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  • Author or Editor: J.D. Obermiller x
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A series of experiments was undertaken to compare the performance of an axial fan air blast sprayer equipped with air induction (AI) or conventional (C) nozzles in medium density apple (Malus ×domestica) orchards. Performance was compared by assessing 1) spray coverage within the canopy at four levels of across-row wind speeds, 2) ground deposits from airborne drift under still conditions, and 3) biological efficacy of a postbloom thinning spray and a seasonal “high-risk” fungicide program where thiophanate methyl was not included. Spray coverage was reduced by up to 50% with increasing across-row wind speeds, the most dramatic reductions occurring between 0 and 4 mph and at heights greater than 8 ft in the canopy. AI nozzles resulted in a 2-fold increase in spray coverage in the canopy of trees in the row immediately adjacent to the sprayer compared with C nozzles regardless of across-row wind speed. AI nozzles resulted in significantly less airborne drift compared with C nozzles under still conditions in an open field and within a mature ‘Cripps Pink’/ ‘M.7’ orchard planted in rows 20 ft apart. Sprayer efficiency, measured as the proportion of total spray volume that was intercepted by the tree canopy, was higher for an airblast sprayer fitted with AI nozzles (38%) than for C nozzles (26%). The efficacy of a postbloom thinning spray (100 ppm benzyladenine plus carbaryl at 1 lb/100 gal) applied to ‘Morgan Spur Red Delicious’/‘M.111’ trees planted at a between-row spacing of 15 ft was slightly greater when applied with AI nozzles compared with C nozzles. However, application of a “high-risk” fungicide program with AI nozzles resulted in a higher incidence of fruit with flyspeck (Zygophiala jamaicensis) at harvest compared with C nozzles. These inconsistencies were related to the combined effects of nozzle type, orchard row spacing, and canopy density on spray deposition on trees in the second row from the sprayer or to possible effects of nozzle type on droplet density on the target. AI nozzles should provide equivalent or possibly improved coverage and biological efficacy compared with C nozzles in well-managed orchards planted at distances of 18 ft or less between rows. However, when orchard rows are spaced greater than 18 ft apart, AI nozzles will result in reduced spray coverage and chemical efficacy compared with C nozzles because of a reduction in spray carry-over to adjacent rows as a result of reduced airborne drift.

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The normal window for application of thinning chemicals in apple extends from bloom until 3 weeks after bloom, when the fruit reach a mean diameter of ≈16 mm. After this time fruit are generally insensitive to standard chemical thinning sprays. The potential for the photosystem II (PSII) inhibitor metamitron and the ethylene precursor 1-aminocyclopropane carboxylic acid (ACC) to thin apple fruit after the traditional thinning window was investigated in field experiments over three years. A standard rescue thinning spray of carbaryl plus ethephon plus naphthaleneacetic acid (NAA) reduced fruit set of Gale ‘Gala’ if applied when the mean fruit diameter was 18, 20, and 27 mm in 2010, 2011, and 2012, respectively. The thinning activity of 400 mg·L−1 ACC was equivalent to the standard rescue thinning spray in 2010, whereas 350 mg·L−1 metamitron reduced fruit set more effectively than either the standard or ACC in 2010. Application of 400 mg·L−1 ACC plus 350 mg·L−1 metamitron when the mean fruit diameter was 18 mm reduced fruit set to almost no crop in 2010. The combination of metamitron plus ACC exhibited thinning activity after application at 25 and 33 mm mean fruit diameter in 2011 and 2012, respectively. Increased ethylene evolution was found in detached ‘GoldRush’ fruit 24 h after applications of ACC from 11 mm to 27 mm mean fruit diameter, but not when ACC was applied at 31 mm mean fruit diameter. Ethylene evolution was much higher after application of ACC at the 11 mm or 17 mm mean fruit diameter stage compared with application when fruit diameter was 23 mm or 27 mm. The thinning activity of ACC was related to the period of maximum ethylene response. The effects of delayed applications of ACC and metamitron on fruit set tended to be greater when these two chemicals were combined, suggesting that the creation of a carbohydrate stress and the capacity to convert ACC to ethylene are both required to trigger abscission of apple fruit larger than 18 mm in diameter.

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A series of four experiments were undertaken to evaluate the effects of individual and combined applications of prohexadione-Ca (P-Ca) and GA4+7 primarily on fruit russet, but also on fruit set, fruit weight, early season shoot growth, and fruit maturity of ‘Golden Delicious’ apples (Malus × domestica Borkh.). A single application of P-Ca (138 to 167 mg·L−1) at petal fall (PF) reduced the severity of russet in three of the four experiments; however, multiple applications of 20 ppm GA4+7 at 10-day intervals beginning at PF generally reduced russet more effectively than P-Ca. P-Ca did not reduce the efficacy of GA4+7 sprays for russet reduction. However, GA4+7 sprays reduced the inhibitory effects of P-Ca on shoot growth measured 30 days after PF. A single application of P-Ca at PF had no effect on mean fruit weight at harvest. Fruit size was lowest for the combined P-Ca and GA4+7 treatment in every experiment, although there was a significant interaction between P-Ca and GA4+7 sprays on mean fruit weight in only one experiment. There were no consistent effects of P-Ca and GA4+7 sprays, alone or in combination, on fruit maturity parameters at harvest. These data show that a single application of P-Ca at PF reduced russet severity, and the effects of P-Ca and GA4+7 sprays on russet can be additive. The economic benefits resulting from a reduction in russet severity after combined P-Ca and GA4+7 sprays will need to be balanced against their occasional negative effect on fruit size. Chemical names used: prohexadione-calcium [3-oxido-4-propionyl-5-oxo-3 cyclohexenecarboxylate formulated as Apogee (27.5% a.i.)].

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Experiments were conducted in commercial apple (Malus ×domestica) orchards in the southeastern U.S. between 1998 and 2006 with the primary objective of evaluating the effects of naphthaleneacetic acid (NAA) and ethephon on return bloom. NAA increased return bloom in six of 10 experiments, whereas ethephon increased return bloom in four of seven experiments. Four biweekly applications of 5 ppm NAA during June and July (early summer NAA) increased return bloom more consistently than fewer applications. Four weekly preharvest applications of 5 ppm NAA increased return bloom of ‘Delicious’ and ‘Golden Delicious’ as effectively as early summer applications. Combining NAA and ethephon in early summer sprays did not consistently increase return bloom compared with either material alone. The flower cluster density of ‘Golden Delicious’ in the year of treatment had a negative effect on return bloom that was more pronounced on control trees than trees sprayed 5 weeks after bloom with 444 ppm ethephon (48 fl oz/acre Ethrel). Combining four early summer sprays of 316 ppm ethephon (24 fl oz/acre Ethrel) with 15 ppm gibberellin A4 + A7 (GA4+7) increased return bloom of ‘Cameo’ but had no effect on return bloom of ‘Mutsu’ or ‘Golden Delicious’. Growth regulator treatments did not have a consistent effect on fruit firmness in the year of treatment. Naphthaleneacetic acid or ethephon treatments in the on year of a biennial bearing cycle can promote return bloom of apple spurs. However, the positive effect on return bloom may be minimal in cultivars with a strong natural tendency toward biennial bearing or when bloom or initial fruit set are heavy in the year of treatment.

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The effects of foliar applications of the photosystem II (PSII) inhibitor metamitron on chlorophyll fluorescence and fruit set were compared in peach and apple trees. Metamitron increased dark-adapted chlorophyll fluorescence, measured as a reduction in Fv/Fm values, in both peaches and apples. Maximum suppression of the normalized ratio of variable fluorescence to maximum fluorescence (Fv/Fm) in peaches occurred 1 to 2 days after application and Fv/Fm values recovered by 7 days after treatment. The effects of metamitron on chlorophyll fluorescence were more persistent in apples compared with peaches. Fv/Fm values in apple declined within 2 days of treatment and did not start recovering until 5 days after treatment or longer. Concentrations of metamitron greater than 200 mg·L−1 were phytotoxic to peach leaves, reducing the leaf chlorophyll concentration as determined by SPAD measurements. At 300 mg·L−1, metamitron reduced fruit set in apple but not in peach. Inclusion of a non-ionic surfactant (Silwett L-77) with metamitron greatly increased its negative effect on Fv/Fm, quantum photosynthetic yield of PSII (ΦPSII), and relative electron transport rate (ETR). These results suggest that metamitron may be a useful thinner in apple but not in peach. Additional information is needed to understand how combining metamitron with existing thinning chemicals might enhance their activity. In particular, caution may be necessary if metamitron is applied as a tank mixture with commercial thinning products that have been formulated with a wetting agent.

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The variation in natural fruit drop of ‘Scarletspur Delicious’/‘M.7’ (M.7) apple (Malus ×domestica) trees in a commercial orchard over a period of 11 consecutive years was visualized using box and whisker plots. Delaying harvest until 1 week after the normal harvest date resulted in fruit drop ranging from 2% to 33% depending on the year. The effects of aminoethoxyvinlyglycine (AVG) and naphthaleneacetic acid (NAA) on fruit drop and fruit firmness at normal and delayed harvests was monitored each year. AVG and NAA programs tended to mitigate fruit drop most effectively in years when natural fruit drop was heavy. AVG delayed the loss of fruit firmness, whereas a preload NAA program delayed firmness loss in fruit that were harvested 3 weeks after the normal harvest date only. A standard NAA program for drop control did not accelerate softening of ‘Scarletspur Delicious’ during the first 3 weeks after the normal harvest date. Growers should closely monitor fruit maturity and stem loosening during the harvest window each year to minimize the risk of major losses due to fruit drop. When timely harvest is not possible, perhaps due to unforeseen weather events or constraints in labor availability, or poor management, then use of harvest management aids such as AVG or NAA becomes critical on cultivars prone to fruit drop.

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A series of experiments were undertaken to compare the effects of individual and combined applications of GA4+7 and prohexadione-Ca (P-Ca) on scarf skin and fruit quality parameters on red strains of `Rome Beauty' and `Gala' apples. Three applications of GA4+7 at 10-day intervals beginning at petal fall (PF) significantly reduced scarf skin severity in all experiments. A single application of P-Ca at PF had no effect on scarf skin in one experiment but reduced scarf skin severity in two further experiments. Combining P-Ca with the first of three GA4+7 sprays as a tank mix reduced the severity of scarf skin more effectively than either material alone in two of three experiments at P < 0.05 and in all three experiments at P < 0.10. Combining P-Ca with the first application of GA4+7 as a tank mix generally reduced scarf skin as effectively as applying P-Ca and the first GA4+7 spray two days apart, although in one experiment, greater scarf skin control was achieved when P-Ca was applied 2 days after the first GA4+7 spray. A single application of P-Ca at PF consistently reduced, and three applications of GA4+7 consistently increased, mean fruit weight at harvest compared with the control. The economic benefits as a result of reducing scarf skin severity with P-Ca and GA4+7 sprays will need to be balanced against the negative effect of P-Ca on mean fruit weight. There is no antagonism between early season P-Ca and GA4+7 sprays for scarf skin control, and P-Ca may increase the efficacy of GA4+7 sprays for scarf skin control in apple.

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Experiments were conducted to compare the effects of different preharvest and postharvest 1-methylcyclopropene (1-MCP) treatment combinations on ‘Law Rome’ and ‘Golden Delicious’ apple fruit. Preharvest 1-MCP sprays had minimal effects on maturity as determined by flesh firmness, starch index, internal ethylene concentration, and soluble solids concentration. Fruit internal ethylene concentration and firmness loss after 30- to 40-days storage at 0 °C plus 7 days at 20 °C were reduced by preharvest and postharvest 1-MCP treatments. The positive effects of preharvest 1-MCP on postharvest quality of ‘Law Rome’ declined in fruit that were harvested 3 days or more after spraying, whereas preharvest 1-MCP continued to have a positive effect on postharvest fruit quality of ‘Golden Delicious’ that were harvested up to 9 days after spraying. The loss in postharvest effects of preharvest 1-MCP treatment on ‘Law Rome’ at delayed harvests was reinstated by exposing fruit to gaseous 1-MCP on the day of harvest. These findings suggest that attached apple fruit of some cultivars may be capable of rapidly generating new ethylene receptors.

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Three experiments were undertaken to evaluate the effects of different preharvest 1-methylcyclopropene (1-MCP) spray treatments on apple (Malus × domestica Borkh.) fruit maturity at harvest and quality after long-term storage in a regular atmosphere or controlled atmosphere (CA). Trees were sprayed within 7 days of the anticipated harvest date (H) and fruit for long-term storage were sampled at either H in the case of ‘Law Rome’ or at harvest dates that were delayed by up to 21 days (H + 21) in the case of ‘Golden Delicious’ and ‘Law Rome’. Preharvest 1-MCP sprays within 7 days of H reduced fruit drop, internal ethylene concentration, and starch index and reduced firmness loss during long-term storage of fruit at delayed harvest dates but had only minor effects on fruit maturity at H. Preharvest 1-MCP sprays reduced the incidence of superficial scald on ‘Law Rome’ apples more effectively than either diphenylamine or CA storage. Application of 1-MCP within 7 days of H may be used to delay harvest date, thereby allowing continued fruit growth without a concomitant advance in fruit maturity and to reduce firmness loss and superficial scald during long-term storage both for normal and delayed harvests.

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