Managing implies control. This paper discusses managing pecan nut growth from two perspectives, controlling nut volume or size and filling the volume with kernel of acceptable quality. This paper is a combination and extension of previous papers on managing nut growth (Sparks, 1995a, 1995b, 1995c; Sparks and Weber, 1993). Understanding the stages of nut growth is a perquisite to intelligent management of nut growth. Accordingly, pecan fruit growth stages are discussed first.

Pecan fruit growth occurs in four stages, elongation, expansion, kernel filling, and shuck opening or dehiscence. Variations in timing of the stages occur from year to year and more between early and late maturing cultivars. However, generalizations can be made. Under southeastern United States conditions, fruit elongation occurs June through July 15; fruit expansion occurs July 15 through August 15; kernel filling, in September; and shuck dehiscence, in October.

June is the primary month of fruit elongation. During this month much of the final length of the nut is established. July is the month of major fruit expansion. During this interval, the elongated cylinder increases in diameter. Nut size is fixed by mid-August when shell hardening has begun. The nut does not continue to increase in volume once hardening has occurred. Consequently, June through mid-August is the major nut sizing period in the southeastern United States (Fig.1) and nut size has been finalized at the end of this period.

The fruit continues to increase in size after shell hardening, but, this increase in fruit size is due to an increase in shuck thickness, not in nut volume. Although kernel filling occurs over about a 6-week period, most of it occurs in the 4 weeks (Davis and Sparks, 1974) of September. Date of shuck dehiscence or nut maturity varies with springtime temperatures (Sparks, 1989), but usually occurs within the first two weeks of October. For the most part, the stages of fruit and thus nut growth occur one stage at a time with very little overlapping of the stages. The discreteness of the stages allows the grower an opportunity to manage each growth stage individually. Management requires a knowledge of the factors influencing nut growth.

The five major factors governing nut growth are pests (insects and diseases), temperature, soil moisture, nut volume, and crop load. Nitrogen, potassium, zinc, and other nutrients also affect nut growth but their effects are minor and difficult to detect except under severe deficiency.

Insects and diseases

Pecan weevil, hickory shuckworm, stink bugs, and pecan nut casebearer are major insects affecting pecan nut growth. Pecan scab is the major disease affecting pecan nut growth.

Temperature

Observations indicate nut volume and kernel development increase with temperature. Nuts with maximum nut volume are produced during hot seasons provided other factors, especially soil moisture, are optimum. The hot 1999 growing season in Georgia is an excellent example. In well-irrigated orchards, nut volume was large. The apparent effect of temperature on kernel filling is often masked because other factors (discussed later) have a greater effect on kernel development than temperature as such.

Soil moisture

Soil moisture affects all phases of nut growth; that is, nut volume and shape, shell thickness, kernel filling, and shuck dehiscence. These effects are discussed in order.

Nut volume and shape: Nut volume and shape are primarily a function of soil moisture except when scab damage occurs. The relationship of nut volume to soil moisture is illustrated in Figure 2. The relationship is especially striking considering each data point is a different year. That nut volume is correlated with rainfall across years reemphasizes the point that nut volume is primarily dependent on soil moisture. Obviously, the grower can control nut volume by water management in arid climates and humid climates, but to a much lesser extent in humid than arid climates. In the southeastern United States, the effect of variation in soil moisture on nut volume (Fig. 3) is a common occurrence.

Final volume and shape of the nut depend on the availability of soil moisture during the elongation and expansion stages of the sizing period (Fig. 4). Optimum rainfall or irrigation throughout the nut sizing period produces a large nut with a normal shape. Soil moisture deficit throughout the elongation and expansion stages minimizes nut elongation and expansion. The result is a small nut with a normal shape. Soil moisture deficiency during the elongation stage, but optimum moisture during the expansion stage results in a short, fat nut, that tends to be round. On the other hand, when soil moisture is optimum during elongation but deficient during the middle of the expansion stage, the basal or stem end of the nut does not fully expand relative to the blossom end. The result is a nut with an obovate shape or the nut is said to have a pinched butt. The obovate shape occurs, but not often. The size and shape of the nut at maturity can easily be used to track the soil moisture history that existed during the two stages of nut sizing.

Shell thickness: Soil moisture affects shell thickness in that the shell is thinner under deficit soil moisture conditions (Table 1). However, even under severe drought status as occurred during 2000, the moisture effect is relatively small; that is, the shell was only about 11% thinner than nuts that developed under adequate soil moisture.

Kernel filling: As with nut volume, the role of soil moisture on kernel percentage is dramatic (Fig. 5). Depending on the level of soil moisture, kernel quality may range from poor to excellent. Additionally, the data in Figure 5, along with other data (Sparks, 1996), suggest 1), the critical period for kernel development in the southeastern United States occurs during the first 2 weeks in September and 2), during this period, the tree needs about 1.25 inches of water per acre per week.

The correlative data in Figure 5 are confirmed by experimental data (Table 2). Application of water at the beginning of the first week in September resulted in substantially better kernel quality compared with no irrigation. However, application of 1.5 inches of water (0.75 inches per week for the two-week period) did not result in maximum kernel quality. A high quality Stuart should have a kernel percentage of about 48.0, not 46.5. Thus, the 1.5 inches of water applied was not enough.

The two-week period in which water is especially critical for kernel development is about 4 to 5 weeks before nut maturity. The 4 to 5-week rule can be used to detect the critical period of kernel development for pecans grown in geographical regions other than the southeastern United States.

Shuck dehiscence: Although the first two weeks in September are especially critical for kernel development, this does not mean that adequate soil water is not important beyond this period. Adequate soil moisture through the time of nut maturity is essential for proper shuck opening (Table 3). There have been years in Georgia where sufficient soil moisture was available for kernel development but the shuck did not properly open because a severe soil moisture deficit occurred after the critical period of kernel development. The classic example occurred in 1987. In that year, the shuck did not open and instead turned black, dried, and stuck to the shell. Mechanical shuck removal was required before the nut could be cracked and shelled. Improper shuck opening, due to inadequate soil moisture, becomes especially important in pecan growing regions with a hot climate and low elevation (southwest Texas, Visalia area of California, and parts of Arizona and Mexico). In these regions, premature germination is a perennial problem that is greatly accelerated by poor shuck opening (Sparks, 1993).

Some believe that shuck opening is accelerated by the onset of low temperatures in the Fall. There are no data to support this contention.

Inadequate soil moisture 1-3 weeks before the time of shuck dehiscence makes the fruit more susceptible to a disorder called shuck decline (Fig. 6). The first symptom of shuck decline is a thin, dark, necrotic line on the inner shuck which then spreads to the outer surface of the shuck. Generally, excessive fruiting is a perquisite for induction of shuck decline. However, at lower levels of excessive fruiting, shuck decline will not occur unless heavy fruiting is coupled with inadequate soil moisture as occurred in Georgia during 1991. The crop load was heavy and drought was severe. Consequently, shuck decline was massive in the block irrigated by drip (2400 gallons water/acre/day) which failed to furnish sufficient water relative to sprinkler-irrigated block (Table 4).

In the southeastern United States, shuck problems are more likely to occur with drip than with sprinkler irrigation. This is because many drip systems were designed to deliver only 2400 gallons water per acre per day. During severe droughts, the 2400 gallon capacity is not sufficient.

Crop load: There are only so many nuts that a pecan tree can adequately fill with acceptable quality (Fig. 7). Once that point is reached nut quality rapidly decreases with additional nuts. The production level beyond which nut quality becomes unacceptable varies with nut volume (discussed later) and with soil moisture. If soil moisture is optimized by sprinkler irrigation based on pan evaporation, 2000 pounds of nuts per acre is near the upper limit that pecan trees can fill with acceptable kernels (Fig. 7) for the major cultivars grown in the southeastern United States. If soil moisture is less than optimum, as can occur from drip irrigation systems, the curve in Figure 7 will break at some point far less than 2000 pounds of nuts per acre. Furthermore, if the orchard is not irrigated and drought conditions exist, the curve will break at very low production levels or all nuts may have unacceptable quality depending on nut size and drought severity.

Nut volume: Large nut size is desirable in the southeastern United States because of the premium paid by the market. However, a large nut can be a detriment to kernel quality. The data in Table 5 support this contention. Small nuts filled better than large nuts although trees producing the large nuts were well-irrigated during the severe 2000 drought in Georgia.

Although the data in Table 5 suggest a small nut is easier to fill than a big nut, the data do not prove this. To prove a small nut is easier to fill than a big nut, trees must have similar nuts per tree, similar rainfall in September, but different nut volumes produced by differential rain during the June - August 15 nut sizing period. The data in Table 6 are from trees with similar nuts per tree, similar September rain but different nut volumes. The small nut had better quality, confirming that small nuts fill better than large nuts.

Small nuts fill better than large nuts simply because more energy is required to fill a large nut than a small nut. The inverse relationship between nut size and kernel development is often apparent in nuts from cultivars versus seedlings. The small nut of seedlings is usually better filled than the larger nut of cultivars. Native pecans characteristically produce small nuts. Quality is usually good, as frequently pointed out by growers of native pecans. Furthermore, cultivars that have a small nut, for example Elliott, Schley, and Western Schley, usually have better quality than cultivars that produce a large nut, such as Mohawk and Mahan, or even a medium size nut, as in the case of Stuart.

In the southeastern United States where pecans are not native, the rainfall pattern is a setup for a large volume nut and potentially poor quality. This can be inferred from the comparison of the average rainfall pattern (Fig. 8) in the native pecan area of Grimes County, Texas versus the nonnative area of Dougherty County, Georgia. In the native pecan area, rainfall and growth and development of pecan coincide in a manner that results in good tree growth and nut quality. Leaf and shoot growth coincide with increasing rainfall. During June rainfall decreases and the annual low occurs in July and remains low in August. The low rainfall during the nut sizing period ensures a low volume nut which is easier to fill. Good kernel filling is further ensured by the increase in rainfall during the September nut filling period. In contrast, in Georgia, the increase and large amount of rainfall during the June-Aug. 15 nut sizing period tends to maximize nut volume. Poor filling of the large nut becomes a major threat because of declining rainfall which usually begins in mid-August and continues at a steady rate through the critical kernel filling month of September as well as through the shuck dehiscence period of early to mid-October. Thus, the frequent occurrence of poor kernel filling in the southeastern United States is not surprising. In fact, calculations from Georgia's long-term rainfall records indicate that during 50% of the years, rainfall during the first 2 weeks of September will be less than the 2.5 inches needed for good kernel filling. This means that there is a 50% chance that potential profit will be reduced or a loss will be incurred in non-irrigated orchards. Furthermore, there is a 25% chance that there will be less than 0.75 inches of rainfall and unacceptable quality will result. Obviously, farming non-irrigated pecan orchards in the southeastern United States is an extremely risky business venture. Because of the high risk, economical inputs in non-irrigated orchards should be minimal. The probability of losing is too great to do otherwise.

Year in and year out, inadequate soil moisture during the critical two week period of kernel development is the number one reason for poor quality in the southeastern United States. In Georgia alone, maintaining adequate soil moisture during the kernel development period would easily increase that state's long term annual pecan production by 25%.

Interaction between nut volume and production: In years of high or excessive production, a small nut is an advantage, especially in orchards with marginal soil moisture during kernel development. The importance of a small nut in a year of excessive production was illustrated during 1993 when Georgia produced a record 150 million pounds. A drought during the nut sizing period resulted in a small nut. In extreme cases in non-irrigated orchards, nut count for Stuart was near 100 per pound. Quality was acceptable in spite of the high production and only moderate soil moisture during kernel development. A large nut would have resulted in a disaster in non-irrigated orchards. A large nut also becomes a detriment in well-irrigated orchards if production is excessive. For example, during 1999, nut size was unusually large in well-irrigated orchards. If these orchards had excessive production, quality was marginal at best.

The advantage of a small nut holds across cultivars. If all other factors are equal, higher per acre production with acceptable kernels quality is more predictable with cultivars with a small nut than with those with a large nut. For example, the prolific Moneymaker, Western Schley, Elliott, Moore, and Curtis will produce higher per acre production than similarly prolific but large nut cultivars such as Success, Mobile, Mahan, and Mohawk. Likewise, production of nuts with moderate sized nuts, such as 'Stuart' and 'Desirable,' is less than prolific cultivars that produce small nuts.

Regulating crop load to increase kernel fill: The suppressive effect of crop load on kernel fill (Fig. 7) can be resolved by mechanical fruit thinning (Table 7). This section discusses the results following application of a mechanical fruit thinning program in a commercial orchard near Albany, Ga.

The data are from a 160 acre, Desirable-Cape Fear orchard over a 9-year period, 1992 -2000. Adequate soil moisture was maintained by sprinkler irrigation. Insect and disease control was good and well above average. All data are for marketable nuts; that is, the data are post cleaning plant.

From 1992 to 1994 the orchard was in a severe alternate bearing cycle (Fig. 9). In 1995, mechanical fruit thinning was initiated and continued annually thereafter. Alternate bearing, for all practical purposes, was eliminated. Production per acre was good considering that Desirable and Cape Fear produce a moderately large volume nut. The lowest production, about 1100 pounds per acre, occurred during 1998. During that year, nuts per pound was 48 and percentage kernel was only 48.4. The large nut indicates that soil moisture was good during nut sizing. The low percentage kernel indicates a problem during kernel filling, probably insufficient soil moisture. Production was higher during 1999 and 2000 than in other years. The high production during these two years probably reflected the high quality of the growing seasons. Both seasons were hot and dry. Hot and dry seasons are ideal for pecan production provided sufficient water is available as was the case in this orchard. During 1999 nuts per pound was 43 and cracking percentage was 51.6. Similarly, during 2000, nuts per pound was 49 and percentage kernel was 50.7. These results indicate that a large nut volume can be filled with acceptable kernel during a heavy crop year.

In Table 8, the results from the fruit thinning versus no fruit thinning are compared in greater detail. In Table 8, results from 1993 and 1994 (one "on" and one "off" year) are compared with 6 years of annual fruit thinning. These 6 years represent three potential cycles of alternate bearing. Compared to no fruit thinning, mechanical fruit thinning substantially increased production per acre, nut size and percentage kernel. Kernels per acre were increased by 172 pounds. At a conservative two dollars per pound, mechanical fruit thinning increased gross income by $344.00 per acre. Although many of the original fruits were removed in mechanical fruit thinning, thinning increased the number of marketable nuts per acre. Increase in marketable nuts occurs for two reasons. One, during a heavy "on" year, the tree has more nuts than it can adequately fill and nuts with poor quality are blown out during the harvesting and cleaning processes. Two, return bloom is greater following mechanical fruit thinning than following no fruit thinning.

The argument can be made that the comparison of two years of production (1993 and 1994) with the next 6 years of production is a risky parallel. However, comparison of the 1993-1994 alternate bearing cycle with the next (1995-1996) alternate bearing cycle(Table 9 ) produced the same results as from the longer-term comparison (Table 8). The initiation of mechanical fruit thinning in this orchard was approached with some apprehension by the owner. As a result, a check of three long rows was included. Trees that were not mechanically thinned during the "on" year produced poor kernel quality and poor return bloom the following year (data not presented).

The results (Fig. 9, Tables 8 and 9) indicate mechanical fruit thinning in a well-irrigated orchard allows annual, high production of large nuts with acceptable nut fill. The early southeastern United States market, which is often high-priced, demands large, well-filled nuts.

Regulating nut size to increase nut fill. Because nut volume is mainly a function of soil moisture (Fig. 2), growers with irrigation can regulate nut size to varying degrees depending on the amount and timing of rains. Soil water management to regulate nut size should be based on crop load and type of irrigation. If the crop is off, but still commercial, the objective should be to irrigate for maximum nut size in order to make up for less than desired number of fruits per tree. There is little risk in maximizing nut size on trees with a low crop load. A large nut usually can be filled if production is off, even with marginal soil moisture during kernel development. In fact, a small nut during a low production season may be spilt if soil moisture is optimum during kernel development. The nut splits near the time of nut maturity and does so because nut volume is not large enough to accommodate the kernel.

The decision of how much to control nut size by irrigation comes when crop load is near optimum. The decision depends, in turn, on the water delivery capacity of the irrigation system. Unless the crop is excessive, maintaining adequate soil moisture during kernel development of the major cultivars of the southeastern United States will largely override the negative effect of large nut size on nut quality. Thus, quality is ensured, in most years, in orchards with solid-set sprinklers and in which irrigation is scheduled by pan evaporation.

The quality problem comes in orchards with drip irrigation systems because of insufficient water supply. During nut filling, pecans require about 5,000 gallons of water per acre per day (1.25 inches per acre per week) (Fig. 5). Most drip systems in the southeastern United States deliver about 2400 gallons of water per acre per day. This is only about half of the water needed to fill a large crop if drought conditions exist during kernel development. On the average, the major month of kernel development, September, is the second driest month of the season. Thus, in drip-irrigated orchards with a commercial or an excessive crop, the system should be used to intentionally restrict nut volume (deficit irrigation) and maximize soil water during kernel development with the hope of producing a kernel with acceptable quality.

A gamble exists in attempting to regulate nut size with deficit drip irrigation. September may have above average rain and the extra rain plus drip may be sufficient to fill a large nut in spite of moderately high production. Regardless, for several years, some growers have successfully used drip irrigation to limit nut size and produce acceptable quality during years of high production. The irrigation scheduling varies, but basically involves increasing the irrigation frequency as the season progresses. Specifically, as the water requirement for fruit growth increases, the irrigation is increased. That is, the drip irrigation system is operated 4 hr per day in May, 6 in June, 8 in July, 10 in August, and maximized up to 20 hr in September. Some judgment has to be used in deficit irrigation. If a severe drought occurs during the nut sizing period, an adjustment will have to be made. Another option with deficit irrigation is to run the system only in September. However, this may not result in maximum use of the water due to insufficient time to develop an adequate root mass near the emitter. Some growers prefer to use drip irrigation to maximize nut size regardless of crop load and take the chance that, between water from the drip irrigation and rains, soil moisture will be sufficient to fill a large nut. The latter irrigation management will result in poor kernel in years of large nuts, high production, and dry Septembers. Regardless of the pros and cons of regulating nut size, soil moisture during the June-August 15 period exerts a dominating influence on final crop production by affecting nut size, and, in some situations, indirectly by influencing kernel quality.

Deficit irrigation to control nut size has been discussed primarily from the standpoint of drip irrigation. However, if crop load is excessive in sprinkler-irrigated orchards and the grower chooses not to remove the excess fruit, deficit irrigation should also be used in sprinkler-irrigated orchards. The objective of deficit irrigation in either drip or sprinkler-irrigated orchard is to obtain acceptable quality in the current season without regard to return bloom. If the crop is excessive and not mechanically fruit thinned, the extra kernel development and accompanying increased stress obtained from deficit drip or sprinkler irrigation may accentuate alternate bearing the following year.

In summary, nut volume, crop load, and adequate soil water during kernel filling mainly determine kernel quality. Mechanical fruit thinning is the best management program in pecan orchards with adequate irrigation. This program will result in annual production of a large crop with optimum size and quality of the nuts. If fruit thinning is not employed, suppressing nut volume by deficit irrigation and optimizing soil water during kernel development will improve kernel quality and allow acceptable quality even when crop load is relatively high. However, this practice should be used as a last ditch effort because of the probability of increasing alternate bearing in the trees.

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