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  • Author or Editor: D.C. Elfving x
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Excessive vegetative growth and its control remain significant and often costly problems facing deciduous fruit growers. The term “vegetative growth” is often understood to mean “shoot growth”, since the most visible manifestation of vegetative growth is the development of new shoots each year. Other forms of vegetative growth also occur in deciduous fruit trees, although their impact on productivity is less clearly understood. In this paper, “excessive vegetative growth” will refer to any condition that results in an undesirable amount of shoot growth.

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Some cultivars of deciduous tree fruit, especially in apple and pear, tend to come into bearing slowly. Bioregulators have been used to stimulate flowering and cropping in slow-to-crop cultivars, but success has been variable. Improved flowering may not necessarily lead to increased cropping. Reduction in vegetative vigor as a result of bioregulator treatment is often, but not always, associated with increased flowering. Bioregulators that act by interfering with gibberellin biosynthesis or that generate ethylene in plant tissues have proven effective for increasing flowering under a variety of conditions. Few studies have demonstrated that exogenous bioregulator applications to poorly cropping fruit trees can be used to produce sustained and economically significant improvement in fruiting while maintaining satisfactory fruit quality.

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Dormant heading-back of terminal extension shoots on scaffold limbs on 2-year-old `Empire'/M.26 EMLA apple trees (Malus domestica Borkh.) in Apr. 1985 increased shoot growth from 1- and 2-year-old limb sections. Removing competing shoots to restore a single terminal extension shoot on each scaffold on half the trees in each pruning treatment in May 1985 had little influence on shoot growth. Annual trunk enlargement was reduced in 1985 and 1986 by heading-back pruning in 1985. Trunk cross-sectional area in Fall 1989 remained smaller for trees only headed back once, in 1985. Yields were decreased in 1986 through 1989 by heading-back treatments applied in 1985.

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Applications of 6-benzylamino purine (BA) alone or together with gibberellic acid (GA4+7) increased lateral-shoot development in 1- to 3-year-old apple trees (Malus domestica Borkh.). Cultivars differed in response based in part on their habit of growth. Supplementary surfactant made BA alone as effective in shoot-growth induction as BA + GA4+7 with or without surfactant. Induced shoots achieved lengths of up to 40 cm, and the pattern of shoot-length distribution generally was similar to that of untreated trees. Follow-up pruning was required in direct proportion to increases in lateral-shoot formation. In a 1981 timing study, trees ceased to be responsive to exogenous BA in mid-June. Application of BA or BA + GA4+7 at various times during the preceding 3 weeks induced a constant number of lateral shoots from growing points established the previous season. Increased numbers of lateral shoots originating from newly formed buds were produced with later applications within the 3-week responsive period.

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In three trials over 3 years, foliar BA applications for fruitlet thinning of `Empire' apple (Malus domestica Borkh.) trees produced small and inconsistent effects on flesh firmness at harvest and after air storage. Soluble solids concentrations at harvest and after air storage were consistently increased by BA alone or together with GA4+7 [Promalin (PR)], and were also increased by CB in one trial. Starch hydrolysis was slightly delayed by BA applications in 1990. Ethylene evolution at harvest was increased by NAA in 1988 and slightly increased by PR applied 29 days after full bloom (DAFB) in 1990, while poststorage ethylene evolution was stimulated by BA and PR treatments in 1990 except BA at 29 DAFB. Incidence of poststorage disorders was low and largely uninfluenced by thinning treatments. Chemical names used: N-(phenylmethyl)-1H-purine-6-amine [benzyladenine (BA)]; BA plus gibberellins A4 and A7 (GA 4+7) [Promalin (PR)]; 1-naphthaleneacetic acid (NAA); 1-naphthalenyl methylcarbamate [carbaryl (CB)].

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Annual yields per tree for `Starkspur Supreme Delicious' apple (Malus domestica Borkh.) trees on nine size-controlling rootstock were related linearly to number of fruit per tree at harvest each year, independent of rootstock. Mean fruit weight was inversely and less closely related to number of fruit per tree when adjusted for tree size (fruit load). Annual yield-fruit count data for 9 years analyzed together showed that the number of fruit per tree was the principal factor determining yield, regardless of rootstock or tree age. A curvilinear relationship between yield and fruit count per tree during 9 years suggests that the sink strength of an apple crop is nearly, but not precisely, proportional to the number of fruit per tree.

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Benzyladenine (BA) applied postbloom at 125 and 250 mg·liter-1 thinned `Empire' apple (Malus domestica Borkh.) trees below commercial crop levels but thinned less than thidiazuron (THI) at 62 and 125 mg·liter-1. Ethephon (ETH) applied up to 250 mg·liter-1 reduced fruit set only slightly. When BA was tank-mixed with ETH, thinning was the same as with BA alone. Although THI thinned more, BA resulted in a larger increase in fruit weight. Seed development was nearly eliminated by THI, but was unaffected by either BA or ETH. Thinner effects on foliar nutrient concentrations were associated with changes in fruit load but not shoot growth. The effects of BA and ETH on fruit-flesh nutrient concentrations were similar to their effects on foliar nutrient concentrations. Although THI thinned strongly and produced large changes in foliar nutrient concentrations and seed count, THI resulted in virtually no changes in fruit-flesh nutrient concentrations. Chemical names used: N-(phenylmethyl)-1 H-purine-6-amine (benzyladenine); 2-chlorophosphonic acid (ethephon); N-phenyl-N'-1,2,3-thiadiazol-5-ylurea (thidiazuron).

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Three commercial `Lapins' sweet cherry (Prunus avium) orchards were used for this study during three crop seasons. Orchards were selected based on the historical average date of commercial harvest. The difference in commercial harvest date among the three orchards was 5 to 7 days. Three harvests were carried out in each orchard each year: 1) beginning 4 to 5 days before commercial harvest, 2) at commercial harvest, and 3) 4 to 5 days after commercial harvest. Fruit quality was determined after 0, 7, 14, and 21 days of storage. Harvesting fruit up to 5 days later than normal commercial harvest resulted in increases in fruit weight and soluble solids content along with no loss of firmness or change in acidity. Pedicel color did not change as harvest was delayed. Changes in visual ratings of both fruit and pedicel appearance with delayed harvest were detectable in only 1 of 3 years. Neither pitting nor bruising was influenced by harvest date. The amount of pitting or bruising present was related more to the year of harvest than to harvest date. Delaying harvest a short time beyond the normal commercial harvest date could enhance consumer appeal and increase fruit value. Storage time after harvest resulted in reduced fruit and pedicel appearance, but only beyond 14 days of storage.

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Beginning in 1982, daminozide (DZ) was applied annually for 5 years to whole, 5-year-old `Northern Spy'/MM.106 (Malus domestics Borkh.) trees: a) shortly after bloom, b) together with ethephon (ETH) 6 to 7 weeks after bloom, or c) after harvest. Controls were unsprayed. One-half of the trees receiving each growth regulator treatment were summer-pruned after terminal-bud formation each year. Postharvest DZ reduced shoot numbers, mean shoot length, trunk enlargement, and fruit size, but had little or no effect on bloom, fruit set, or yield. Postbloom DZ, summer DZ plus ETH, and summer-pruning reduced vegetative growth and time required for dormant-pruning, but only postbloom DZ and summer DZ plus ETH increased spur density in the tree. Postbloom DZ and summer DZ plus ETH increased both flowering and cropping in 3 of the 5 years, with little effect on fruit set. Fruit size was reduced only in years when cropping was enhanced. Total yields (1982-86) were increased 34% and 36% by postbloom DZ and summer DZ plus ETH, respectively. Summer-pruning had no effect on fruit size in any year, but reduced yields in 1984 and 1986. Year-to-year fluctuation in yield was unaffected by any treatment. Growth-control treatments had no direct effect on foliar or fruit macronutrient concentrations. Chemical names used: butanedioic acid mono (2,2-dimethylhydrazide) (daminozide); 2-chloroethylphosphonic acid (ethephon).

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Postbloom applications of benzyladenine (BA) thinned young fruitlets of mature `Empire' apple trees (Malus domestica Borkh.) as well as or better than NAA or carbaryl (CB). BA increased fruit weight more effectively than either NAA or CB. Promalin (PR) was less effective than BA for both thinning and fruit-weight increase. In 1990, both BA and PR reduced fruit set up to 29 days after full bloom, but PR showed less thinning activity. BA and NAA produced independent and additive thinning responses when tank-mixed. Effects of all thinners on foliar mineral-nutrient concentrations were associated with changes in fruit load. BA increased return bloom as much or more than NAA or CB. PR did not affect return bloom. Chemical names used: N -(phenylmethyl)-1 H -purine-6-amine [benzyladenine (BA)]; BA plus gibberellins A, and A, [Promalin (PR)]; 1-naphthaleneacetic acid (NAA); 1-naphthalenyl methylcarbamate [carbaryl (CB)].

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