintain consistent production for the follow season, it is necessary to produce substantial shoot growth beyond each developing cluster of nuts. Indeed, during heavy years of production, pistachio trees will manifest a paucity of vegetative growth, and the growth which develops appears sparse, stunted and of low density. Between mid-June and July, when the pistachio tree is pushing hard to mature the crop, you can commonly observe a sudden yellowing of leaves which surround a cluster of nuts. This is because the tree compensates and draws the balance of energy reserves from surrounding tissues.

Another primary energy source, besides surrounding leaves, are the roots. The roots of perennials are known to be the main storage organ for high energy molecules, such as starch. To verify my hypothesis of that crop production was robbing energy from plant tissues, I examined the roots of the pistachio trees from mid-June through July. Invariably, I observed the tender root tips undergoing attrition, visibly dying back.

This was also a period in the life of the tree that l observed (through a corresponding study of Verticillium wilt) the primary entrance of the Verticillium wilt fungus, making the transition from mere attachment to the root cortex, to entering the vascular tissues of the roots.

A primary requirement for disease resistance resides in the plant’s ability to maintain a high reserve of carbohydrates, which allow for bursts of high metabolism to produce ‘walling-off’ compounds within a short period of time. As almost all the reserves and current production of energy and carbohydrates in the pistachio trees was being diverted to maturing the crop, natural disease resistance was at an extremely low ebb.

I might add that the tips of young, developing roots (observed undergoing attrition) represent the primary centers for the production of one of the key growth hormones, the cytokinins. Thus, additional growth factors imparted from otherwise sufficient levels of cytokinins, instrumental in development and set of fruit buds, were being affected and minimized at this time. In July and August, I examined trees for the season’s resultant set of fruit buds. In almost all cases, there was either an absence or minimal shoot growth to hold fruit buds, with many trees pushing forth less than one inch of shoot growth for the season. In addition, many of the fruit buds that did set aborted and fell off before the end of the season.

The marginal energy harvest was barely sufficient to meet the demands for the current on-year crop, leaving a deficiency in energy to support necessary vegetative growth that would otherwise differentiate and hold fruit buds for the following season. The energy required for vegetative growth was instead being utilized for developing and maturing the crop load, the tree being forced to draw on stored reserves from both surrounding canopy as well as from root tissues. In the absence of sufficient current season shoot growth and the necessary differentiation of fruit buds for the following season, the tree can only bear a minimal and much-reduced crop the next season—alternate bearing.

Alternate bearing is not a biological phenomenon, but an energy-related one.

Some of the most sophisticated research in alternate bearing has been conducted by the Pecan Growers Association. Realizing that alternate bearing is an energy-related phenomenon, pecan researchers have devised a monitoring criterion whereby a pecan nut requires a minimum of 13 compound leaves to avoid alternate bearing. That is, 13 compound pecan leaves will harvest the minimum radiant energy necessary to fill out and mature the nut crop, while supporting development of vegetative tissues and differentiation of next year’s fruit buds.

It is a characteristic of nut growers to try and preserve the entirety of the crop that is set. In contrast, a fruit grower will thin the crop to achieve a state of energy flow that optimizes crop quality. In so doing, the fruit grower relieves the tree of undue energy stress. This is part of the reason why we rarely see alternate bearing problems in fruit trees.

Where we do observe alternate bearing in fruit trees (as with Fuji apple culture), examination of cultural activities reveals that many of the practices reduce the radiant energy harvesting capacity of the tree. These practices include extensive summer pruning, leaf pulling, girdling, and minimization of fertilization programs. The value of understanding this model of alternate bearing is that the principles of energy dynamics apply to many maladies found in plants, animals and man.

Avoiding alternate bearing is a simple matter of ensuring the availability of sufficient energy.

First, define the energy demand of the crop and supporting tissue. To approximate this, calculate the energy contained in the final crop, and add 50% to account for necessary supporting tissue. For example, a mature pistachio tree can produce 90 pounds of wet nuts, which have a total energy demand of about 82,000 kilocalories. Thus, to sustain this crop as well as the necessary plant tissues, about 123,000 kilocalories of energy are needed.

Then, determine whether the current stature of the tree (or vine, or plant) allows for the harvesting of the required energy. If not, what can be done to improve the energy harvest? Practices that can increase a tree’s energy harvesting capacity include:

• Pruning to allow for even sunlight in the canopy.
• Thinning the crop to produce a crop load that can be supported.
• Determine the minimum mineral levels needed to produce the crop, while accounting for the availability factor of the soil. Example: To fulfill the 5% nitrogen requirement needed to produce 3,000 pounds of pistachio nuts in 1 acre, with an approximate soil/application efficiency of 35%, you need 3000 x 0.05 ÷ 0.35 = 428 pounds of applied nitrogen per acre.
• Ensure that irrigation is optimal to minimize water stress.
• Implement necessary pest and disease controls.

There are a number of other non-standard practices that can increase plant energy harvest. These include improving the mineral availability of the soil by increasing microbial activity, ensuring that necessary minerals are present, optimizing the pH, supplementing with foliar nutrition, and providing anti-stress foliars in anticipation of periods of heat stress.

There are a variety of issues that can hinder energy harvest, such as nematode infestations, soil-borne diseases, the presence of herbicides and heavy metals, poor quality water, and poor soil characteristics. Remedying these issues is critical to ensure adequate energy harvest.

(Thumbnail source: Pixabay, licensed via Pixabay license)