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Thomas M. Kon, Melanie A. Schupp, Hans E. Winzeler, and James R. Schupp

Blossom thinning can confer significant benefits to apple growers, including increased fruit size and annual bearing. However, current blossom thinning practices can damage spur leaves and/or fruit. We evaluated the use of short duration forced heated air treatments [thermal shock (TS)] as a blossom thinning strategy for ‘York Imperial’. Using a variable-temperature heat gun, TS treatments were applied to solitary blossoms 24 hours after pollination. Effects of output temperature (five levels) and treatment duration (four levels) were evaluated using a completely randomized design with a factorial treatment structure. Short duration treatments (0.5 and 1.0 seconds) were ineffective for arresting pollen tube growth in vivo. TS temperature required to inhibit stylar pollen tube growth was inconsistent across years. In 2014, TS temperatures ≥56 °C inhibited pollen tubes from reaching the style base at 2.0 and 4.0 second durations. However, in 2015, TS temperatures ≥81 °C at 4.0 seconds prevented pollen tubes from reaching the style base. Inconsistent effects of TS across years were attributed to treatments being applied too late due to optimal conditions for pollen tube growth during the intervening 24-hour period after pollination. Excessive injury to spur leaf tissue was observed at temperatures higher than 84 °C and 70 °C (2.0 and 4.0 seconds, respectively). Pollen tube growth was reduced or arrested at temperature and duration combinations that caused minimal visible injury to spur leaves. Identifying and exploiting structural differences between apple blossoms and vegetative spur leaves may provide insight for the future development of TS or other attempts at developing selective thinning technologies.

Free access

Peter H. Dernoeden, Cale A. Bigelow, John E. Kaminski, and John M. Krouse

Smooth crabgrass [Digitaria ischaemum (Schreber) Schreber ex Muhlenb.] is an invasive weed of cool-season turfgrasses. Previous research has demonstrated that quinclorac is an effective postemergence herbicide for crabgrass control, but performance has been erratic in some regions. Furthermore, quinclorac may elicit objectionable levels of discoloration in creeping bentgrass (Agrostis stolonifera L.). The objectives of this 3-year field study were to determine optimum rates and timings of quinclorac applications that provide consistent levels of effective crabgrass control and to assess creeping bentgrass quality responses to quinclorac. To evaluate crabgrass control, quinclorac was applied in early-, mid- and late-postemergence timings at various rates to a perennial ryegrass (Lolium perenne L.) turf. Similar treatments were applied to creeping bentgrass to determine if application timing and rate influenced the level and duration of discoloration. Quinclorac was applied alone or was tank-mixed with either urea (N at 6.1 kg·ha-1) or chelated iron (Fe)+nitrogen (N) (FeSO4 at 1.1 kg·ha-1+N at 2.2 kg·ha-1) to determine if they would mask discoloration. Crabgrass control generally was more effective in the early- and midpostemergence application timings. A single application of quinclorac (0.84 kg·ha-1) was effective where crabgrass levels were moderate, but sequential (i.e. multiple) applications were required where crabgrass levels were severe. The most consistent level of crabgrass control where weed pressure was severe occurred with three, sequential quinclorac (0.37 or 0.42 kg·ha-1) applications. Creeping bentgrass exhibited 2 to 11 weeks of unacceptable discoloration in response to sequential quinclorac applications. Chelated Fe+N was more effective than urea in masking discoloration. In general, chelated Fe+N tank-mixed with quinclorac masked discoloration and turf had quality equivalent to untreated bentgrass on most, but not all rating dates. Chemical names used: 3,7,-dichloro-8-quinolinecarboxylic acid (quinclorac).

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N.R. Bhat, Thomas L. Prince, Harry K. Tayama, and Stephen A. Carver

Free access

Creighton L. Gupton and James M. Spiers

An experiment arranged in a randomized complete block design with four replications of two cultivars × six pH levels × four Zn levels was conducted to determine if Zn caused leaf chlorosis in rabbiteye (Vaccinium ashei Reade cv. Climax) and southern highbush (mostly V. corymbosum L. cv. Bladen) blueberry. `Bladen' accumulated more foliar Mn and Zn than `Climax', but Fe concentration was similar in the two cultivars. Leaf chlorosis ratings were similar for the two cultivars. Solution pH had no significant effect on Mn, Zn, or Fe leaf concentration or degree of chlorosis. Zinc level in the nutrient solution affected leaf concentration of Mn and Zn but not of Fe. A significant linear increase in chlorosis resulted from increasing Zn solution concentration from 30 to 120 mg·L–1. We conclude that high levels of Zn may induce leaf chlorosis in rabbiteye and southern highbush blueberry.

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John D. Lea-Cox and James P. Syvertsen

We studied whether foliar-applied N uptake from a single application of low-biuret N-urea or K NO to citrus leaves was affected by N source, leaf age, or whole-shoot N content. In a glasshouse experiment using potted 18-month-old Citrus paradisi (L.) `Redblush' grapefruit trees grown in full sun, 2- and 6-month-old leaves on single shoots were dipped into a 11.2 g N/liter (1.776% atom excess N-urea) solution with 0.1% (v/v) Triton X-77. Two entire trees were harvested 1.5,6,24, and 48 hours after N application. Uptake of N per unit leaf area was 1.6- to 6-fold greater for 2-month-old leaves than for older leaves. The largest proportion of N remained in the treated leaf, although there was some acropetal movement to shoot tips. In a second experiment, 11.2 g N/liter (3.78% atom excess) urea-15N and 3.4 g N/titer (4.92% atom excess) KNO solutions of comparable osmotic potential were applied to 8-week-old leaves on 5-year-old `Redblush' grapefruit field-grown trees of differing N status. Twenty-four percent of the applied N-urea was taken up after 1 hour and 54% after 48 hours. On average, only 3% and 8% of the K NO was taken up after 1 and 48 hours, respectively. Urea increased leaf N concentration by 2.2 mg N/g or 7.5% of total leaf N after 48 hours compared to a 0.5 mg N/g increase (1.8% of total leaf N) for KNO. Foliar uptake of N from urea, however, decreased (P < 0.05) with increasing total shoot N content after 48 hours (r = 0.57).

Open access

Thomas M. Kon, Melanie A. Schupp, Hans E. Winzeler, and James R. Schupp

The use of short-duration applications of thermal energy (thermal shock; TS) as an apple blossom thinning strategy was investigated. Effects of TS temperature and timing on stigmatic receptivity, pollen tube growth in vivo, and visible leaf injury were evaluated in multiple experiments on ‘Crimson Gala’. TS treatments were applied to blossoms and spur leaves using a variable temperature heat gun. TS temperatures ≥86 °C had a strong inhibitory effect on pollen tube growth on the stigmatic surface and in the style. TS temperatures >79 °C reduced average pollen tube length to less than the average style length. Timing of TS treatment (0 or 24 hours after pollination) was not an influential factor, indicating that effective TS temperatures reduced pollen tube growth up to 24 hours after the pollination event. The onset of thermal injury to vegetative tissues occurred at similar TS temperatures that inhibited pollen tube growth in vivo. Excessive leaf injury (>33%) was observed at 95 °C, suggesting relatively narrow differences in thermal sensitivity between reproductive and vegetative tissues. Inconsistent TS temperatures and/or responses were observed in some experiments. Ambient air temperature may have influenced heat gun output temperatures and/or plant susceptibility. While results suggest some promise, additional work is required to validate and further develop this concept.

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Joseph E. Beeler, Gregory R. Armel, James T. Brosnan, Jose J. Vargas, William E. Klingeman, Rebecca M. Koepke-Hill, Gary E. Bates, Dean A. Kopsell, and Phillip C. Flanagan

Trumpetcreeper (Campsis radicans) is a native, perennial, weedy vine of pastures, row crops, fence rows, and right-of-ways throughout most of the eastern United States. Field and greenhouse studies were conducted in 2008 and 2009 near Newport, TN, and in Knoxville, TN, to evaluate aminocyclopyrachlor-methyl and aminopyralid alone and in mixtures with 2,4-D and diflufenzopyr for selective trumpetcreeper control when applied postemergence in an abandoned nursery. These treatments were compared with commercial standards of dicamba and a prepackaged mixture of triclopyr plus 2,4-D. In the field, aminocyclopyrachlor-methyl alone controlled trumpetcreeper 77% to 93%, while aminopyralid alone only controlled trumpetcreeper 0% to 20% by 12 months after treatment (MAT). The addition of diflufenzopyr or 2,4-D to aminocyclopyrachlor-methyl did not improve trumpetcreeper control in the field; however, the addition of 2,4-D to aminopyralid improved control of trumpetcreeper from 50% to 58%. All aminocyclopyrachlor-methyl treatments controlled trumpetcreeper greater than or equal to dicamba and the prepackaged mixture of triclopyr plus 2,4-D. In the greenhouse, aminocyclopyrachlor and aminocyclopyrachlor-methyl applied at 8.75 to 35 g·ha−1 controlled trumpetcreeper 58% to 72% by 1 MAT. When both herbicides were applied at 70 g·ha−1, aminocyclopyrachlor controlled trumpetcreeper 64%, while aminocyclopyrachlor-methyl controlled trumpetcreeper 99%, similar to dicamba.

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U. Hartmond, J.D. Whitney, J.K. Burns, and W.J. Kender

Two field studies were conducted to evaluate the effect of metsulfuron-methyl and 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMN-pyrazole) on abscission of `Valencia' orange [Citrus sinensis (L.) Osbeck] during the 3-month harvest season. Solutions of metsulfuron-methyl at 0.5, 1, and 2 mg·L-1 active ingredient (a.i.) were applied at 10-day intervals beginning on 13 Feb. and ending 18 May 1998. Early in the harvest season, 1 or 2 mg·L-1 metsulfuron-methyl significantly reduced fruit detachment force (FDF) 14 days after application. Metsulfuron-methyl was less effective during a 4- to 6-week period following bloom (“less-responsive period”). After this period, metsulfuron-methyl regained the ability to loosen fruit. Applications of 2 mg·L-1 a.i. were more effective than 1 mg·L-1 in reducing FDF and causing leaf drop, but 0.5 mg·L-1 a.i. had little or no effect on FDF. Flowers and leaflets on developing shoots and young fruit completely abscised with 1 and 2 mg·L-1 a.i. Defoliation and twig dieback was extensive at all concentrations and spray dates, eliminating metsulfuron-methyl as a commercially viable abscission agent for citrus. In a separate experiment CMN-pyrazole at 50 and 100 mg·L-1 a.i. and metsulfuronmethyl at 0.5 mg·L-1 a.i. were applied to `Valencia' trees to determine fruit removal with a trunk shake and catch harvesting system. Application of both abscission materials before and after the “less-responsive period” resulted in a 10% to 12% increase in fruit removal when compared to control trees. Less than a 35% reduction in FDF was sufficient to significantly increase fruit removal. Only 100 mg·L-1 a.i. CMN-pyrazole significantly increased fruit removal when applied during the “less-responsive period.” Chemical names used: Methyl-2-(((((4-Methoxy-6-Methyl-1,3,5-Triazin-2-yl)-Amino)Carbonyl) Amino)Sulfonyl)Benzene (Metsulfuron-methyl); 5-Chloro-3-methyl-4-nitro-1-H-pyrazole (CMN-pyrazole).

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T.R. Willard, C.M. Peacock, and D.G. Shilling

The effects of sethoxydim, cloproxydim, and fluazifop on photosynthesis and growth of St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze `Floralawn'], bahiagrass (Paspalum notatum var. saurae Parodi `Pensacola'), and centipedegrass [Eremochloa ophiuroides (Munro.) Hack.] were evaluated to determine if photosynthesis could be used as a rapid, nondestructive measure of relative susceptibility. Field and greenhouse studies were conducted using infrared CO2 analysis to estimate photosynthesis. Under field conditions, St. Augustinegrass was susceptible to sethoxydim and fluazifop applications, as indicated by a 40% and 38% reduction in apparent photosynthesis, respectively. Bahiagrass incurred a respective 62% and 51% reduction in apparent photosynthesis from sethoxydim and fluazifop application. Growth of these species, as measured by foliage dry weight, was also inhibited by both herbicides. Centipedegrass growth was unaffected by sethoxydim, but was reduced 48% by fluazifop. Under greenhouse conditions, centipedegrass apparent photosynthesis was reduced by sethoxydim and cloproxydim (41% and 51%, respectively), while fluazifop caused a 71% reduction. Growth of centipedegrass was significantly reduced only by fluazifop (83%). These studies indicated that in vivo photosynthetic measurements may provide a sensitive, rapid, and nondestructive method for determining the susceptibility of turfgrasses to postemergence grass herbicides. chemical names used: 2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio) propyl]-3-hydroxy-2-cyclohexen-l-one (sethoxydim); (E,E) -2-[1-[[(3-chloro-2-propenyl) oxy]imino]butyl] -5-[2-(ethylthio) propyl]-3-hydroxy-2-cyclohexen-l-one (cloproxydim); and butyl ester of 2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]-propanoate(fluazifop).

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Krista C. Shellie and Robert L. Mangan

`Dancy' tangerines (Citrus reticulata Blanco) were harvested after color break and exposed to high-temperature forced air (HTFA) at 45C for 3.5 or 4 h to kill Mexican fruit fly [Anastrepha ludens (Loew)] larvae. Heat-treated and control fruit were stored subsequently for 2 weeks at 4C. Tangerines harvested after color break (naturally degreened) tolerated exposure to HTFA in a similar fashion as tangerines harvested before color break and degreened by postharvest exposure to ethylene. Titratable acidity (TA) was significantly lower after heat treatments. Flavor, soluble solids concentration, external appearance, incidence of decay, percent juice yield, percent weight change, and flavedo color of heat-treated fruit were not different from nonheat-treated, control fruit. Exposure to HTFA is a viable alternative to methyl bromide for disinfestation of `Dancy' tangerine.