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  • Author or Editor: Michael W. Kilby x
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The pecan is native to North America and is cultured as a major crop in both the United States and Mexico. In the early part of this century, pecans were thought of as a secondary crop grown in the southern geographic section of the United States. Increased demand for use as a nutritious food has resulted in expansion of the industry into the desert Southwest and California. Adaptive cultivars and irrigation coupled with the lack of diseases and insects has been instrumental in industry development in the West. As the industry has matured during the latter part of the century, pecan culture has improved into a strong crop enterprise business. Orchard management technique and orchard development concepts have been refined, resulting in increased production and awareness. In recent years, production in Mexico has impacted the U.S. price structure and pecan industry economy. The alternate-bearing nature of pecans also impacts prices received by growers. The aging of pecan trees has resulted in serious dilemmas, such as increased tree size and shading. This situation requires techniques such as tree thinning or hedge pruning to enhance annual production and improve nut quality. Various ramifications and parameters of these management practices will be discussed.

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Phymatotrichum omnivorum (P.O.) is a soil-borne fungus ubiquitous to the alkaline soil of Texas, New Mexico, Arizona, and Mexico. This fungus causes serious economic loss to grapevines in Arizona, ranging from high to low desert environments. In order to determine the relative resistance to P.O., rootstocks of various species combinations were planted in a calcareous soil which had a history of P.O.; the primary Vitis species included in the trial were champini, candican, berlandieri, repestris, and vinifera. One-year-old rooted cuttings were planted in a randomized block design with one plant per plot with six replications. Common names of rootstocks planted included Freedom, Dogridge, Oppenheim 4 (SO4), Harmony, Champanel, and 5BB. The vinifera used as the control was `Sauvignon Blanc'. Vines were allowed to grow and die for two years. All of the rootstocks exhibited greater resistance to P.O. when compared to the vinifera control. The rootstock exhibiting the greatest resistance was Harmony, with a low of 18% mortality. Other rootstocks showed a loss of approximately 33% over the duration of the trial.

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A thinning-timing study of apple, (Malus domestica Borkh. cv. `Royal Gala'), grown on M7a rootstock, was initiated during the Spring of 1992 to determine the optimum and last effective thinning date to affect `Royal Gala' apple size. Trees planted in 1987 in sandy-loam soil and trained on a three tier Ebro trellis system were used in this study. Thinning-timing treatments were begun at full bloom and continued weekly for four weeks making a total of five thinning dates and an non-thinned control. Each treatment's flowers and/or fruits were hand thinned to 15 - 20 centimeters between fruit on tree limbs. Measurements of the diameter of marked apples from each treatment were made weekly during the growing season to plot growth curves. Results show a significant difference of larger fruit size and weight of second and third thinning treatment dates as compared to the other treatment dates and the control.

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N-dimethylamino succinamic acid (Alar) labeled as C14 radioisotope was used to study the absorption and translocation of the chemical in young tung trees. The lower epidermis of the leaves was treated with known quantities of the compound and at various time intervals samples of treated leaves and other portions of the plants were assayed for radioactivity.

One hour after treatment the labeled compound had been absorbed by the lamina and translocated to the petiole. Alar was found in all parts of the plant 24 hr after treatment. Four days after treatment, peak accumulation was reached in most of the 14 sampling locations throughout the plant with the terminal growing point showing the greatest accumulation in micrograms per gram of fresh tissue. Significant quantities of labeled Alar were found in stems on all sampling dates.

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The objective of our experiment was to determine if the application of two deer repellents to six grape cultivars (Vitis vinifera L.) caused significant phytotoxic effects, production losses, or altered the sensory characteristics of wine. We evaluated fifteen single vine plants from six different cultivars in a randomized block design that included the two repellent treatments and an untreated control. During spring 1997, we applied repellents biweekly from budbreak until flowering (2 Apr. to 14 May). Plantskyyd was applied more frequently than recommended by the product label (for trees) due to rapid emergence of unprotected shoot growth in vineyards. Hot Sauce and Plantskydd caused some initial minor phytotoxicity during 1997, however, the yield and phytotoxicity of treated plants were similar to controls by harvest. A panel detected a significant difference in the color, aroma, or taste of `Chardonnay' wine made from grapes treated with repellents compared to wine made from untreated control grapes (P = 0.001 for Hot Sauce; P = 0.05 for Plantskydd). We conclude that Hot Sauce and Plantskydd did not cause serious production losses or phytotoxic effects for the six cultivars treated. However, both Hot Sauce and Plantskydd significantly altered the sensory attributes of Chardonnay wine, which may preclude the use of chemical repellents in wine grape vineyards under the experimental conditions applied in our study.

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The effect of soil banding zinc sulfate and zinc (Zn)-EDTA was evaluated over a period of 4 years on established ‘Wichita’ pecans [Carya illinoinensis (Wangenh.) K. Koch] growing in alkaline, calcareous soil. Treatments evaluated were ZnSO4 applied at 74 kg Zn/ha and Zn-EDTA at 19 kg Zn/ha. These materials were applied just once on 23 Mar. 2005. Fertilizers were injected in two bands placed 1.2 m from either side of the trunk of the tree and 18 cm deep. Treatments were replicated four times in a randomized complete block design. Data collected included foliar Zn concentrations throughout the season, midseason foliar nutrient concentrations, leaflet growth, nut yield, and nut quality. Significant differences in foliar Zn levels were found 1 month after application of Zn-EDTA. Differences also were noted during the next 3 years on ≈25% of the sampling dates. Yields of in-shell pecans averaged 2800 kg·ha−1 during the 3 years of harvest but were unaffected by treatments. Nut quality also was unaffected. Nut kernel percentage was very high, ranging from 61.2% to 63.6% during the study. Growth, measured as leaflet area and trunk cross-sectional area, was unaffected by Zn application. Chlorophyll index ranged from 47.5 to 48.0 in 2007 and from 44.7 to 45.4 in 2008 and was unaffected by applied treatments. Zn-EDTA increased Zn uptake slightly by ‘Wichita’ pecan trees in alkaline, calcareous conditions during 3 years after one soil band injection. Ongoing research on potted pecan trees (with the same soil used in the present study) suggests that Zn-EDTA can very effectively increase Zn uptake if placed in close proximity to the tree roots. Additional research is needed to refine application and placement methods in these types of soils to produce a more effective and consistent response.

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Measurement of nutrients in leaf tissue is a practical method of monitoring the nutritional status of perennial crops such as pecan (Carya illinoinensis, Wang. C. Koch). Accurate interpretations require known standard concentrations for the crop and region. To determine standard concentrations for pecans, focusing on those grown in the desert southwest, we conducted a survey of 135 `Western Schley' pecan trees in Arizona for 2 years. Leaf nutrient concentrations and yield were collected for each tree. Leaf nutrient concentrations from the highest yielding trees (50th yield percentile) were used to calculate a mean and CV for each nutrient. Results were compared with data from New Mexico, Georgia, and Sonora, Mexico. Relatively large differences were noted in mean K, Ca, B, Cu, Fe, Mn, and Zn levels. Nutrient interpretation ranges were calculated based on Arizona population statistics using the balance index method.

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The crop water stress index (CWSI), based on the relationship between the canopy temperature of a well-watered plant in full sunlight and the atmospheric water content, numerically quantifies water stress. A 4-year study was established to determine the long-term effect of water application levels on production, nut quality characteristics, and growth of pecans [Carya illinoinensis (Wangenh.) C. Koch cv. Western Schley]. Highest yields were attained when trees were relatively nonstressed (CWSI ≤ 0.08). Trees subjected to moderate water stress before irrigation (CWSI ≥ 0.20) showed reduced yield, nut weight, and tree growth, although water-use efficiency increased. With water management practices resulting in maximum yield, nut size, and tree growth (CWSI ≤ 0.08), tree water use varied up to 44% in the same orchard, depending on crop load and yearly climatic variations.

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