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  • Author or Editor: Sparks Darrell x
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A multiple regression model (R 2 = 0.945) was developed from historical data, 1971-92, to predict pecan [Carya illinoensis (Wangenh.) C. Koch] production in an arid climate at high elevations. Dependent variables were year of production as a measure of production trend, number of degrees below 0.6°C following budbreak as a measure of late spring freeze injury, and number of degrees below 0.6°C preceding nut maturity as a measure of early fall freeze injury. Year of production was the dominant factor influencing production. Freezing temperature following budbreak had about two times more effect on production than freezing temperature preceding nut maturity. Pecan production under arid conditions at high elevations depends on fewer variables (three) than previously shown for humid conditions (eight variables).

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A multiple regression model was developed from historical data, 1945-92, to predict pecan [Carya illinoensis (Wangenh.) C. Koch] production in a humid climate. Variables were production trend (year of production), previous year's production, and climatic indices for the previous and current year. Production trend was used to measure change in production with time. Previous year's production was the index of alternate bearing. Variables for previous year's climate were heating degree-days for April-October and cumulative rainfall during May-July and 1-15 Sept. Variables for current year's climate were cumulative rainfall during April-August and 1-15 Sept. The indicator used for scab [Cladosporium caryigenum (Ell. & Langl.) Gottwald] infection was the highest cumulative sum of 2 or more days of consecutive rain occurring in May, June, or 1-15 July. The R 2 for the model was 0.908. Production trend was the most important factor influencing production during the 1945-92 study period. Importance of the other variables in decreasing order were previous year's rainfall in May-July, consecutive rainy days, previous year's production, current year's 1-15 Sept. rainfall, previous year's heating degree-days, previous year's rainfall for 1-15 Sept., and current year's rainfall during April-August. Previous year's conditions had a greater effect on production than current year's. The recent decline in pecan production in the southeastern United States is due to an unfavorable change in climate.

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Chilling and heating effects on budbreak of pecan [Carya illinoinensis (Wangenh.) K. Koch] trees were examined by linear regression analyses from experimental data and from records of budbreak dates over a wide geographic range. The results demonstrate that budbreak in pecan is under the interactive control of heating and of chilling. Heat required for budbreak varies inversely with chill accumulation, and budbreak may occur with no chilling once sufficient heat accumulates. Variability in budbreak increases dramatically when there are fewer than ≈ 100 chilling degree days. Heating degree days with daily minima <2.2C are inefficient; 3.9C is the most efficient heating and chilling base. At base 3.9C and the daily minimum heating temperature of 2.2C, heating degree days required for budbreak of a composite of cultivars can be predicted from chilling degree days over a wide geographic range by the relationship, Log Y = 2.7190-0.0216 √X.

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Pecan (Carya illinoinensis) scab was evaluated following unusually extended rains in 1994. Strengths and weaknesses of a variety of scab management practices were studied in five orchards, Scab control was more effective on trees with adequate sunlight exposure than in dense orchards or with fungicide applied after rain than by preset intervals. Triphenyl tin hydroxide, a nonsystemic fungicide, was most effective when applied within 24 h after rain; but, the systemic fungicide, propiconazole, was effective when applied within 6 days after rain. Fungicides must be applied consistently after rain for maximum scab control.

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Effects of applied P on pecan [Carya illinoensis (Wangenh.) C. Koch] were studied in the greenhouse and field. Objectives were to study P effects on vegetative and nut growth, leaf scorch and/or premature defoliation, and nutrition. Elements were analyzed for both total and extractable (2% acetic acid) fractions. In the greenhouse, deficiency symptoms occurred when leaf P (dry-weight basis) was 0.08%; vegetative growth was maximum when P was 0.19% to 0.22%. In the field, applied P increased nut growth and decreased leaf scorch and premature defoliation. Leaf P values for maximum nut growth and minimum leaf scorch and/or defoliation are greater than 0.14% and 0.16%, respectively. Except when P was within the visible deficiency range, applied P had little direct influence on the concentration of other essential elements under either greenhouse or field conditions. Generally, extractable P was a slightly better indicator of the P status of the tree than total P, but the differences are of doubtful physiological significance.

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The relationship was analyzed between historical annual rainfall and pecan [Carya illinoinensis (Wangenh.) K. Koch] homogeneity in selected hardwood populations along two river systems in the native habitat of the species. Tree species other than pecan (sympatric species) were more abundant with increasing rainfall in that the more homogenous pecan populations were located in geographic areas with the least rainfall. These results are the first to establish that pecan stand homogeneity across geographic areas is inversely related to the amount of annual rainfall. Variation in soil texture within geographic areas was also strongly correlated with variation in pecan homogeneity and pecan density. Pecans occur principally on loamy bottom lands and grow on clayey bottom lands in less abundance. A hypothesis related to growth partitioning between root and shoot is proposed to account for pecan's survival advantage, and thus higher stand homogeneity, with decreasing rainfall in native pecan areas. Conversely, the decrease in stand homogeneity with increasing rainfall is proposed to be due to increased forestation of sympatric species on clayey sites that are not optimum for pecan. Across the rainfall gradients, pecan's shade intolerance is suggested to be minimized by differential site requirements for pecan and its sympatric species.

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Pecan [Carya illinoensis (Wang) K. Koch] seed were germinated in perlite and treated with either a complete nutrient solution or a nutrient solution minus B, Ca, Cu, Fe, K, Mg, Mn, N, P, S, or Zn. Omitting any single nutrient suppressed seedling growth and induced deficiency symptoms for all nutrients except Fe, Mn, and Cu. Corresponding leaf concentration data associated with deficiency symptoms and normal growth agreed closely with proposed standards.

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‘Curtis’ pecan [Carya illinoensis (Wang) K. Koch] seeds were germinated in perlite and treated with either deionized water, a complete nutrient solution or a nutrient solution minus B, Ca, Cu, Fe, K, Mg, Mn, N, P, S, or Zn. All seedlings receiving deionized water died back, 23% died back in the minus B treatment, 82% when Ca was omitted, and none when treated with a complete nutrient solution.

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Foliar applied Mg (0.0%, 0.24%, 0.48%, or 0.96% MgSO4-7H2O) was compared to root-supplied Mg (Hoagland's solution) in pecan [Carya illinoensis (Wan- genh.) C. Koch] seedlings. Foliar-applied Mg suppressed, but did not prevent, Mg deficiency symptoms and increased leaf concentration of Mg and seedling growth compared to plants grown without Mg. Leaf Mg and growth from foliar sprays were substantially less than for plants with root-supplied Mg. Compared to root-supplied Mg, omitting Mg increased the leaf concentration of P, K, Cu, and Zn; decreased Ca and Mg; and had no effect on N, Fe, Mn, B, and Al. The nutritional imbalances induced by Mg deficiency were alleviated by foliar-applied Mg.

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Abstract

Response from foliar-applied P (0.0%, 0.50%, 0.75%, or 1.00% P from KH2PO4) was compared to that from root-supplied P (Hoagland's solution) in pecan [Carya illinoinensis (Wangenh.) C. Koch] seedlings. Compared with no applied P, foliar-applied P suppressed or prevented P deficiency symptoms; increased the P concentration in the leaf, trunk, and root; and increased tree growth. However, P in all 3 organs and growth of plants treated with foliar sprays were less than for plants with root-supplied P. Furthermore, P sprays eventually produced leaf scorch. Compared to root-supplied P, omitting P affected N, P, K, Ca, Mg, Fe, Mn, B, Cu, Zn, Na, and Al in the plant. These imbalances induced by P deficiency were only partially alleviated by foliar-applied P.

Open Access