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  • Author or Editor: Bruce W. Wood x
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Alternate bearing is a major economic problem for producers of pecan nuts [Carya illinoinensis (Wangenh.) K. Koch], yet a fundamental understanding of alternate bearing remains elusive. Nut yields (over a period of up to 78 years) from a commercial-like orchard of 66 cultivars was used to calculate alternate bearing intensity (I). Best-fit regression analysis indicates no association between I and fruit ripening date (FRD) or nut volume; although, there was moderate association with post-ripening foliation periods (PRFP) in that I tends to decrease as the length of the PRFP decreases. Multiple regression models indicated that FRD and nut volume were poor predictors of I: however, PRFP possessed significant inverse predictive power. Late-season canopy health, as measured by percentage of leaflet retention, decreased as FRD approached early-season ripening. Late-season photoassimilation rate was high er on foliage of trees with late FRDs than those with mid- or early-season ripening dates. These data provide new insight into the complex nature of alternate bearing in pecan and provide evidence for modifying the existing theories of alternate bearing of pecan.

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Abstract

The heavy levels of sooty mold commonly present on pecan [Carya illinoensis (Wangenh.) C. Koch] foliage in the autumn prompted an evaluation of its influence on net photosynthesis (Pn) of pecan leaves. Extra heavy sooty mold levels were observed to block light penetration to the leaf surface by up to 98%. Heavy mold levels suppressed leaflet Pn by up to 70% with suppression due to a blockage of photosynthetically active radiation (PAR). An observed 4°C increase in abaxial leaf surface temperature may also contribute to this suppression. The results indicate a possible need to introduce sooty mold control methods into orchard management programs.

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A 4-year field study on pecan [Carya illinoinensis (Wangenh.) K. Koch] provided indirect support of the supposition held by some U.S. pecan growers that air-blast foliar sprays of potassium nitrate (KNO3) plus surfactant enhances nut yield. While these treatments did not measurably influence yield components, foliar K nutrition, or net photosynthesis, they did suppress “yellow-type” aphid populations. While air-blast sprays of water alone suppressed aphid populations, the inclusion of KNO3 plus surfactant provided an additional level of suppression.

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Overcrowding in young high-density pecan [Carya illinoensis (Wangenh.) C. Koch] orchards has prompted a study of tree transplanting and evaluation of survival and tree performance. Shoot growth and nut production characteristics of 13-year-old `Stuart' and `Farley' pecan trees subjected to different stubbing and pruning treatments and then transplanted with a large tree spade indicated that transplants can survive with little or no pruning if moved when dormant. Shoot regrowth was proportional to the degree of pruning, and nut production was inversely proportional to the degree of pruning.

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Mouse-ear (ME) is a potentially severe anomalous growth disorder affecting young pecan [Carya illinoinensis (Wangenh.) K. Koch] trees in portions of the Gulf Coast Coastal Plain of the southeastern United States. A survey of its incidence and severity found it to be commonly exhibited by replants on second-generation orchard sites, or where mature pecan trees previously grew. While most frequently observed as a replant problem, it also occasionally occurs at sites where pecan has not previously grown. The disorder is not graft transmissible and is only temporarily mitigated by pruning. Degree of severity within the tree canopy typically increases with canopy height. Several morphological and physiological symptoms for mouse-ear are described. Important symptoms include dwarfing of tree organs, poorly developed root system, rosetting, delayed bud-break, loss of apical dominance, reduced photoassimilation, nutrient element imbalance in foliage, and increased water stress. The overall symptomatology is consistent with a physiological deficiency of a key micronutrient at budbreak, that is influenced by biotic (e.g., nematodes) and abiotic (e.g., water and fertility management strategies) factors. A comparison of orchard soil characteristics between ME and adjacent normal orchards indicated that severely affected orchards typically possessed high amounts of soil Zn, Ca, Mg, and P, but low Cu and Ni; and were acidic and sandy in texture. The Zn: Cu ratio of soils appears to be a major factor contributing to symptoms, especially since ME severity increases as the Zn: Cu ratio increases. However, Ni may also be a factor as the Zn: Ni ratio is also larger in soils of ME sites. It is postulated that the “severe” form of mouse-ear is primarily due to the physiological deficiency of copper at budbreak, but may also be influenced by Ni and nematodes.

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Mouse-ear (ME) is a severe growth disorder affecting pecan [Carya illinoinensis (Wangenh.) K. Koch] trees from southeastern U.S. Gulf Coast Coastal Plain orchards. Slight to moderate ME was substantially corrected by foliar sprays of either Cu or GA3 shortly after budbreak, but sprays were ineffective for severely mouse-eared trees. Applications of Cu, S, and P to the soil surface of moderately affected trees corrected deficiencies after three years. Incorporation of Cu or P in backfill soils of newly planted trees prevented ME, whereas incorporation of Zn or Ca induced ME and Mn was benign. The severe form of ME, commonly exhibited by young trees, appears to be linked to a physiological deficiency of Cu and/or Ni at the time of budbreak. It likely occurs as a replant problem in second-generation orchards due to accumulation of soil Zn from decades of foliar Zn applications to correct Zn deficiency.

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Mouse-ear (ME) is a potentially severe anomalous growth disorder affecting pecan [Carya illinoinensis (Wangenh.) K. Koch] trees. It is especially severe in second generation sites throughout much of the Gulf Coast Coastal Plain of the southeastern U.S., but can also occur in potted nursery trees. Orchard and greenhouse studies on trees treated with either Cu or Ni indicated that foliar applied Ni corrects ME. ME symptoms were prevented, in both orchard and greenhouse trees, by a single mid-October foliar spray of Ni (nickel sulfate), whereas nontreated control trees exhibited severe ME. Similarly, post budbreak spring spray applications of Ni to foliage of shoots of orchard trees exhibiting severe ME prevented ME symptoms on subsequent growth, but did not correct morphological distortions of foliage developed before Ni treatment. Foliar application of Cu in mid-October to greenhouse seedling trees increased ME severity the following spring. Post budbreak application of Ni to these Cu treated MEed seedling trees prevented ME symptoms in post Ni application growth, but did not alter morphology of foliage exhibiting ME before Ni treatment. Thus, high leaf Cu concentrations appear to be capable of disrupting Ni dependent physiological processes. Foliar application of Ni to ME prone trees in mid-October or soon after budbreak, is an effective means of preventing or minimizing ME. These studies indicate that ME in pecan is due to a Ni deficiency at budbreak. It also supports the role of Ni as an essential plant nutrient element.

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The discovery of nickel (Ni) deficiency in field plantings of pecan [Caryaillinoinensis (Wangenh.) K. Koch] (Wood et al., 2004) has led to efforts to identify appropriate management approaches to correct tree deficiency and to identify the causes for Ni deficiency. Evaluation of several inorganic and organic forms of Ni have indicated that solutions from all sources function well to correct deficiencies when timely applied as a foliar spray to affected trees at Ni concentrations >10 mg·L-1. Addition of urea, ammonium nitrate, or nicotinic acid to Ni spray solutions increased apparent foliar uptake from Ni sprays. The lower critical level of Ni, based on foliar analysis, appears to be in the 3-5 mg·L-1 dw range, with the upper critical level appearing to be >50 mg·L-1 dw. The cause of Ni deficiency in soils possessing plenty of Ni is associated with excessive amounts of one or more metals (e.g., Ca, Mg, Fr, Mn, Cu, and Zn) that inhibit Ni uptake and/or utilization. Root damage by nematode feeding and cool/dry soils during early spring also contributes to Ni deficiency. Foliar application of Ni to foliage in the autumn and subsequent appearance of Ni in dormant season shoot tissues indicates that Ni can be mobilized from senescing foliage to dormant season shoots and is therefore available for early spring growth. Evidence indicates that pecan has a higher Ni requirement than most other crop species because it transports nitrogenous substances as ureides. Thus, there is evidence that Ni-metalloenzymes are playing either a direct or indirect role in ureide and nitrogen metabolism. It is postulated that crop species that are most likely to exhibit field level Ni deficiencies are those that transport N as ureides. Candidate crops will be discussed.

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Insufficient fruit retention limits profitability of certain pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivars. The present study examined efficacy of aminoethoxyvinylglycine (formulated as ReTain®; Valent BioSciences, Libertyville, IL), a natural ethylene inhibitor, for increasing crop-load through increased fruit retention in pecan trees grown at three distinct locations within the U.S. pecan belt. Several years of field studies found that timely postpollination ReTain® sprays [132 mg·L−1 a.i. (11.7 oz./acre)] to canopies could increase fruit retention of ‘Desirable’ and increase crop yield by 16% to 38% in trees carrying a “moderate to heavy” crop. ReTain® did not detectably increase fruit retention on trees carrying a “light” crop-load. The ReTain®-associated increase in yield of “heavy” crop-load trees did not necessarily decrease subsequent year yield. ReTain® appears to offer commercial potential as a crop-load management tool for ‘Desirable’ through regulation of Stage II drop (i.e., June-drop), but may not be efficacious for all cultivars.

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