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Leonardo Lombardini, Astrid Volder, Monte L. Nesbitt and Donita L. Cartmill

After an outbreak of blotch leafminer (Cameraria caryaefoliella) on field-grown pecan (Carya illinoinensis) trees in 2010, an experiment was conducted to evaluate the consequences of the injury on carbon assimilation and photosynthetic efficiency, and, in particular, to assess if low-to-moderate injury induces a compensatory increase in photosynthesis. Gas exchange and light-adapted fluorescence were measured on non-injured portions of the leaflet lamina adjacent to the injured area as well as on portions of leaflets that included leafminer injury. Results indicate that damage of the photosynthetic apparatus did not extend beyond the injured areas by leafminers. Furthermore, although a strong relationship between the proportion of leafminer injury and area-based net CO2 assimilation rate of injured leaflet tissue was found, there was no evidence that pecan leaves were able to compensate for leafminer injury by upregulating CO2 assimilation in leaflet tissue that was unaffected.

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William D. Goff, Monte L. Nesbitt and Cathy L. Browne

Fourteen pecan (Carya illinoensis) clones with desirable traits selected from preliminary screenings were evaluated for scab resistance, foliage condition and foliage retention. No fungicide or insecticide sprays were applied in order to increase pest and disease pressure and to better assess suitability of the selections for low input plantings. Most clones were equal to or better than `Elliott', the resistant standard cultivar and were superior to `Desirable', the susceptible standard cultivar, in scab incidence and foliage condition.

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Monte L. Nesbitt, William D. Goff and Larry A. Stein

Pecan (Carya illinoinensis) cultivars are commercially propagated by grafting and budding. The whip graft, bark graft and four-flap graft, the most frequently used techniques for pecan grafting, require dormant scions, collected and stored for 60 to 120 days before the spring grafting season. Poor graft success is sometimes attributed to poor handling and storage of the scionwood. Moisture content of packing material, sealing cut ends of the scions with wax, and use of polyethylene bags was evaluated in 1998 and 1999. Scions were collected in early February each year, and stored for 60 to 70 days in a household refrigerator at 2 °C (35.6 °F) under different treatment regimes. Scion viability was tested by bark grafting on limbs of mature pecan trees. Moisture of the scions was affected each year by the amount of water added to packing material and by sealing the cut ends, but the differences did not impact graft success. In 1998, graft success rate was equally good among scions stored in polyethylene bags with different amounts of added moisture, whether cut ends were sealed or not. Graft success in 1999 was affected by an interaction of sealing the cut ends, packing material and location of grafting.

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Monte L. Nesbitt, Larry Stein and William D. Goff

Pecan is a difficult species to propagate by grafting. The whip graft, bark graft, and four-flap graft, the most often-used techniques for pecan grafting, require dormant scions, collected and stored for 60 to 120 days prior to spring-season grafting. Poor graft success is often blamed on handling and storage environment of the scionwood. Moisture content of packing material, waxing of cut ends, and use of polyethylene bags was evaluated in a controlled experiment in 1998 and 1999. Scions were cut in early February each year, and stored for 60 to 70 days in a household refrigerator under different treatment regimes. Scion viability was tested by bark grafting mature pecan trees in Fairhope, Ala., and Uvalde, Texas. In 1998, graft success rate was equally good among scions stored in polyethylene bags with different amounts of added moisture, whether cut ends were waxed or not. Moisture loss of the scions during storage was affected each year by the amount of water added to packing material and by waxing the cut-ends, but the differences did not impact graft success. An interaction of not waxing the cut ends and very wet packing material reduced graft success at Fairhope, Ala., but not Uvalde, Texas, in 1999.

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Monte L. Nesbitt, Robert C. Ebel and William A. Dozier Jr

Satsuma mandarin is presently the primary citrus crop for citrus growers in south Alabama, south Mississippi, and for certain new plantings in northwest Florida. Current growth in satsuma mandarin hectarage in Alabama is similar to the historical hectarage expansion that occurred during decades or clusters of years with a low incidence of lethal, freezing temperatures. Commercial groves currently range in size from 100 to 2000 trees and use various freeze protection strategies, including wind breaks, overstory frost protection with pine or pecan trees, under-tree and scaffold branch irrigation, and high tunnel polyethylene-covered greenhouses. The various methods of freeze protection require adjustments in cultural management practices, including spacing, pruning, irrigation, and fertilization. The primary rootstock used and recommended for its cold-hardiness and edaphic adaptation is Poncirus trifoliata. Some groves use ‘Swingle’ citrumelo, mainly because it is grown and propagated in Louisiana, where it is valued for its higher salt tolerance. ‘Owari’ was the original cultivar introduced to the United States from Japan in the late 19th century and is still the main cultivar in Alabama today, although earlier maturing cultivars, including ‘Brown Select’, ‘Early St. Ann’, and ‘LA Early’, are being introduced to extend the marketing season. Cultivars from Japan and China, including ‘Okitsu Wase’, ‘Miyagawa Wase’, and ‘Xie Shan’, are currently being evaluated for suitability in the region. Three general methods of culture with regard to spacing and pruning are discussed. Nitrogen application rates are typically low to moderate, yet leaf nitrogen levels surveyed in groves in 2005 were generally optimal or high with respect to published sufficiency levels for mature citrus in Florida.

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Monte L. Nesbitt, J. Benton Storey, D. Lyda and L.J. Grauke

Rootstock resistance to soil-borne phytopathogenic fungi, such as Phymatotrichum omnivorum (Shear) Duggar, is an important factor in disease control. Measurement of natural rootstock resistance is often based on plant survival/mortality percentage, and /or growth data. Fungal colonization of host roots in disease screening experiments may not be uniform for many reasons, causing variability in host response. Quantification of fungal colonization is needed in order to better understand rootstock performance. Ergosterol, a structural sterol in cell membranes of fungi, is not found in higher plants, and can thus be a measure of fungal colonization. Ergosterol was extracted from roots of pecan seedlings artificially inoculated with P. omnivorum and grown in an environmental growth chamber. Analysis of extracts with HPLC revealed that seedlings which were killed in screening, or had low root performance ratings, had increased levels of ergosterol. Non-inoculated controls also contained Ergosterol. indicating contamination and possible competition by other fungi.

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Monte L. Nesbitt, J.B. Storey, S.D. Lyda and L.J. Grauke

Phymatotrichum Root Rot, caused by Phymatotrichum omnivorum (Shear) Duggar, imposes severe losses upon dicotyledonous horticultural crops in the southwestern United States and northern Mexico. Rootstock resistance could benefit pecan (Carya illinoensis) production in affected growing areas; however, erratic growth habit of this pathogen and site variability prevents effective field screening. We have developed a containerized screening method for horticultural crops, using a commercial soilless growing medium. In sterile cultures, 2.5 × 60 cm glass tubes containing Metro Mix 500 yielded more grams of P. omnivorum sclerotia than cultures grown in Houston Black Clay, a traditional medium for cotton research. Preliminary screening with Okra (Abelmoschus esculentus) in non-sterile Metro-Mix 500 resulted in 75% mortality of inoculated plants in 30-35 days. Preliminary screening with 12-week-old, open-pollinated `Apache' and `Moore' pecan rootstocks has resulted in 25% mortality of inoculated plants in 150 days. Pecan seedlings with visible taproot infection appear to delay mortality by adventitious root formation.

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Monte L. Nesbitt, N.R. McDaniel, Robert C. Ebel, W.A. Dozier and David G. Himelrick

Several microsprinkler treatments were tested on 5-year-old satsuma mandarin orange (Citrus unshiu Marc.) trees to compare survivability of trunks and scaffold limbs in severe freezes. Three damaging freeze events occurred during winter, with two in 1995-96 and one in 1996-97. Air temperature dropped to -9.4, -5.6, and -6.7 °C, respectively. Almost 90% of the foliage was dead on the control plants after the first freezing event and 98% after the second. A single microsprinkler 1.6 m high in the canopy delivering 90.8 L·h-1 reduced injury; only 54% of the canopy was dead after the first freeze and 71% after the second. There was slightly more shoot-tip dieback on the plants in the microsprinkler treatments than on the control plants after the first two freezes. The amount of limb breakage by ice was minor. The third freeze killed 34% of the canopy in the control plants, but only 26% in the plants in the microsprinkler treatments. Use of microsprinklers increased yield in 1996, but yield for all treatments was very low. Yield for all treatments fully recovered in 1997, averaging 153 kg/tree. Although no death of scaffold limbs or trunks occurred, these results demonstrate that microsprinkler irrigation reduces damage to foliage and increases yield somewhat in severe freezes.

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Monte L. Nesbitt, N.R. McDaniel, Robert C. Ebel, W.A. Dozier and David G. Himelrick

Several microsprinkler treatments were tested on 5-year-old satsuma mandarin orange (Citrus unshiu Marc.) trees to compare survivability of trunks and scaffold limbs in severe freezes. Three damaging freeze events occurred during winter, with two in 1995–96 and one in 1996–97. Air temperature dropped to –9.4, –5.6, and –6.7 °C, respectively. Almost 90% of the foliage was dead on the control plants after the first freezing event and 98% after the second. A single microsprinkler 1.6 m high in the canopy delivering 90.8 L·h–1 reduced injury; only 54% of the canopy was dead after the first freeze and 71% after the second. There was slightly more shoot-tip dieback on the plants in the microsprinkler treatments than on the control plants after the first two freezes. The amount of limb breakage by ice was minor. The third freeze killed 34% of the canopy in the control plants, but only 26% in the plants in the microsprinkler treatments. Use of microsprinklers increased yield in 1996, but yield for all treatments was very low. Yield for all treatments fully recovered in 1997, averaging 153 kg/tree. Although no death of scaffold limbs or trunks occurred, these results demonstrate that microsprinkler irrigation reduces damage to foliage and increases yield somewhat in severe freezes.

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Monte L. Nesbitt, Robert C. Ebel, Douglas Findley, Bryan Wilkins, Floyd Woods and David Himelrick

Containerized `Owari' satsuma mandarin (Citrus unshiu Marc.) on Poncirus trifoliata `Flying Dragon' rootstock were exposed to one of two acclimation regimes (cold acclimated and unacclimated) and frozen in a computer-controlled freezer to five different low temperatures. Whole plant survival was measured and compared to the results of four leaf and stem injury assays. Acclimating plants in growth chambers at 20 °C day and 10 °C night for 14 days, followed by 15 °C day and 4 °C night for 14 to 21 days resulted in an 81% and 80% increase in leaf and stem survival, respectively, when frozen to a low of -8 °C. Electrolyte leakage and phenolic leakage assays effectively detected changes in percent leaf survival, but the TTC stain assay, using leaf disks, did not. Stem survival was best predicted by the TTC assay, using the phloem as the indicator tissue for survival. Electrolyte leakage and phenolic leakage were also reliable assays for predicting stem survival, although survival percentages were different at the same electrolyte leakage values reported in other studies. The callus growth assay accurately predicted survival for cold acclimated satsuma mandarin stems only. Chemical name used: triphenyl tetrazolium chloride (TTC).