Excessive Stage II fruit drop (i.e., June drop) often limits profitability of certain pecan [Carya illinoinensis (Wangenh.) C. Koch] cultivars. Anecdotal evidence indicates that one cause of drop is linked to a nutrient element deficiency. This study examines the consequences of improving tree potassium (K) nutrition on fruit drop, nutmeat yield, and kernel quality (i.e., % kernel) in ‘Desirable’ orchards in which generally accepted foliar analysis standards indicate satisfactory tree K nutritional status (i.e., 0.75% or greater to 2.5% K/dry weight). Multiyear field studies of two such orchards found that elevating leaf and fruit K concentration through soil banding of potash over drip irrigation emitters: 1) increased fruit retention by reducing Stage II fruit drop; 2) increased in-shell nut yield; and 3) increased nut quality by increasing percentage kernel. Potash applied through soil banding elevated foliar and fruit K concentration by ≈ 0.1% to 0.4% units within a few months post-application depending on the amount applied; however, the beneficial effects of a single potash soil band application diminished after the first year. A comparison of the K concentration of retained fruit versus abscised fruit during the Stage II fruit drop window found that retained fruit possessed endogenous K concentrations of 1.2% to 1.7% (dry weight basis) in one orchard and 1.45% to 1.9% in a second orchard, whereas aborted fruit possessed K at 0.65% to 1.2% in one orchard and 0.75% to 1.2% in a second orchard, respectively, thus establishing ≈ 1.25% K as a “drop threshold” under conditions of this study. The total K concentration of retained fruit is typically 0.25% to 0.50 K/dry weight greater than dropped fruit. Considerable K-associated late-spring fruit drop can occur in ‘Desirable’, although early- to midsummer leaf analysis indicates trees were K-sufficient, hence implying that young fruit likely possesses a higher K requirement than does foliage. These K-associated benefits to trees meeting accepted K sufficiency criteria is evidence that K nutrition management of ‘Desirable’ pecan merits r-evaluation and possibly pecan K nutrition in general.
Bruce W. Wood, Lenny Wells and Frank Funderburke
M. Leonard Wells and Bruce W. Wood
This study examines the relationship between foliar nitrogen:potassium (N:K) ratio and in-shell yield of pecan [Carya illinoinensis (Wangenh.) K.Koch]. Regression analysis of linear and curvilinear relationships between leaflet N:K ratio and in-shell yield identified associations relevant to orchard nutrition management. Analysis revealed that ON (heavy crop) year N:K ratio correlates with ON year yield (r2 = –0.69), OFF (light crop) year yield (r2 = +0.34), 2-year average yield (r2 = −0.52), and difference between ON and OFF year yields (r2 = –0.69) below the optimum yield level (less than 1800 kg·ha−1) for southeastern U.S. pecan orchards. Pecan yield therefore appears to be associated with N:K ratio. This study suggests that a decline in pecan yield is associated with high N:K ratios in the ON year, thus meriting further investigation into the relationships of N and K to yield. It is suggested that pecan orchards be managed such that foliage contains a N concentration of 2.5% to 2.9% and a K concentration of 1.3% to 1.5% while maintaining the N:K ratio at ≈2:1 for maximization of pecan yields in the southeastern United States over the long term.
Bruce W. Wood and Charles C. Reilly
Orchard trees of pecan [Carya illinoinensis (Wangenh.) K. Koch] were subjected to combinations of cultural practices inducing differential physiological states so as to assess the potential for culture-related impact on damage to trees by key arthropod pests. Leaf N concentration, leaf water status, and crop load all affected foliar damage by black pecan aphids [BPA; Melanocallis caryaefoliae (Davis)] and pecan leaf scorch mite [PLSM; Eotetranychus hicoriae (McGregor)], as well as second-flush shoot growth. Damage to first-flush foliage in the late season by BPA generally diminished as leaf water status and leaf N concentration increased, but intensified with a reduction in crop load. Conversely, foliage damage by PLSM increased with elevated leaf water status and N concentration, but was unaffected by crop load. First- and second-order interactions for all combinations of cultural treatments conferring differential physiological states affected damage by pests and induction of second-flush shoot growth. Arthropod-induced defoliation on trees receiving highly favorable cultural practices—those producing high leaf N, high leaf water availability, and low crop load—was greater than on trees receiving minimal or lesser cultural inputs. Thus, cultural practices influencing leaf water status, N status, or crop load potentially act and interact to produce both desirable and undesirable side-effects on damage incurred by certain arthropod pests and therefore merit consideration in efforts to develop improved integrated pest management strategies.
Patrick J. Conner and Bruce W. Wood
Genetic variation among pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivars was studied using randomly amplified polymorphic DNA (RAPD) markers. Using a combination of primers, a unique fingerprint was produced for each of the pecan genotypes studied. The genetic relatedness between 44 cultivars was estimated using more than 100 RAPD markers. Genetic distances based on the simple matching coefficient varied from 0.91 to 0.59. The phenetic dendogram developed from cluster analysis showed relatively weak grouping association. However, cultivars with known pedigrees usually grouped with at least one of the parents and genetic similarity estimates appear to agree with known genetic relationships. Using RAPD information in determining genetic relationships among pecan cultivars with unknown or questionable pedigrees and the integration of that knowledge into the breeding program is discussed.
E.A. Baldwin and Bruce W. Woods
Pecans (Carya illinoinensis) are full of unsaturated fatty acids, which are subject to oxidative cleavage. This results in the development of rancid off-flavors, which render the nuts unmarketable. For this reason, pecans must be stored under costly refrigerated conditions. Furthermore, pecans usually undergo retail distribution and marketing at ambient conditions, which promote development of off-flavors. Application of cellulose-based edible coatings reduced off-flavor, and improved overall flavor scores while adding shine to the nuts during 14 months of storage under ambient conditions. Development of rancidity involves hydrolysis of glycerides into free fatty acids, oxidation of double bonds of unsaturated fatty acids to form peroxides and then autooxidation of the free fatty acids once the peroxides reach a sufficient level to perpetuate this reaction. One of the products of autooxidation is hexanal which is, thus, a good indicator of rancidity. Analysis of pecans by gas chromatography revealed that hexanal levels were reduced in coated nuts by 5- to over 200-fold compared to uncoated controls, depending on the coating treatment. Some of the coating treatments affected nut color, but overall flavor and appearance were improved by certain formulations.
Bruce W. Wood and Charles C. Reilly
Foliar feeding by the black pecan aphid [Melanocallis caryaefoliae (Davis)] can cause tremendous economic losses. Evaluations of black aphids on pecan genotypes indicates that both antixenosis and antibiosis-like resistance mechanisms exists. Tests for antixenosis indicated that aphids possess clear preferences for certain genotypes over others and that this preference can be dependent on a water-soluble chemical component of the leaf surface. Aphids also exhibited a “conditioning preference,” in which they preferentially feed on genotypes from which they originated. Antibiosis tests indicated that pecan genotypes influence the reproductive success of aphids already possessing a feeding adaptation to those same pecan genotypes; therefore, an evaluation of 30 cultivars for antibiosis indicated that populations developed only 20% as fast on `Choctaw' and `Alley' as on `Desirable' and `Success'. No cultivar was observed to essentially prevent aphid reproduction.
Bruce W. Wood and Charles C. Reilly
Water stage fruit split (WS) is an erratic and complex problem often causing major crop losses to susceptible pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivars. This study identified two episodes of WS for `Wichita' pecan—a highly susceptible cultivar. The previously recognized precipitation-induced fruit splitting is the major episode; however, a previously unrecognized precipitation-independent, minor episode can also occured before the major episode. This minor episode was associated with the low solar irradiance and high relative humidity—conditions commonly associated with August rains. The crop characteristics of affected trees also influenced WS in that WS increased as crop load per tree increased. Fruits were also more likely to exhibit WS if located within the lower tree canopy. Treatment of foliage with an antitranspirant immediately before split-inducing conditions increased WS. Maintenance of moist soils for ≈2 weeks before WS-inducing conditions substantially reduced WS-related crop losses. These findings help to explain the erratic nature of WS and indicate that maintenance of trees in a well-watered state for ≈2 weeks before the initiation of shell hardening may substantially reduce WS-related crop losses in certain years.
M. Lenny Wells and Bruce W. Wood
Water-stage fruit-split (WSFS) is a relatively common and often major problem of certain pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivars. This study evaluates the possibility that the malady can be influenced by improving tree micronutrient nutrition. Foliar sprays of boron (B) and nickel (Ni) to WSFS-susceptible fruit of ‘Cape Fear’ and ‘Sumner’ are evaluated based on the possibility that either B or Ni potentially affects the severity of WSFS exhibited by trees. Although the incidence of WSFS on ‘Cape Fear’ was unaffected by micronutrient sprays, the severity of WSFS was substantially reduced in each of the 3 study years by foliar B application and in 2005 by foliar Ni application. Repeated foliar sprays of Ni also reduced WSFS of ‘Sumner’ fruit. These data indicate that improving either B or Ni nutrition can potentially reduce crop loss resulting from WSFS in certain orchard situations and provides evidence that insufficient availability of B or Ni to developing ovary tissues potentially predisposes developing fruit to WSFS when environmental triggers occur.
Patrick J. Conner and Bruce W. Wood
Genetic variation among pecan [Carya illinoinensis (Wangenh.) C. Koch] cultivars was studied using randomly amplified polymorphic DNA (RAPD) markers. Using a combination of primers, a unique fingerprint is presented for each of the pecan genotypes studied. The genetic relatedness between 43 cultivars was estimated using 100 RAPD markers. Genetic distances, based on the similarity coefficient of Nei & Li, varied from 0.91 to 0.46, with an average value of 0.66 among all cultivars. The phenetic dendrogram developed from cluster analysis showed relatively weak grouping association. However, cultivars with known pedigrees usually grouped with at least one of the parents and genetic similarity estimates appear to agree with known genetic relationships.
Bruce W. Wood and William R. Joyner
Observations of net assimilation rates (`A') by pecan sun and shade leaves in relation to various levels of solar irradiation, the light adaptation characteristics of these leaf types, the role of clouds in suppressing the penetration of solar irradiation, and the abundance of cloud cover in the southeastern U.S. during the growing season, suggest that nut production throughout the U.S. pecan belt is being limited by insufficient sunlight with the southeastern U.S. (comprising about 2/3 of the commercial U.S. pecan production) being especially impacted. In support of this hypothesis, regression analysis showed cultivar-type nut production for Georgia from 1977-1989 to be significantly (P<.0001, R2 = 0.79) associated with sunlight levels ≥ 3000 Wh m-2d-1 from mid August to early October for the same year. This is taken as evidence that the amount of sunlight reaching the canopy seems to be a major factor that should be considered in relation to orchard site selection and canopy management techniques.