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  • Author or Editor: Xiaoe Yang x
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Bloom in individual citrus (Citrus) trees often continues for more than 1 month in south Florida, with even greater bloom duration within most orchard blocks because of variation in bloom timing between trees. Prolonged bloom contributes to variable fruit maturity as harvest approaches and increases severity of postbloom fruit drop (PFD) disease (caused by Colletotrichum acutatum). Hydrogen cyanamide (cyanamide) has been effective in accelerating bloom in various deciduous fruits, and its potential use in citrus was investigated in this preliminary study. Cyanamide was applied at a range of concentrations, from 0% to 1.0% a.i., to potted trees of six citrus types reflecting fairly broad diversity in commercial citrus that was readily available as seed [alemow (Citrus macrophylla), ‘Duncan’ grapefruit (Citrus paradisi), sour orange (Citrus aurantium), ‘Smooth Flat Seville’ sour orange hybrid (C. aurantium hybrid), ‘Swingle’ citrumelo (C. paradisi × Poncirus trifoliata), and ‘Sun Chu Sha’ mandarin (Citrus reticulata)] in Dec. 1999 while trees were quiescent. Phytotoxicity increased with cyanamide rate, with some damage at 0.125% cyanamide on most tested plants, and large variation among citrus types. All cyanamide rates hastened flushing. Airblast application of cyanamide (0, 0.025%, 0.05%, and 0.10%) was made to mature trees of ‘Valencia’ and ‘Navel’ sweet orange (Citrus sinensis) in Ft. Pierce, FL, on 27 Jan. 2000. On 15 Feb. and 28 Feb. additional trees received cyanamide at 0.05%. There was considerable defoliation, which increased linearly with cyanamide rate. Flushing and flowering were unaffected by cyanamide compared with controls except in February where cyanamide applied at 0.05% increased flowers per tree in ‘Valencia’ sweet orange, and in contrast, 0.1% cyanamide on 27 Jan. reduced ‘Navel’ sweet orange flowering. Cyanamide application to ‘Valencia’ sweet orange on 28 Feb., after initial flowering but 16 days before peak bloom, significantly reduced yield per tree but there were no other effects on cropping. In these trials, cyanamide was not an effective agent for hastening bloom in south Florida citrus with applications late January through February. Further work is needed to determine whether December applications of cyanamide to trees in the field may be more effective in concentrating subsequent flush and bloom.

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Two years of field experiments were conducted in eastern New York to evaluate the efficacy of a multi-step thinning approach on reducing crop load (no. fruit per cm2 trunk cross-sectional area) and increasing fruit size of 'Empire' apple (Malus ×domestica Borkh.). Applications of Endothall (ET) at 80% bloom, NAA + carbaryl (CB) at petal fall (PF), and Accel™ + CB at 10 mm king fruitlet diameter (KFD), alone and in all combinations, were compared to a nonthinned control and to the application of NAA + CB at 10 mm KFD (commercial standard). In both 1996 and 1997, orthogonal contrasts indicated the multi-step treatment significantly increased fruit size, reduced cropload, and reduced yield compared to single applications. Effects on cropload of consecutive treatments were largely predicted by multiplying effects of individual treatments. Although all thinning treatments except for NAA + CB at PF in 1997 significantly reduced cropload, no single treatment thinned sufficiently to ensure good return bloom. Compared to NAA + CB at 10 mm KFD, multi-step thinning with NAA + CB at PF followed by Accel™ + CB at 10 mm KFD produced bigger fruits in both years, and resulted in a higher percentage of spurs carrying a single fruit in 1996. When fruit size was evaluated after removing the effect of cropload (cropload adjusted fruit weight), NAA + CB at PF, Accel™ + CB at 10 mm, and the two applied sequentially, resulted in greater cropload adjusted fruit weight than the nonthinned control in both years, whereas NAA + CB at 10 mm did not. Contrast analysis of treatments with and without ET showed no significant effect of including ET on fruit size, though total cropload was reduced at P = 0.10 and total yield was reduced (P = 0.03 in 1996 and P = 0.12 in 1997). No deleterious effects from multi-step treatments have been observed. All thinning treatments significantly increased return bloom in 1996 and 1997 compared to the control with little difference observed between treatments. Chemical names used: naphthalene acetic acid (NAA); 1-naphthyl-N-methylcarbamate [carbaryl (CB)]; 6-benzyladenine [BA (Accel™)]; 7-oxabicyclo (2,2,1) heptane-2,3 dicarboxylic acid [ET (Endothall™)]

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Thinning with BA reportedly increases size of 'Empire' fruit more than does thinning with NAA because of enhancement of cell division by BA. This study was conducted to determine the phenological stage at which BA application provides maximum fruit weight relative to degree of cropload reduction. In all years, treatments were applied at a range of timings: petal fall (PF), 5-, 10-, or 15-mm king fruitlet diameter (KFD). For each thinner, the same concentration was used throughout the study. In 1994, only Accel® at 75 mg·L-1 was evaluated. In 1995, NAA (7.5 mg·L-1) + carbaryl (600 mg·L-1), Accel®, and a BA-only formulation were compared, but BA alone was applied only at PF, 10- and 15-mm KFD. In 1996, Accel® and NAA were compared with and without carbaryl at all timings. Most treatments reduced cropload and enhanced fruit weight. When data for all 3 years were combined, Accel® or BA increased cropload-adjusted fruit weight (CAFW) in 8 of 10 treatments made at 10- or 15-mm KFD, PF treatments never increased CAFW, and only one of four applications at 5-mm KFD increased CAFW. In contrast, NAA + carbaryl increased CAFW in four of four treatments applied at PF or 5-mm KFD, but in only one of four treatments at 10- or 15-mm KFD. Accel® was less effective than NAA in reducing fruit clusters to a single fruit per spur in most comparisons, either with or without carbaryl. Return bloom varied greatly across years, but was always influenced by application time and types of thinners. In 1994 and 1996, return bloom was closely related to cropload the previous year. Although return bloom was very low for most treatments in 1995, 10- and 15-mm KFD applications of NAA + carbaryl increased it three-fold in comparison with other treatments (NAA + carbaryl at PF or 5 mm or BA at 10-mm KFD) that had similar effects on cropload. Chemical names used: 6-benzyladenine (BA); naphthaleneacetic acid (NAA).

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