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Stephen M. Southwick and James T. Yeager

Heavy fruit set of apricot (Prunus armeniaca) cultivars grown in California often require hand thinning to insure that adequate fruit size is obtained. Alternatives to costly hand thinning would be welcome. GA treatments made during flower bud initiation/differentiation have been previously shown to inhibit the development of floral and vegetative buds in a number of different tree fruit species. The effects of post-harvest limb and whole tree aqueous gibberellic acid (GA) sprays on flower and fruit production were investigated over a 3 year period in `Patterson' apricot. Limb treatments indicated the potential for utilizing postharvest GA sprays to reduce the number of flowers produced in the following season. Harvest fruit size (June 1989) was increased by a 100 mg·liter-1 GA whole tree spray applied 7 July 1988 when compared to non-thinned and hand thinned trees. Yield per tree was reduced by that GA spray, but not enough to show statistical differences. No abnormal tree growth responses have been observed in GA-sprayed trees to date. These results and those from the 1989 and 1990 growing seasons will be presented in effort to identify a role for whole tree postharvest GA sprays in a chemical thinning program suitable for commercial apricots.

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Stephen M. Southwick and James T. Yeager

Sweet cherries produce vigorous upright growth from Apr.-Sept. and are slow to bear in California. Our tree training objectives include earlier bearing, easier harvesting, high productivity of good quality fruit. `Bing' cherry on mazzard and mahaleb rootstock were planted in 7 blocks and trained 6 ways. One group was headed 12-18 inches above the bud union and 4 branches were retained at the 1st dormant pruning. Lateral buds were treated with promalin at bud-break to induce lateral shoot formation. Trees were spring-summer pruned to reduce terminal growth. At the second dormant pruning, strong shoots were removed and lateral shoots were treated with promalin to induce spur formation. Trees were treated likewise through the 3rd dormant season and produced a fair crop in the 4th season. Central leader trees were created by tying/weighting limbs, dormant and summer pruning, and retaining less vigorous limbs as well as utilizing promalin. Slow growing trees tended to bear fruit more rapidly. Both training methods yielded fruit in the 4th season while traditional pruning procedures produced few fruit. Data and procedures will be presented to document these practices.

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Stephen M. Southwick and James T. Yeager

Hand-thinning is required every season to ensure large fruit size of `Loadel' cling peach in California. Hand-thinning is costly. Chemical thinning could help to lower costs of hand-thinning. Armothin® {[N,N-bis2-(omega-hydroxypolyoxyethylene/polyoxypropylene)ethyl alkylamine], AKZO-Nobel, Inc., Chicago; AR} was sprayed at 80% of full bloom (FB), FB and FB + 3 days. The spray volume was 935 liters·ha–1. Concentrations of AR were 1%, 3%, and 5% AR applied at FB. No damage to fruit was noted. Leaf and fine shoot phytotoxicity were seen at 5% AR. The amount of time needed and number of fruits thinned were reduced by those same treatments. Salable yield and fruit size after AR treatments equaled those found on hand-thinned controls. Armothin® shows promise for chemical thinning of peach when used as a bloom spray that damages flowers, thereby reducing fruit set. An experimental use permit was issued for use of AR for stone fruit thinning in California during 1995.

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Stephen M. Southwick, James T. Yeager, and Kitren G. Weis

`Loadel' cling peach [Prunus persica (L.) Batsch] trees were sprayed with Release® LC (Abbott Laboratories, North Chicago, Ill.) in 1993. Preharvest (harvested 16 July) sprays of 50, 75, 100, and 120 ppm applied on 15 June improved fruit firmness without altering fruit maturity (flesh color by commercial standards) in 1993. In the following 1994 season, flower number per centimeter of shoot length was reduced by sprays ranging from 50 to 120 ppm applied on 15 June and 9 July. No hand-thinning was required on trees treated on 15 June. Trees treated 9 July had 50% fewer fruit removed than on untreated trees, where more than 3000 fruit were removed by hand-thinning. Salable yield was higher than untreated control trees where Release® LC had been applied at 50 ppm on 15 June and 9 July. Fruit size equaled those of hand-thinned controls. As concentration increased on 15 June, salable yield decreased linearly. Fruit size (diameter and individual weight) increased with reductions in salable yield. Interestingly, fruit were evenly distributed along shoots after Release® LC treatment, similar to those found after hand-thinning. Release® LC will be available for commercial chemical thinning of stone fruit in California during 1995. Additional results from peach and other stone fruit will be presented.

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Warren C. Micke, Joseph A. Grant, and James T. Yeager

`Gala', the third most widely planted apple cultivar in California, requires early and precise thinning to produce good fruit size. Thus, chemical thinning would be ideally suited for this cultivar. However, the normally prolonged bloom for apples in California makes timing of chemical thinning applications difficult. In 1995 and 1996 trials, several chemical thinning treatments provided significantly reduced fruit set on `Gala' compared to the untreated control. Three treatments showed promise for commercial use: 1) carbaryl, two applications at petal fall and again at 10-15 mm diameter of the king fruit; 2) carbaryl plus NAD at petal fall; and 3) carbaryl plus 6-benzyladenine and GA4+7 (Accel®), two applications at petal fall and at ≈10 mm diameter of the king fruit. These treatments generally gave reduced fruit set per 100 flower clusters, fruit set per fruiting cluster and/or numbers of fruit removed by follow-up hand-thinning. None of these treatments showed evidence of phytotoxicity, and some increased fruit size over the untreated control.

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Warren C. Micke, Mark W. Freeman, and James T. Yeager

A replicated rootstock trial for almond was established in 1986 in the central San Joaquin Valley, a major almond growing area for this most widely planted tree crop in California. `Nonpareil', the major cultivar in California, was used for this trial with `Fritz' grown as the pollenizing cultivar. Two standard rootstocks for almond, `Nemaguard' and `Lovell' peach, were compared to two newer peach-almond hybrid rootstocks, `Bright's' and `Hansen'. After eight years both hybrid rootstocks produced significantly larger trees than the peach rootstocks, based on trunk cross-sectional area. Trees on hybrid rootstocks frequently produced greater yields than those on peach rootstocks; although, differences were not always significant. However, there were generally no significant differences in production per trunk cross-sectional area (yield efficiency). Thus, increased production by trees on hybrid rootstock was the result of larger tree size and not an inherent increase in productive efficiency of the tree itself. Since trees on hybrid rootstock should be planted further apart than those on peach, production per hectare should not be significantly increased, at least under good growing conditions as represented in this trial.

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Stephen M. Southwick, Kitren G. Weis, and James T. Yeager

Hand thinning fruit is required every season to ensure large fruit size of `Loadel' cling peach [Prunus persica (L.) Batsch] in California. Chemical thinning may lower costs of hand thinning. A surfactant, Armothin {[N,N-bis 2-(omega-hydroxypolyoxyethylene/polyoxypropylene) ethyl alkylamine]; AKZO-Nobel, Chicago; AR}, was sprayed at 80% of full bloom (FB), FB, and FB + 3 days. The spray volume was 935 liters/ha. Concentrations of AR were 1%, 3%, and 5% (v/v). An early hand thinning in late April, a normal hand thinning at 13 days before standard reference date (early May), and a nonthinned control were compared to bloom-thinned trees for set, yield, and fruit quality. AR resulted in no damage to fruit; however, slight leaf yellowing and burn and small shoot dieback were seen at the 5% concentration. Fruit set, and therefore, the number of fruit that had to be hand thinned, were reduced with 3% AR applied at 80% FB and 5% AR applied at all bloom phenophases (stages of bloom development). Thinning time was reduced by 37% (5% AR applied at 80% FB), 28% (5% applied at FB), and by 20% (3% applied at 80% of FB), compared to the normally hand-thinned control. Although AR resulted in early size (cross suture diameter and weight) advantages, at harvest there were no significant differences in fruit size among all AR treatments and the normally hand-thinned control. Total and salable yields of AR treatments and the normally hand-thinned control were equal. Armothin shows promise for chemical thinning of peach when used as a bloom thinner.

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Warren C. Micke, Joseph A. Grant, Maxwell V. Norton, and James T. Yeager

Under California conditions `Granny Smith' apple does not “self-thin” sufficiently to promote good return bloom nor to provide fruit size desired for the fresh market. Preliminary studies conducted during 1985-87 indicated that 1-naphthyl N-methylcarbamate (carbaryl), 1-naphthaleneacetic Acid (NAA), and 1-naphthaleneacetamide (NAD) could be useful for thinning `Granny Smith'. Detailed studies conducted in 1988 and 89 using dilute handgun applications demonstrated that all 3 materials provided reasonable thinning as shown by fruit set counts. NAA and NAD tended to slow fruit growth as compared to carbaryl. Carbaryl tended to uniformly thin clusters while NAA and NAD were more likely to remove all the fruit from some clusters and few fruit from others, especially in 1988. Compared to the control, all materials applied in 1988 improved return bloom in 1989 with carbaryl having a slightly greater effect than NAA and NAD. As a result of these studies carbaryl at 1.7 to 2.2 kg (active ingredient) per ha as a dilute application is being suggested for grower trials in California.

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Warren C. Micke, Joseph A. Grant, Maxwell V. Norton, and James T. Yeager

Under California conditions `Granny Smith' apple does not “self-thin” sufficiently to promote good return bloom nor to provide fruit size desired for the fresh market. Preliminary studies conducted during 1985-87 indicated that 1-naphthyl N-methylcarbamate (carbaryl), 1-naphthaleneacetic Acid (NAA), and 1-naphthaleneacetamide (NAD) could be useful for thinning `Granny Smith'. Detailed studies conducted in 1988 and 89 using dilute handgun applications demonstrated that all 3 materials provided reasonable thinning as shown by fruit set counts. NAA and NAD tended to slow fruit growth as compared to carbaryl. Carbaryl tended to uniformly thin clusters while NAA and NAD were more likely to remove all the fruit from some clusters and few fruit from others, especially in 1988. Compared to the control, all materials applied in 1988 improved return bloom in 1989 with carbaryl having a slightly greater effect than NAA and NAD. As a result of these studies carbaryl at 1.7 to 2.2 kg (active ingredient) per ha as a dilute application is being suggested for grower trials in California.

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Stephen M. Southwick, James T. Yeager, Joseph Osgood, Richard Buchner, William Olson, and Maxwell Norton

Ten new marianna root-stocks [Prunus cerasifera Ehrh. × P. munsoniana Wight & Hedr.(?)] derived from open pollination of `Marianna 2616' (M series) were planted in 1987 and evaluated at four commercial orchard locations in California (Tehama, Butte, Sutter, and Merced counties) with `Improved French' prune (P. domestica L.) as the scion. These rootstocks were compared to three standard rootstocks: `Marianna 2624', myrobalan seedling (P. cerasifera Ehrh.) and `Myrobalan 29C'. Leaf potassium (K) and nitrogen (N), tree growth, fruit production and fruit quality were measured. Selection M40 in particular had high leaf N, high leaf K (equal to `Marianna 2624' and better than the myrobalan standards), higher yield efficiency per tree, fruit size, drying characteristics, and few root suckers when compared to the three standard rootstocks. M40 is being considered for patent and release by the Pomology Department at the University of California, Davis. Selection M58 had the highest yield efficiency of any tested rootstock. Several selections had characteristics that would make expanded planting worth considering.