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- Author or Editor: Darrell Sparks x
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.
The modified Mitscherlich plant growth model was used to quantify the threshold leaf Zn level influencing nut yield and vegetative growth, on an orchard basis, for pecan [Carya illinoinensis (Wangenh.) C. Koch]. Four indices of tree performance, including percentage of trees without deficiency symptoms, vegetative growth, nut yield, and trees without deficiency symptoms plus nut yield, were analyzed with regard to leaf Zn concentration. Data available from published and unpublished sources on any single performance index were combined for mathematical modeling. The threshold value for leaf Zn was determined to be ≈50 μg·g-1 for these tree performance indices. Thus, nut yield and vegetative growth in an orchard will be reduced with a leaf Zn concentration below ≈50 μg·g-1, but will not be affected above this value.
A modification of the chilling and heating model for pecan budbreak was used to describe the interactive effects of chilling and heating on the date of first entry of the pecan nut casebearer (PNC; Acrobasis nuxvorella Neunzig) into the pecan [Carya illinoinensis (Wangenh.) K. Koch] fruit. Selected data from unpublished and published sources were used to construct the model. Base temperatures of 9.4 and 13.9C for chilling and heating, respectively, provided the best fit (r 2 = 0.981) for the model used to predict PNC activity. An inverse relationship [1/Y = 0.0037259(1 – 0.1e–0.0028069x – 574.9638969)] was found between chilling (1 Dec. through February) and heating (beginning 1 Feb.) degree-days accumulated until entry of first-generation PNC into the pecan fruit. This model can be used to predict entry of first-generation PNC larvae into fruit over a range of geographic and climatic conditions and pecan genotypes. Model validation using 1994 data from two sites in Texas suggests precision is sufficient to use the model as a guide in managing nut casebearer control.
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.
The inter-relationship of precocity, prolificacy, and kernel percentage was studied in pecan [Carya illinoensis (Wangenh.) C. Koch]. Prolificacy was highly correlated with precocity, but the relationship was not one to one. Increased precocity resulted in proportionately smaller increase in prolificacy. Variability in kernel percentage increased with prolificacy.
Pecan [Carya illinoinensis (Wangenh.) C. Koch] is indigenous to the Mississippi River drainage system of the United States. Climate in the native pecan region ranges from humid to semiarid and from mild to harsh winters. Rainfall is bimodal with peaks in March to April and in August to September. Pecan is site specific and is the climax tree species on loamy, well drained, first bottom river land with a relatively high water table. Detrimental effects from pecan's shade intolerance from its more vigorous, sympatric species are minimized as these species are specific to differ sites. Pecan's deep and phreatophytic rooting habit ensures soil moisture during drought periods and facilitates pecan's survival in semiarid regions. Root development in the humus-surface layer ensures nutrient uptake from the most nutrient rich layer of the soil and, when the lower soil profile is saturated, aeration for the roots and water and nutrient uptake. The bimodal rain pattern replenishes soil profile moisture and its timing ensures seed germination, stand establishment, well-developed seed, and minimal drought stress. Natural selection for freeze tolerance and for minimum fruit development time allows survival in areas with harsh winters and short growing seasons. Regulation of seed germination and budbreak by heating and chilling results in pecan being native in cold and warm climates, greatly increasing the native range. The northern limit for pecan is dictated by heat units; the southern limit is restricted by lack of bimodal rains and vivipary. Reproductive stress is caused by the high lipid content of seed, but is counteracted by a long juvenile growth period of the seedling, by a small nut size and low percentage kernel, and by “off” production years of the tree. Nut and percentage kernel decrease as the growing season decreases which contributes to species survival in geographical regions with a short growing season. Selection for small nuts with low percentage kernel is enhanced by predators. Tree reserves are depleted by heavy production during “on” years and are replenished during “off”years. Perpetuation of pecan forests is apparently from sib/half sib seedlings following predator satiation while dissemination into new areas may be mainly by predators. Pecan and its pests successfully co-exist. Major defense against fruit feeders is escape in time, leaf feeders by biological associations and accommodation, and leaf diseases by confrontation. Heterozygous progenies from cross-pollination provide ample genetic diversity for continuous pecan selection to endure pressures imposed throughout a wide climatic range. Ecological adaptions within native pecan forests should be used in developing and maintaining commercial pecan orchards.
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.
Freezing and damaging temperatures were imposed on `Desirable' pecan [Carya illinoinensis (Wangenh) K. Koch] trees before budbreak and again during the beginning of pistillate anthesis. Freezing temperatures imposed before budbreak resulted in abnormal flowering; freezing temperatures during anthesis did not. Abnormal flowering depends on both a critical temperature (about -1.7 to -2.2C) and a critical stage of pistillate flower bud development within the 8- to 10-day interval before budbreak.
Abstract
Fruit abortion in three cultivars of pecan [Carya illinoensis (Wangenh.) C. Koch] was compared during the 1986 drought. Fruit abortion in ‘Caddo’, ‘Western’, and ‘Cape Fear’ was, respectively, 36, 28, and 6 times greater from drought-stressed than from irrigated trees. These results may reflect cultivar differences in drought tolerance and/or differences in available soil moisture. The data also suggest that the third fruit drop in pecan is sensitive to water stress.
Abstract
Rosette was the major pecan [Carya illinoensis (Wangenh.) C. Koch] production problem prior to the discovery, in 1932, that the disorder was a symptom of Zn deficiency. A comparison of pecan production during the 10-year period prior to 1933 and the 10-year period following 1933 was made to determine the apparent effect of the use of Zn on the growth rate of pecan production from orchards in native and non-native pecan areas. Following the discovery of the cause of rosette, the rate of pecan production increased in non-native but not in native areas. The differential response is proposed to have been due primarily to less severe Zn deficiency in orchards in native than non-native areas. Alternatively, correction of Zn deficiency would have been less likely in native area orchards because of the general ineffectiveness of soil applications of Zn on calcareous soils.