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Michael W. Smith and Charles T. Rohla

Typical damage, cleanup, and recovery from four ice storms beginning in Dec. 2000, with the latest in Dec. 2007, are reported for pecan (Carya illinoinensis). Damage levels were amplified as radial ice accretion increased. Cultivar affected the amount of damage incurred. Trees less than 15 ft tall typically had the least damage. Trees 15 to 30 ft tall incurred as much or more damage than larger trees and cleanup costs were greater. Production potential was directly related to canopy loss during the first growing season. The time to recover full production potential varied with the severity of canopy loss. Cleanup costs depended upon the amount of canopy damage incurred, tree spacing, tree size, and the amount of pruning needed to remove hanging and damaged limbs from the tree.

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Eric T. Stafne, Charles T. Rohla and Becky L. Carroll

Pecan (Carya illinoinensis) shells are waste products that are occasionally used for mulch in ornamental landscape settings, yet most shell waste is left in piles near the shelling facility or discarded by other methods. If another use for this waste product could be developed, it may add income for pecan producers and provide peach (Prunus persica) growers with another option for weed control. A block of ‘Loring’ peach trees grafted onto ‘Halford’ rootstocks was planted at a spacing of 18 × 22 ft in Feb. 2005 at the Cimarron Valley Research Station near Perkins, OK, to determine the effect of pecan shell mulch on peach trees. Five treatments were imposed: no weed control except mowing (MOW), weed-free 6- × 6-ft area maintained with glyphosate herbicide (SPRAY), 6-ft × 6-ft × 2-inch deep mulch (TWO), 6-ft × 6-ft × 4-inch deep mulch (FOUR), and 6-ft × 6-ft × 6-inch deep mulch (SIX). Yields in 2008 were poorest in the MOW treatment (13.2 kg/tree and 93 fruit/tree). All other treatments did not differ. Soluble solids concentration as a measure of fruit quality and fruit weight was unaffected by treatment. Tree height, pruning weights, and trunk cross-sectional area were similar with the exception that MOW was lower for all three growth measurements beginning in 2007. Pecan mulch appears to have the potential to reduce soil pH. Foliar analysis for nitrogen (N), potassium (K), and zinc (Zn) showed treatment differences in 2006. No treatment differences were evident in 2007 and 2008 for K and Zn, but in 2008, FOUR had greater N than MOW. Tree mortality increased with pecan mulch depth. MOW, SPRAY, and TWO had little tree loss (0%–5%), whereas FOUR and SIX had 15% and 35% mortality, respectively. Tree mortality was attributed to record rains in 2007 coupled with longer soil moisture retention under the deeper mulch.

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Charles T. Rohla, Michael W. Smith and Niels O. Maness

Alternate bearing pecan trees [Carya illinoinensis (Wangenh.) C. Koch] were hand-thinned annually to 1, ≤2, or ≤3 fruit/cluster or not thinned when the ovule was about one-half expanded. Return bloom was monitored on (1) vegetative shoots, (2) bearing shoots without a second growth flush in the terminal position on 1-year-old branches, (3) bearing shoots without a second growth flush in the lateral position on 1-year-old branches, and (4) bearing shoots with a second growth flush that were primarily in the terminal position. Yield and nut quality were determined in addition to nonstructural carbohydrate, organically bound nitrogen (N), and potassium (K) concentrations in the roots and shoots during January. Fruit thinning improved return bloom but had little effect on weight/nut, kernel percent, or kernel grade. Fruit thinning had either a modest or no effect on nonstructural carbohydrates, organically bound N, and K concentrations. Vegetative shoots and bearing terminal shoots produced a similar number of flowers/1-year-old branch and percentage of flowering current-season shoots. Bearing lateral shoots produced fewer flowers than vegetative shoots most years and fewer flowering current-season shoots during one year. Shoots with a second growth flush produced more flowers/1-year-old branch and a larger percentage of flowering current-season shoots than did vegetative shoots 2 of 3 years. These data indicate fruit thinning of overloaded trees improved return bloom, but the lack of interactions between thinning treatment and shoot type suggests that the number of fruit/cluster was less important than total crop load in determining nut quality and return bloom. Thus removal of entire fruit clusters appears as effective as thinning fruit within a cluster to maintain adequate nut quality and promote return bloom. Nonstructural carbohydrates, organically bound N, and K were not limiting factors in bearing consistency because they were not depressed in unthinned trees. Nonstructural carbohydrates, organically bound N, and K concentrations were not closely linked to alternate bearing because return bloom was enhanced by thinning, but thinning did not affect their concentrations.

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Charles T. Rohla, Michael W. Smith and Niels O. Maness

Whole fruit clusters of `Pawnee' pecan [Carya illinoinensis (Wang.) C. Koch.] were collected from three shoot types: terminal and lateral shoots without a secondary growth flush and shoots that had an early-season secondary growth flush. Fruit per cluster were counted and nuts were individually harvested, weighed, shelled and graded. Bloom the following year was determined for the same shoots where clusters were collected. Wafers (cotyledons that failed to develop) were not associated with cluster size or shoot type. When wafers were included in the data, nut weight, kernel percentage and return bloom were not affected by cluster size or shoot type. However, when wafers were excluded from the data there were significant relationships of cluster size and shoot type with the dependent variables. Cluster size on lateral shoots was negatively related to nut weight and kernel percentage. Cluster size on terminal shoots without a secondary growth flush was inversely related to kernel percentage, but not related to nut weight. When shoots had a secondary growth flush, cluster size was not related to kernel percentage or nut weight. There was a positive linear relationship between cluster size and total kernel weight for the three shoot types. Return bloom of terminal shoots without a secondary growth flush was negatively related to cluster size, but cluster size did not affect return bloom of the other shoot types. The number of shoots that developed the following year was positively related to cluster size for terminal and lateral shoots, but not for shoots with a secondary growth flush. Shoots with a secondary growth flush produced substantially more shoots with larger fruit clusters the next year than the other shoot types.

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Michael W. Smith, Charles T. Rohla and Niels O. Maness

The current theory of pecan [Carya illinoinensis (Wangenh.) C. Koch] alternate bearing is the “growth regulator–carbohydrate theory” in which flowering is first controlled by growth regulators produced by fruit and leaves, and then by the size of the carbohydrate pool near budbreak. Lack of nitrogen (N) reserves has also been proposed to be limiting after large crops, thus reducing return bloom. Annual production was determined for 12 individual trees for 3 years. Return bloom was monitored on four previous-season shoot types: 1) vegetative shoots, 2) bearing terminal shoots without a second growth flush, 3) bearing lateral shoots without a second growth flush, and 4) bearing shoots that were primarily in the terminal position with a second growth flush. Nonstructural carbohydrates, organically bound N, and potassium (K) concentrations were determined in roots and shoots. Regression analysis was used to determine the effect of yield on subsequent nonstructural carbohydrates, N, and K in the roots and shoots, and their postyield concentrations on subsequent flowering. Alternate bearing was evident because there were reductions of 18%, 16%, and 18% in the percentage of current season shoots flowering for every 10 kg/tree production increase in the previous season's yield in 2002, 2003, and 2004 respectively. Flower production in 2002 decreased by 2.6 flowers/1-year-old branch and 1.6 flowers/1-year-old branch in 2003 for each 10 kg/tree increase in production. The third year of the study, neither previous season shoot type nor yield affected subsequent flower production. The previous year's shoot type did not affect the percentage of current season shoots flowering; however, the previous year's shoots that had a second growth flush produced more flowers the following year than the other shoot types. Results suggested that crop load was not related to nonstructural carbohydrates, N, or K in the roots and shoots during January in these well-managed trees. Stored nonstructural carbohydrates, N, and K were also not related to return bloom. These data suggest that the current “growth regulator–carbohydrate theory” may not be valid in these well-managed trees. Nonstructural carbohydrates, K, and organically bound N do not appear to be critical factors regulating flowering.

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Michael W. Smith, Charles T. Rohla and William D. Goff

Pecan (Carya illinoinensis) leaf elemental concentrations are the industry standard to guide fertility programs. To provide meaningful information, a standard index tissue collected at a specific development stage is required along with established elemental sufficiency ranges. We report pecan leaf elemental sufficiency ranges used in Oklahoma that were developed based on research in Oklahoma and elsewhere. In addition, fertilizer recommendations, based on various leaf elemental concentrations, are included.

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Frederic B. Ouedraogo, B. Wade Brorsen, Jon T. Biermacher and Charles T. Rohla

When trees with taproots are grown in containers, the taproot typically spirals around the bottom of the container. Currently, there is no consensus on what is the best thing to do about it. Pecan (Carya illinoinensis) grafted container-grown trees were transplanted under three treatment conditions. The treatments were container trees 1) planted straight from the container with no root pruning and no disturbance, 2) planted with taproots pruned so they no longer curled around the bottom of the container and with minimal disturbance of the root ball, and 3) planted with taproot pruned and all potting medium removed. Trunk size and shoot growth were recorded for each tree in each year during the first 4 posttransplant years (2010–14). There was no statistically significant effect of the treatments on trunk size. Shoot growth was slowed during the first year for trees with taproot pruned and potting medium removed, but there was no significant effect over the entire 4-year period.

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Abby ShalekBriski, B. Wade Brorsen, Jon T. Biermacher, Charles T. Rohla and Will Chaney

Although irrigation is a common practice in pecan (Carya illinoinensis) orchards, the effects of different methods of irrigation on young tree growth, nut quality, and nutrient uptake have not been estimated. Five irrigation systems and one nonirrigated control system were established. Tree performance was characterized by change in trunk diameter, weight per nut, average kernel percentage, and total trunk diameter growth. Nutrient uptake was determined by foliar levels. The five irrigation systems were a microsprinkler with a 35-ft diameter, a microsprinkler with a 70-ft diameter, two subsurface driplines irrigating for 2 days/week alternating between water for 2 hours and no water for 2 hours, two subsurface driplines irrigating 1 day/week for 20 hours continuously (LI2), and four subsurface driplines irrigating for 10 hours continuously for 1 day/week (LI4). Irrigation systems affected foliar levels of potassium (K), boron (B), and manganese (Mn) levels. Irrigation system did not affect change in trunk diameter or kernel percentage. A spatial Durbin error model was estimated to use trunk diameter estimates from all trees in the orchard. This model found the trunk diameters of nonirrigated and LI4 system trees to be significantly less than those trees that were irrigated by the LI2 system. When observations were pooled over all years, LI4 trees had individual pecan nut weights that were significantly less than all other systems.

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Charles T. Rohla*, Michael W. Smith, Niels O. Maness and William R. Reid

Whole fruit clusters were collected from three shoot types: terminal and lateral shoots without secondary growth, and shoots with secondary growth. Fruit per cluster was counted and nuts were individually weighed, shelled and graded. Return bloom of the same shoots was measured. Results indicated that cluster size of lateral bearing shoots was negatively related to next year's average kernel weight, nut weight, and kernel percentage. However, only kernel percentage was related to cluster size on terminal bearing shoots, and none of these parameters were related to cluster size on shoots with secondary growth. Cluster size and total kernel weight per shoot were positively related for the three shoot types. Return bloom of terminal shoots was negatively related to cluster size, but cluster size did not affect return bloom of the other shoot types.

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Charles T. Rohla, Michael W. Smith, Niels O. Maness and William Reid

The most significant horticultural problem facing pecan producers is alternate bearing. Four pecan [Carya illinoinensis (Wangenh.) C. Koch] cultivars were chosen, two with low to moderate and two with severe alternate-bearing tendencies, to compare selected characteristics related to irregular bearing. The cultivars were Colby and Peruque (low to medium alternate-bearing tendency) and Osage and Giles (high alternate-bearing tendency). Vegetative shoots and fruit-bearing shoots in the terminal and lateral position on 1-year-old branches were tagged in October, and flowering was determined the next spring. Shoot and root samples were collected while dormant and then analyzed for organically bound nitrogen (N), potassium (K), and nonstructural carbohydrate concentrations. As expected, ‘Colby’ and ‘Peruque’ had a lower alternate-bearing tendency than ‘Giles’ and ‘Osage’. Cultivars with a low alternate-bearing tendency had a larger return bloom on the bearing shoots in the terminal position than the other shoot types. Cultivars with a high alternate-bearing tendency had a lower return bloom on bearing terminal shoots than vegetative shoots. Bearing shoots in the lateral position usually had a lower return bloom than the other shoot types regardless of cultivar. Neither root nor shoot N, K, or nonstructural carbohydrate concentrations appeared to be closely related to the alternate-bearing characteristics of the four cultivars. The unique characteristic identified for low alternate-bearing cultivars was their ability to produce as many or more flowers and flowering shoots the next year on previously bearing terminal shoots compared with previously vegetative shoots. In high alternate-bearing cultivars, return bloom of bearing terminal shoots was suppressed relative to their vegetative shoots.