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Bruce W. Wood, William Goff and Monte Nesbitt

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Bruce W. Wood and Charles C. Reilly

Bearing pecan [Carya illinoinensis (Wangenh.) K. Koch] trees overly stressed by crop load and premature autumn defoliation either died or were severely damaged by -3°C in mid-November. Orchard damage was associated with death of tree roots during the dormant season. Exposure of stressed trees to -5°C in mid-March produced an atypical, but distinct, bottom-to-top-of-canopy gradient in bud death and reduced growth of shoots and foliage that was consistent with the pattern of reduced carbohydrate reserves of associated support shoots. Additionally, the foliage of damaged trees contained higher concentrations of N, P, K, Ca, Mg, Mn, Fe, and B. Trees did not exhibit traditional symptoms of cold damage, thus these findings extend cold injury diagnostic criteria to include both root and tree death during the dormant season and also a distinct gradient in shoot death during early spring. Damage by cold appears to be preventable by avoiding excessive tree stress due to overcropping and premature defoliation.

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Bruce W. Wood and Larry J. Grauke

We report the composition of the rare-earth (REE) metallome component of the foliar ionomes of pecan (Carya illinoinensis) and other North American Carya and how accumulation of specific REEs relate to ploidy level and to accumulation of essential divalent nutrient elements. REE accumulation within the foliar ionomes of 12 Carya species, growing on a common site and soil, indicates that REEs accumulate according to the Oddo-Harkins rule with Ce, La, Nd, and Y (Ce > La > Nd > Y) being the dominant REEs with accumulated concentration typically being La > Ce > Nd > Y > Gd > Pr > Sm > Dy > Er > Yb > Ho > Tb >Tm > Sc >Lu. Carya species quantitatively differ in accumulation of REEs with all but C. aquatica accumulating at much greater concentrations than non-Carya tree species and with tetraploid Carya accumulating to approximately twice the concentration as diploid Carya. Carya tomentosa was an especially heavy accumulator of REEs at 859 μg·g−1 dry weight, whereas C. aquatica was especially light at 84 μg·g−1. Accumulation of REEs was such that any one element within this elemental class was tightly linked (generally r ≥ 0.94, but 0.81 for Ce) to all others. Accumulation of REEs is negatively correlated with Ca accumulation and positively correlated with Mn and Cu accumulation in diploid Carya. In tetraploid Carya, accumulated Mg, Ca, and Fe is positively associated with foliar concentration of REEs. Total concentration of REEs in pecan's foliar ionome was 190 μg·g−1, about equivalent to that of Mn. Circumstantial evidence suggests that one or more of the physiochemically similar REEs increases physiological plasticity and subsequent adaptive fitness to certain Carya species, especially tetraploids. Because all tetraploid Carya are high REE accumulators and are native to more xeric habitats than diploids, which typically occupy mesic habitats, it appears that REEs might play a role in Carya speciation and adaptation to certain site-limiting environmental stresses. REEs appear to play an unknown metabolic/physiological role in pecan and most Carya species, especially tetraploids; thus, their nutritional physiology merits further investigation.

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Andrew P. Nyczepir and Bruce W. Wood

Peach tree short life (PTSL) is associated with the presence of ring nematode, Mesocriconema xenoplax, and poor orchard management practices. The ability of postplant nickel (Ni) foliar application to suppress M. xenoplax population density and thereby prolong survival of peach trees on a PTSL site infested with M. xenoplax was investigated from 2004 to 2011. For this study, the site was divided into plots, which received the following treatments: 1) Ni (foliar-applied); 2) methyl bromide fumigation (MBr); and 3) an untreated control. Peach trees were planted into all plots in Mar. 2005 and the foliar Ni treatment was applied three times in 2005 and 2006. Nickel did not detectably suppress M. xenoplax populations as compared with MBr fumigation. The protective effect of MBr fumigation in suppressing M. xenoplax population density persisted for 27 months after orchard establishment. Trees receiving multiple foliar Ni applications at 0.45 g·L−1 over 2 years, while exposed to M. xenoplax, exhibited greater PTSL mortality than trees growing in untreated or MBr-fumigated plots. These results suggest that foliar applications of Ni to peach trees, growing on a PTSL site, should be used with caution in commercial orchards because these treatments can deleteriously disrupt tree metabolic/physiological processes sufficient to increase the incidence of PTSL tree mortality.

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Bruce W. Wood and Robert D. Marquard

Self-pollination was estimated in three Georgia pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards. Selfing in two large orchards lacking an interplanted complementary pollinizer (one orchard being comprised of `Curtis' and the other `Moneymaker') was estimated to be at least 3% and 49%, respectively. A `Cheyenne' orchard containing `Stuart' as a complementary pollinizer at 5% density was estimated to have had at least 14% and 42% of ripened nuts derived from selfing in two consecutive years. These estimates suggest self-pollination may reduce yield in pecan orchards in the southeastern United States.

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Bruce W. Wood and Charles C. Reilly

This study reports on sudden death (or decline) of mature and apparently healthy pecan trees [Carya illinoinensis (Wangenh.) K. Koch]. Observations suggest that death and damage is due to winter cold injury (although the season's low was only -5 °C). The severity of this cold injury-like form of sudden death is closely associated with nut crop load (i.e., grams of kernels per square centimeter of trunk cross-sectional area) and premature defoliation. Both dead and declining trees not only produced relatively heavy crops, but also exhibited substantial premature pest-induced defoliation the previous autumn. The near absence of sugars and starch in roots and shoots of dead or declining trees at budbreak and the relatively high levels in healthy trees indicates that diminished assimilate reserves during the dormant season were the key factor causing death or decline. The diminished assimilate reserves prevented the accumulation of assimilate reserves necessary for maintaining live roots throughout the dormancy and prevented proper cold acclimation of shoot tissues. Distinct symptoms of sudden tree death or decline compared to typical cold damage are: a) a distinct top-to-bottom gradation of tree damage, with an increased proportion of dead shoots and shoots supporting abnormally small foliage being near the base of the canopy; b) dessicated and tan appearance of inner bark and phloem of the main trunk rather than brown coloration so typical of classical cold injury; c) death of roots by time of budbreak; and d) absence of resprouting from the trunk or root collar. These observations indicate that pecan trees can suddenly die due to being overly stressed for assimilates and that economic losses previously attributed to injury by severe winter cold sometimes may be due to depleted assimilate reserves during the dormant season as a result of overcropping and premature defoliation.

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Bruce W. Wood and Charles C. Reilly

The host-parasite interaction between the black pecan aphid (BPA) [Melanocallis caryaefoliae (Davis)] and pecan [Carya illinoensis (Wangenh.) K. Koch] was investigated. Three years of field observations of the ability of BPA populations to induce chlorotic blotches, or visual damage, on 32 pecan cultivars revealed considerable variation in cultivar susceptibility to BPA damage. Among the most commonly grown cultivars, `Sioux', `Cape Fear', `Farley', `Cowley', `Grabohls', and `Barton' exhibited the least damage, whereas `Choctaw', `Oconee', and `Sumner' exhibited the greatest, with `Sioux' and `Choctaw' exhibiting the greatest extremes in susceptibility. Subsequent evaluation indicated that the foliage of pecan genotypes can exhibit an antibiotic-like effect, resulting in the suppression of resident BPA populations. However, the relationship between the degree of this antibiotic effect and the degree of damage exhibited by trees, or field tolerance, was negligible (r = -0.10). For example, while `Choctaw' foliage greatly suppressed BPA population growth, this population was able to inflict relatively severe damage to leaves. An evaluation of feeding preference indicated that BPA alate viviparae (winged females) preferentially feed upon host cultivars on which they have been previously feeding. This feeding preference was eliminated by rinsing leaves with distilled water; hence, a water soluble factor(s) appears to be involved in host preference.

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Charles C. Reilly and Bruce W. Wood

Propiconazole, a fungicide, suppressed leaf area of a wide variety of young pecan [Carya illinoinensis (Wangenh.) K. Koch] seedling genotypes but did not reduce leaf area of orchard trees. Leaf area declined linearly as dosage increased from 0.16 to 1.25 mL·L–1. Suppression of leaf area by propiconazole was inversely proportional to leaf age. No reduction of leaf area was detected in orchards where `Cheyenne', `Desirable', and `Pawnee' were treated with three applications (14-day intervals) of fungicide (either propiconazole, fentin hydroxide, or fenbuconazole) from budbreak to early May. Spring application of the three fungicides alone or in combination with zinc sulfate did not influence fruit set. Control of pecan scab [Cladosporium caryigenum (Ell. et Lang) Gottwald] was achieved with either fentin hydroxide or fenbuconazole for the full season, or with early season use of dodine, then propiconazole, and then followed by fentin hydroxide for late-season disease control. Fungicide treatments had no effect on nut weight. These data indicate that fungicides applied to pecan during pollination at commercially recommended dosages and intervals, with or without zinc sulfate, do not adversely influence leaf area or fruit set of orchard trees. Chemical names used: n-dodecylguanidine acetate (dodine); triphenyltin hydroxide (fentin hydroxide); 1-[[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-yl] methyl]-1H-1,2,4-triazole (propiconazole); α-[2-(4-chlorophenyl)ethyl]-α-phenyl-1H-1,2,4-triazole-1-propanenitrile (fenbuconazole).

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Bruce W. Wood, Rufus Chaney and Mark Crawford

The existence of nickel (Ni) deficiency in certain horticultural crops merits development of fertilizer products suitable for specific niche uses and for correcting or preventing deficiency problems before marketability and yields are affected. The efficacy of satisfying plant nutritional needs for Ni using biomass of Ni hyperaccumulator species was assessed. Aqueous extraction of Alyssum murale (Waldst. & Kit.) biomass yielded a Ni-enriched extract that, upon spray application, corrects and prevents Ni deficiency in pecan [Carya illinoinensis (Wangenh.) K. Koch]. The Ni-Alyssum biomass extract was as effective at correcting or preventing Ni deficiency as was a commercial Ni-sulfate salt. Foliar treatment of pecan with either source at ≥10 mg·L–1 Ni, regardless of source, prevented deficiency symptoms whereas treatment at less than 10 mg·L–1 Ni was only partially effective. Autumn application of Ni to foliage at 100 mg·L–1 Ni during leaf senescence resulted in enough remobilized Ni to prevent expression of morphologically based Ni deficiency symptoms the following spring. The study demonstrates that micronutrient deficiencies are potentially correctable using extracts of metal-accumulating plants.