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Michael W. Smith and Becky S. Cheary

Alternate bearing of pecan [Carya illinoinensis (Wangenh.) K. Koch] remains the leading problem of the industry. Several cultural practices have been developed or improved to mitigate alternate bearing. Premature defoliation was one problem identified that substantially decreased return bloom. The objective of this study was to determine the response of individual shoots exposed to various defoliation treatments. In one study, individual vegetative or bearing shoots were hand-defoliated in mid-September. Defoliation was the basal one-half, distal one-half, entire shoot, or not defoliated. Another study applied the same defoliation treatments to bearing shoots in July, August, or September. Defoliation had minimal effects on return bloom and rarely affected the percentage of current-season shoots fruiting the next year. Defoliation date also had little effect on return bloom. These data indicate that individual shoot response to defoliation was not autonomous and has implications for determining crop overload and needed mechanical fruit thinning.

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Michael W. Smith and Becky S. Cheary

Phosphorus (P) and potassium (K) deficiencies were identified in a ‘Pawnee’ pecan [Carya illinoinensis (Wangenh.) C. Koch] orchard. Deficiencies of P and K have traditionally been difficult to correct in pecan. Broadcast application of P rarely results in a positive response. Broadcast applications of K frequently require 3 or more years before a positive response was noted, particularly in soils with high clay content. A 4-year study was initiated on 10-year-old trees using soil band-applied P, K, blended P plus K, and a non-treated control. Rates were based on trunk cross-sectional area adjusted annually for tree growth. Phosphorus application alleviated leaf deficiency symptoms, increased leaf P concentration, and improved flowering. However, P application resulted in darker kernels than the control or K-treated trees. Band-applied K was not associated with observed leaf symptoms but increased leaf K above the recommended minimum level and increased the percentage of number-1 kernels. Band-applied P or blended P plus K suppressed leaf K concentrations. Blended P plus K applied as a band appeared to improve return bloom 2 of 3 years, although no improvement in leaf K concentration was noted.

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Michael W. Smith, Becky S. Cheary and Becky L. Carroll

Nitrogen was applied between 1996 and 2002 to grafted `Mohawk' pecan (Carya illinoinensis Wangenh. C. Koch.) trees at 75 or 150 kg·ha-1 either as a single application in March or as a split application with 60% applied in March and 40% the first week of June. In 1997 and 2001, a spring freeze damaged developing shoots and buds, resulting in a small, noncommercial crop and the June portion of the N application was withheld. Nitrogen was also applied during the first week in October at 0 or 50 kg·ha-1 N if the crop load before fruit thinning in August was ≥40% fruiting shoots. There were few differences in the percentage of fruiting shoots or cluster size associated with N rate or applying N as a single or split application. Leaf N concentrations were either not affected by treatment or the results were inconsistent. Omitting the June application when a crop failure occurred did not affect the percentage of fruiting shoots the following year. October N application either did not affect or reduced the percentage of fruiting shoots the following year, and had no influence on leaf N concentration in July or October. These results indicate that the only advantage of a split N application is the option of withholding the second portion in the event of a crop failure. However, the added expense associated with splitting the N application versus the risk of crop failure must be assessed for each situation to determine if this is a sound economic practice. These data do not support an October N application when the crop is ≥40% fruiting shoots to reduce irregular bearing.

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Michael W. Smith, Becky S. Cheary and Becky L. Carroll

Newly planted pecan (Carya illinoinensis Wangenh. C. Koch cv. Kanza) trees were grown for 5 years in a bermudagrass [Cynodon dactylon (L.) Pers.] sod with vegetation-free circles 0, 0.91, 1.83, 3.66, or 7.32 m in diameter. Trees were irrigated and fertilized to minimize growth differences associated with competition from the bermudagrass. There were no differences in trunk diameter among treatments the first 2 years of the study. During the next 3 years, trunk diameter increased curvilinearly as the vegetation-free circle increased. A vegetation-free circle diameter of 1.83 m produced near maximum tree growth. Although trunk diameter improved slightly as the vegetation-free diameter was increased up to 7.32 m, it was not sufficient to justify the additional expense for herbicides nor exposure of unprotected soil to erosion.

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Michael W. Smith, Becky S. Cheary and Becky L. Carroll

Parameters were defined to germinate pecan [Carya illinoinensis (Wangenh.) C. Koch] seeds in aerated water followed by container planting. Germination was not affected by the ratio of seeds to water in the germination containers. Highest germination rates with the greatest uniformity in germination were obtained with a water bath temperature of 32 °C. Stratification up to 188 days increased the rate of germination, but the largest response was between no stratification and 56 days (6.5 days vs. 2.3 days to reach 50% germination, respectively). Seeds that were germinated in a water bath, then planted in containers, achieved 50% emergence in 4.7 days compared to 12.4 days for direct-planted seed. Emergence was more uniform when seeds were germinated in water before planting compared with seeds that were directly planted in containers (7.0 days vs. 9.5 days between 10% and 90% emergence, respectively). Also, by germinating the seeds before planting, nonviable seeds were eliminated, resulting in 100% emergence compared to 76% emergence when planted directly.

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Michael W. Smith, Becky S. Cheary and Becky L. Carroll

Newly planted pecan (Carya illinoinensis Wangenh. C. Koch) trees were grown for 3 years in a tall fescue (Festuca arundinacea Shreb. CV. Kentucky 31) sod with vegetation-free circles 0, 0.91, 1.83, 3.66, or 7.32 m in diameter. Trees were irrigated to minimize growth differences associated with water competition from fescue. There were no differences among treatments in total shoot growth after 1 year, but trunk growth was increased by vegetation-free areas. During the second year, trees with a 0.91-m-wide vegetation-free area had twice as much shoot growth, and trunks were twice the size of those without a vegetation-free zone. The third year, trees with a 0.91-m-wide vegetation-free circle had 403% more new shoot growth, and trunks were 202% larger than those without a vegetation-free zone. Cumulative shoot growth was up to 559% greater with vegetation control. Tree growth was similar with a 1.83- or 3.66-m-wide vegetation-free circle, and trees in both treatments were larger than trees with 0- or 0.91-m-wide vegetation-free zones. Extending the vegetation-free zone to 7.32 m wide was not advantageous.

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Michael W. Smith, Becky L. Carroll and Becky S. Cheary

`Giles' pecan [Carya illinoinensis (Wangenh.) K. Koch] seedlings were either not mulched or mulched with wood chips arranged in a 1- or 2-m-wide square that was 30 cm deep. Mulch treatments were in factorial combination with two N rates applied as either a single application at budbreak or as a split application at budbreak and 3 weeks later. Tree height was positively related to mulch width each year of the 3-year study, and trunk diameter was positively related to mulch width during the second and third years of the experiment. Leaf P and K concentration during 2 years and leaf N during 1 year of the study were positively related to mulch width. Trees receiving the higher N rate were taller during 2 of 3 years, but leaf N concentration was not affected by N rate. No differences in the parameters measured were observed whether N was applied as a single or as a split application.

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Michael W. Smith, Becky S. Cheary and Becky L. Carroll

Pecan [Carya illinoinensis (Wangenh.) C. Koch] kernels (cotyledon) of ‘Pawnee’ displayed a consistent malady not described previously that was designated as “kernel necrosis.” The most severe form of the problem was blackened, necrotic tissue engulfing the basal one-half to one-third of the kernel. The mildest form was darkened tissue in the dorsal grove at the basal end of the kernel. The problem was first observable during the gel stage of kernel development. No symptoms of kernel necrosis were visible on the shuck (involucre). Kernel necrosis was more prominent on ‘Pawnee’, ‘Choctaw’, and ‘Oklahoma’ than other cultivars observed. At maturity, nuts with kernel necrosis had a larger volume than nuts with normal kernels. There were few differences in elemental concentrations of normal kernels from a severely affected orchard and an orchard with little kernel necrosis, and none of the differences appeared to be associated with this disorder. ‘Pawnee’ kernels with necrosis had more phosphorus, zinc, and manganese than normal kernels. Basal segments of necrotic kernels had more boron and acetic acid-extractable and water-soluble calcium than distal segments or normal kernels. Higher elemental concentrations in basal segments of necrotic kernels did not appear sufficient to cause tissue damage.

Soil from the orchard with severe kernel necrosis had unusually high concentrations of nitrate, expressed as nitrogen (NO3-N), in the soil profile. Groundwater used for irrigation was contaminated with 34 mg·L−1 NO3-N. An experiment on ‘Pawnee’ evaluated three nitrogen (N) rates, 0, 0.8 g·cm2 cross-sectional trunk area applied in March, and 1.6 g + 1.6 g + 1.2 g·cm2 cross-sectional trunk area N applied during the second week in March, first week in June, and first week in September, respectively, on the incidence of kernel necrosis, leaf N concentration, soil NO3 concentration, yield, nut quality, and growth over 5 years. Leaf N was affected by treatment only once during the study. Nitrates accumulated in the soil, increasing 24% in 3 years when no supplemental N was applied, except in the contaminated irrigation water. Kernel necrosis was either unaffected by N treatment or during 1 year, kernel necrosis was highest without supplemental N application. Tree yield, kernel quality, and growth were unaffected by N treatment. Yield fluctuations among years were apparent demonstrating that an abundant N supply did not prevent alternate bearing. Kernel necrosis was a severe problem in one orchard and was identified in several orchards at low frequencies. The cause of kernel necrosis remains unknown.

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Michael W. Smith, Becky L. Carroll and Becky S. Cheary

`Dodd' pecan seedlings [Carya illinoinensis (Wangenh.) K. Koch] were chilled at 6C for 0 to 1800 hours in 300-hour intervals and percent budbreak and days to budbreak recorded. Chilling duration required for ≥ 50% budbreak was 900 hours. Chilling > 900 hours increased budbreak percentage and reduced time to budbreak. `Dodd' seedlings chilled at 1, 5, or 9C for 0 to 2500 hours in 500-hour intervals had more lateral budbreak after 1000 hours of chilling at SC than at 1 or 9C. When chilling hours ranged from 1500 to 2500, 1C increased budbreak of the first lateral bud compared with 5 or 9C. As chilling was increased from 1000 to 2500 hours, the days to budbreak declined, and the uniformity of budbreak increased.

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Michael W. Smith, Becky S. Cheary and Becky L. Carroll

Vegetation surrounding pecan (Carya illinoinensis Wangenh. C. Koch) trees in a 4.3 × 6 m area was either controlled with a nonresidual herbicide for the entire growing season, not controlled, or controlled at certain times during the growing season. After three growing seasons, trunk diameters were suppressed 54% when vegetation was not controlled, 47% when not controlled until 1 Aug., and 37% if not controlled after 1 June compared to entire growing season vegetation control. Trunk diameters were not significantly different from entire season vegetation control when vegetation was controlled from 1 June through fall frost or vegetation controlled from April until 1 Aug. Vegetation in the plots was typically dominated by cool season herbaceous dicots in May and June, and warm-season grasses during August and September.