Alternate bearing is the most significant horticultural problem facing pecan producers. Studies have suggested that stored carbohydrate concentrations during the winter markedly affected subsequent flowering ( Malstrom, 1974 ; Smith and Waugh
Plants set 6.1, 6.4 and 6.7 m apart in each of 3.0-, 3.7- and 4.3-m rows of the muscadine grape, Vitis rotundifolia, Michx., cv. Hunt, were compared for 25 years. The highest plant density, 477 plants/ha, gave the greatest yield per m2 almost consistently for 25 years. The highest yields per plant were obtained from the wider in-the-row spacings. Alternate bearing was rather consistent from 1949 through 1963, but irregular thereafter.
Alternate bearing exerts economic and environmental consequences through unfulfilled yield potential and fertilizer runoff, respectively. We will discuss a systematic biological–statistical modeling management integration approach to address the concert of mechanisms catalyzing alternate bearing. New engineering technologies (precision harvesting, spatially variable fertigation, and mathematical crop modeling) are enabling optimization of alternate bearing systems. Four years of harvest data have been collected, documenting yield per tree of an 80-acre orchard. These results have shown variability within orchard to range from 20–180 lbs per tree per year. Results indicate irregular patterns not directly correlated to previous yield, soil, or tissue nutrient levels, or pollen abundance. Nor does significant autocorrelation of high or low yields occur throughout the orchard, suggesting that genetically dissimilar rootstocks may have significant impact. The general division of high- and low-yielding halves of the orchard may infer a biotic incongruency in microclimates. This orchard does not display a traditional 1 year-on, 1 year-off cyclic pattern. Delineation of causal mechanisms and the ability to manage effectively for current demands will empower growers to evaluate their fertilization, irrigation, male: female ratio, site selection, and economic planning. In comparison to annual crops, the application of precision agriculture to tree crops is more complex and profitable. When applied in conjunction, the aforementioned methods will have the ability to forecast yields, isolate mechanisms of alternate bearing, selectively manage resources, locate superior individuals, and establish new paradigms for experimental designs in perennial tree crops.
Each of 11 insecticides tested as a single spray application on mature leaves of pecan [Carya illinoinensis (Wang.) K. Koch] seedlings reduced net photosynthesis (Pn) 1 day after treatment. Eight of these materials still reduced Pn 9 days after a single spray treatment; however, in a study with 5 materials, all seedlings except those treated with petroleum oil had recovered after 14 days. Recovery of leaf photosynthetic activity generally occurred more rapidly following application of wettable powder formulations than emulsifiable concentrates. Petroleum oil was the most damaging treatment with Pn of treated plants being only 60% of the untreated check 9 days after treatment; however, leaves were nearing recovery after 14 days. The influence of insecticides on pecan irregular bearing is discussed.
All yield components individually correlated with yield. However, percentage of shoots fruiting, shoot fruiting intensity, and tree fruiting intensity were not independent variables; and, in multiple regression analysis, yield correlated only with percentage of shoots fruiting and tree size. The multiple relationship gave a poor estimate of yield since, 1) percentage of shoots fruiting and their relationship to yield varied with the portion of the tree canopy sampled and 2) the relation of yield to tree size was apparently obscured by irregular bearing among individual trees. As indicators of fruitfulness, tree fruiting intensity and percentage of shoots fruiting were essentially equal. Yield variation was governed more by percentage of shoots fruiting than by shoot fruiting intensity. Nut weight varied curvilinearly with yield and nuts per tree.
Supplemental nitrogen applications were tested to reduce irregular bearing and improve yield on drip-irrigated pecan (Caryaillinoinensis Wangenh. C. Koch. cv. Mohawk) trees planted in 1989. Treatments were: 1) no N fertilization; 2) 75 kg·ha-1 N in March; 3) 75 kg·ha-1 N in March plus 50 kg·ha-1 N in August; and 4) 75 kg·ha-1 N in March plus 50 kg·ha-1 N in October. Withholding N did not reduce July leaf N concentration compared to the other treatments until the sixth year of the study. Trees receiving N had similar July leaf N concentrations regardless of treatment throughout the study. October leaf N concentrations were unaffected by treatment, except in 2002, when withholding N suppressed leaf N compared to other treatments. The percentage of fruiting shoots was lower when supplemental N was applied in October compared to August during 2 of 6 years. Otherwise, the percentage of fruiting shoots was unaffected by treatment. Weight per nut and kernel percentage were not affected by treatment, except kernel percentage was lower during 1 year when supplemental N was applied in August compared to October. Kernel grade was usually not affected by treatment.
Controlled low temperature chilling caused flower initiation in container-grown olive (Olea europaea L.) trees at any time of the year. Optimum inflorescence production under controlled conditions occurred after exposure for 70 to 80 days to a diurnal sine wave temperature pattern, with a 2°C minimum and a 15°C maximum. These same temperatures, but changing abruptly from one to another failed to cause inflorescence formation. A constant intermediate temperature of 12.5°C was also effective in causing flower formation, but a continuous constant low temperature (7°C), or continuously high temperatures above 15°C failed to cause infloresence formation. The cv. Ascolano produced inflorescences under a wider range of temperature patterns than did ‘Manzanillo’.
Irregular bearing behavior of olives in California's Central Valley could not be attributed to varying winter chilling patterns. There was no correlation over a 7-year period between amount of winter chilling and subsequent crop size. Sufficient chilling occurs even in warmest winters there for ample flower initiation.
Nitrogen was applied between 1996 and 2002 to grafted `Mohawk' pecan (Carya illinoinensis Wangenh. C. Koch.) trees at 75 or 150 kg·ha-1 either as a single application in March or as a split application with 60% applied in March and 40% the first week of June. In 1997 and 2001, a spring freeze damaged developing shoots and buds, resulting in a small, noncommercial crop and the June portion of the N application was withheld. Nitrogen was also applied during the first week in October at 0 or 50 kg·ha-1 N if the crop load before fruit thinning in August was ≥40% fruiting shoots. There were few differences in the percentage of fruiting shoots or cluster size associated with N rate or applying N as a single or split application. Leaf N concentrations were either not affected by treatment or the results were inconsistent. Omitting the June application when a crop failure occurred did not affect the percentage of fruiting shoots the following year. October N application either did not affect or reduced the percentage of fruiting shoots the following year, and had no influence on leaf N concentration in July or October. These results indicate that the only advantage of a split N application is the option of withholding the second portion in the event of a crop failure. However, the added expense associated with splitting the N application versus the risk of crop failure must be assessed for each situation to determine if this is a sound economic practice. These data do not support an October N application when the crop is ≥40% fruiting shoots to reduce irregular bearing.
Alternate or irregular bearing is the most significant horticultural problem in pecan production. Alternate bearing is typically synchronized over regions by biotic or abiotic stresses and results in high-amplitude cycling ( Gemoets et al., 1976
Irregular flowering and fruiting is a common trait in polycarpic woody species ( Kelly and Sork, 2002 ; Monselise and Goldschmidt, 1982 ). This phenomenon is usually termed alternate (irregular, biennial) bearing in horticultural plants and