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- Author or Editor: C.W. Wood x
Semi-parasitic evergreen mistletoe (Phoradendron flavescens Nutt.) is an increasingly serious weed causing loss of nut yield and tree vigor in pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards of the southeastern United States. Several herbicides and growth regulators were evaluated for efficacy against mistletoe. The dimethylamine salt of 2,4-D proved to be an effective control agent. Ethephon, glyphosate, paraquat dichloride, and polyborate exhibited little or no long-term efficacy. The dimethylamine salt of dicamba also killed mistletoe, but exhibited potential for harming host trees. Dormant season treatment of mistletoe clusters with 2,4-D reduced photosynthesis by about one-third soon after treatment, and by ≈90% from 6 to 16 weeks posttreatment, but clusters did not die until ≈4 months posttreatment. Host limbs, less than ≈3 cm in diameter at the site of mistletoe attachment, usually died within 12 months of 2,4-D treatment of the associated mistletoe cluster. Treatment of entire host trees with 2,4-D did not harm trees if applied prior to ≈1 week of budbreak. Spot treatment of mistletoe clusters, with 2,4-D at 1.2 to 2.4 g·L-1 a.i. (plus 2% crop oil), ≈2 to 3 weeks before budbreak, gave effective long-term control of mistletoe. The inclusion of a crop-oil in the 2,4-D spray greatly increased efficacy. Chemical names used: (2-chloroethyl) phosphonic acid (ethephon).
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.
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).
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.
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.
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.
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.
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.
Fungal leaf scorch, a potentially devastating disease in pecan [Carya illinoinensis (Wangenh.) K. Koch] orchards, was influenced substantially by irrigation and genotype. Three years of evaluating 76 pecan cultivars revealed that all cultivars exhibited scorch symptoms and that at least three classes of scorch susceptibility existed. Severity of symptoms was also much greater in nonirrigated than irrigated trees, and there were substantial differences in the concentrations of free nitrogenous compounds and free sugars in leaves between irrigated and nonirrigated trees.
Zonate leaf spot (ZLS) [Cristulariella moricola (Hino) Redhead (C. pyramidalis Waterman and Marshall)] on pecan [Carya illinoinensis (Wangenh.) K. Koch.]—associated with unusually wet weather during June, July, and August—occurred across much of Georgia during Summer 1994. Scott–Knott cluster analysis indicated that 27 of 36 evaluated genotypes exhibited little or no field susceptibility to ZLS. `Moneymaker' exhibited the greatest susceptibility of all cultivars studied, with `Cape Fear', `Elliott', `Sumner', and `Sioux' segregating to exhibit moderate susceptibility. An evaluation of commercial orchards indicated susceptibility of major southeastern cultivars as `Desirable' < `Stuart' < `Schley' < `Moneymaker'. Control of ZLS in commercial orchards using standard fungicide spray strategies appeared to be generally ineffective.