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Mokhles A. Elsysy, Andrew Hubbard, and Todd C. Einhorn

Bloom and postbloom thinning is essential to managing yield and fruit quality in modern pear production systems. Much of the ‘Bartlett’ pear acreage in the western United States comprises well-established trees planted at low to medium density

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Matthew Arrington, Mateus S. Pasa, and Todd C. Einhorn

50% of full bloom, with two applications improving the response. ATS concentrations of 3% were phytotoxic to other pear cultivars ( Wertheim, 2000 ). Pears are generally thinned during postbloom developmental stages using chemicals that alter the

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D. Michael Glenn, Donald L. Peterson, Daniela Giovannini, and Miklos Faust

Hand-thinning (Prunus persica L. Batsch) “Y”-trained peach trees at bloom and 51 days after full bloom (DAFB) was compared to mechanical fruit thinning 51 DAFB using a spiked-drum and an impact shaker. The spiked-drum shaker removed more fruit from horizontal branches than from vertical branches, yet did not selectively remove either large or small fruit. Bloom thinning by hand increased fruit size compared to postbloom thinning 51 DAFB, and both postbloom mechanical thinning techniques were as effective as postbloom hand thinning. The spiked-drum shaker may be a better thinning technique than the impact shaker because it transfers less shaking energy to the fruit, can be used in high-density plantings, and does not contact the trunk, lessening the potential for tree damage.

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Paweł Wójcik, Anna Skorupińska, and Hamide Gubbuk

L-TRP (pure grade; Sigma-Aldrich, Poznań, Poland) in the following variants: 1) three prebloom sprays, at the stages of green and pink buds and when 5% to 10% flowers were open, 2) three postbloom sprays, at the stage of petal fall and again 14 and

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Max W. Williams

Biennial bearing of apple trees can be overcome either by the use of a blossom chemical thinner or by early application of a postbloom thinner. Carbaryl (Sevin) is a post-bloom fruit-thinning chemical with an effective thinning period of 4 to 5 weeks after bloom. Sevin was compared in 1992 and 1993 with NAA as an early petal-fall spray. Sevin treatments reduced fruit set to one fruit per cluster with no adverse side effects on the foliage. NAA inconsistently reduced fruit set and the remaining fruit were in clusters, The NAA-treated foliage was adversely affected; having small curled leaves. NAA at 10 ppm under-thinned in 1992 and seriously over-thinned in 1993, whereas Sevin treatments were consistent for fruit thinning in both years. Sevin applied at petal-fall or at petal-fall + 7 days effectively reduced fruit set and reduced fruit competition.

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G.H. Neilsen, E.J. Hogue, D. Neilsen, and P. Bowen

Zinc supplied as a fulvic-based Zn compound was absorbed and retranslocated to unsprayed new growth as effectively as zinc sulphate in apple seedlings of low Zn status grown hydroponically in the greenhouse. Similarly, fulvic- and humic-based compounds were as effective as zinc sulphate at improving short-term growth and Zn uptake into new tissues in Zn-deficient apple seedlings, with the best growth occurring at spray concentrations of Zn at 500 mg·L-1. Under field conditions, Zn concentration of peeled and washed `Jonagold' apples at harvest was increased, without phytotoxicity, by two or four postbloom sprays of fulvic Zn. It is therefore possible to use this material safely as an effective Zn-source after bloom. However the mobility of the foliar-applied Zn is limited and any yield response by treated apple orchards of marginal Zn nutrition is unlikely to occur in the short term (within two growing seasons).

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Frank J. Peryea

Postbloom zinc (Zn) sprays are replacing dormant and postharvest sprays as the primary means for applying Zn in commercial apple (Malus ×domestica) orchards. We conducted a multiyear field study comparing the phytoavailability of Zn in 11 commercially available Zn spray products, plus reagent-grade Zn nitrate and a water-sprayed control, applied postbloom at identical Zn concentrations to `Golden Delicious' apple trees. Two sprays were applied per season (mid-May and mid-June), at per-spray rates of either 0.5 lb/acre in 2000 or 1.0 lb/acre in 2001 and 2002. No sprays were applied in 2003 in order to evaluate carry-over effects. The Zn sprays had no effect on fruit number, bitter pit or russeting, or on leaf green color. Zinc concentrations of detergent plus acid-washed leaves (a procedure used to remove surface residues of the Zn sprays) sampled in August and of unwashed winter buds sampled the following January were used as indices of tree Zn status. Leaf Zn concentration generally increased in the order: Zn phosphate < Zn oxide = Zn oxysulfate < chelated/organically complexed Zn ≤ Zn nitrate. There was little consistent difference among chelated and organically complexed Zn products. Leaf Zn concentration varied considerably between seasons, and was not related to Zn application rate. All of the Zn sprays increased leaf Zn concentrations to desirable levels. Because the inorganic Zn-based products typically are substantially less expensive per unit of Zn, it may be less costly and just as effective to use a higher rate of an inorganic Zn product as to use a lower rate of a more expensive chelated or organically complexed Zn product. On the other hand, use of low rates of highly phytoavailable Zn products minimizes release of the nutritionally essential but potentially ecohazardous metal into the environment. There was no detectable lasting effect of the three previous seasons of Zn sprays on leaf Zn in 2003. Similarly, there was no detectable effect in any year of the Zn spray treatments on bud Zn concentration the following winter. These results suggest that the amount of Zn supplied by the sprays at the tested rates was insufficient to promote substantial Zn accumulation within the trees, thereby validating the recommendation for annual application of Zn nutritional maintenance sprays.

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B. Reyes-Gonzalez, G. Mora-Aguilera, S. Osada, D. Tèliz, and U. Diaz

The temporal progress of postbloom fruit drop was characterized in North Veracruz, Mexico, in two commercial orchards with low and high management technology. A total of 200 flowers per orchard were assessed for disease incidence every 2 days during the flowering season. Colletotrichum isolates, putatively similar to C. acutatum, were obtained from diseased flowers and inoculated to Citrus sinensis and Citrus latifolia completing the Koch's postulates. The average Area Under Disease Progress Curve (AUDPC) (625 vs. 688 days, %) and final disease incidence (Yf) (68% vs. 67%) were statistically similar (P = 0.05) to both orchards. Epidemic rates were in the range of 0.05 to 0.16 units, with the highest variability in the high technology orchard. The total epidemic duration was 15 and 18 days for the high and low technology orchards, respectively. The highest absolute increase of disease occurred at 13 days after the beginning of flowering during the full bloom stage. The absolute increase of disease was not correlated (P = 0.05) with nonlagged values of relative humidity (%), maximum temperature (°C) and the number of Colletotrichum spores captured on a Burkard like trap. However, 5-day lagged values of spore numbers were apparently correlated with the highest absolute increase of disease. Similarly, 3- to 5-day lagged values of number of days with at least 85% relative humidity was needed to explain the main spore peaks.

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Luis Pozo and Jacqueline K. Burns

were harvested in early Apr. 2007 and fruit weighed to obtain the 2007 yield. Abscission agents were applied on two dates corresponding to petal fall (6 DAB, 12 Apr. 2007) and fruit set/postbloom abscission (28 DAB, 4 May 2007). These key dates were

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Hui-Qin Chen, Katherine L. Dekkers, Lihua Cao, Jacqueline K. Burns, L. W. Timmer, and Kuang-Ren Chung

Postbloom fruit drop (PFD) of citrus is incited by the fungus Colletotrichum acutatum J. H. Simmonds and may result in young fruit drop and severe yield losses. Previous studies suggested that imbalance of growth regulators such as auxin, ethylene, and jasmonic acid (JA) plays an essential role in young fruit abscission. In this work, we determined the factors associated with fungal-induced fruit drop by testing compounds inhibitory to hormonal transport or biosynthesis. As assessed on sweet orange (Citrus sinensis Osbeck) and grapefruit (C. paradisi Macf.) for 4 years, we found that many auxin transport and action inhibitors such as 2,3,5-triiodobenzolic acid (TIBA), 2-(4-chlorophenoxy)-2-methyl-propionic acid (clofibrate), or quercetin and JA biosynthesis inhibitors such as salicylic acid (SA) and aspirin (methyl-SA) applied 7 d after C. acutatum infection resulted in higher percentages of young fruit retention compared with the water controls. The commercial products ReZist and Actigard, widely used as systemic acquired resistance (SAR) agents, also improved fruit retention. Furthermore, application of gibberellic acid (GA3) on sweet orange, regardless of C. acutatum infection, significantly increased fruit retention. These commercial products may be very useful in managing this destructive disease of citrus in the field.