Six-year-old peach trees [Prunus persica (L.) Batsch] were sprayed with ethephon (100 mg·liter–1) in Oct. 1989, whitewashed in Jan. 1990, and sprayed with dormant oil on one or two dates in Mar. 1990 to study possible interactive effects on flower bud hardiness, pistil growth, time of bloom, and yield. Flower buds from ethephon-treated trees supercooled to a lower temperature [mean low temperature exotherm (MLTE) of –18.5C] than buds from nontreated trees (MLTE of –17.7C) in February; there was no main effect of whitewashing or any interaction with ethephon. No treatment effects on hardiness were detected in March. Ethephon-treated pistils were smaller than nontreated pistils, and pistils from buds on whitewashed trees were smaller than those on nonwhitewashed trees. No main effects or interactions of dormant oil on pistil size were detected. Ethephon and whitewashing delayed bud development during bloom, but prebloom oil application(s) had no effect. Buds from ethephon-treated and whitewashed trees were more tolerant of freezes during bloom than buds from oil-sprayed trees, and yield was enhanced by ethephon and whitewashing. Prebloom oil sprays reduced yield compared with controls. Chemical name used: 2-chloroethylphosphonic acid (ethephon).
Edward F. Durner and Thomas J. Gianfagna
Dennis E. Deyton, Carl E. Sams, and John C. Cummins
Treatments of single applications of 0%, 3%, 6%, 9%, or 12% dormant oil were sprayed on peach (Prunus persica L. Batsch) trees on 6 Feb. 1990. A repeat application of 6% oil plus 6% oil applied 6 days later was also made. Internal CO 2 concentrations of oil-treated buds and twigs were higher than the control the day after treatment and continued to be higher for 6 days. The second application of 10% oil prolonged the elevated CO2 concentration. Applications of 9% or 12% oil delayed flower bud development and bloom. The repeated application of 6% oil delayed bud development and bloom more than a single application of 6% oil. Damage to fruit buds increased as oil concentration increased, but repeated application of 6% oil resulted in less damage than a single application of 12% oil.
R.E. Myers, D.E. Deyton, and C.E. Sams
Spurs of `Starkspur Delicious' trees were dipped in 0, 3, 6, 9 or 12% petroleum oil (dormant oil) or soybean oil emulsions on 26 January 1993. The spurs were cooled at 3C/hr until -9C or kept at 21C. After treatment, the flower buds on spurs were forced at 20C for 11 days and then dissected. The cambium and xylem of the spurs and the interior of the flower buds were rated for damage as indicated by browning. The experiment was repeated at the silver tip stage of buds (early March) except that treated spurs were exposed to 20C, -6C, or -9C. Neither the oil treatments nor low temperature exposure caused visual damage to flower buds or cambium in January. However, the oil treatments damaged flower buds at the silver tip stage (March). Neither petroleum or soybean oil caused visible damage to the xylem or cambium of the spurs.
Dennis E. Deyton, Carl E. Sams, and John C. Cummins
Treatments of dormant oil, at rates of 0, 3, 6, 9, or 12 % (v/v), were sprayed until drip on four year old `Biscoe' peach trees on February 6, 1990. Another treatment was applied as a split application with 6 % applied on the previous application date and a second application of 6% solution applied on February 12. The internal atmosphere of bud and twig was modified by the oil treatment. The internal concentration of CO2 was elevated the morning following treatment and continued higher than the control for seven days. A second application Of 6% oil resulted in additional elevation of internal CO2. External evolution of CO2 of all oil treated twigs was 6 to 18% lower than the control 8 days after treatment. Bud phenology and bloom date of trees receiving higher rates of oil were slightly delayed.
Dennis E. Deyton, Renae E. Moran, Carl E. Sams, and John C. Cummins
Applications of soybean oil to dormant peach [Prunus persica (L.) Batsch] trees were tested for prebloom thinning of flower buds in five separate experiments. Data were combined from experiments in which 2.5% to 20% emulsified soybean oil was sprayed on `Belle of Georgia' or `Redhaven' trees. The number of dead flower buds was concentration-dependent with maximum bud kill of 53% occurring with application of 12% soybean oil. The amount of thinning was fairly consistent from year to year, ranging from 34% to 51% when 10% soybean oil was applied, but was less consistent when 5% was applied, ranging from 6% to 40%. Overthinning by midwinter applications of soybean oil occurred in one experiment when bud mortality on nontreated trees was 40% due to natural causes. Mild to moderate spring freezes occurred in three experiments, but did not reduce yield more in soybean oil–thinned than in nontreated trees. Flower bud survival was improved when trees were sprayed with 10% or 12% soybean oil prior to a –4 °C spring frost. Applications of soybean oil to dormant trees thinned flower buds, reduced the amount of hand thinning required, and hastened fruit maturity.
Kitren Glozer and Charles Ingels
Pear growers in California's Sacramento River Delta and, to some extent, other pear-growing areas in California, use dormant oils for pest control and dormant bud growth stimulation. It is generally believed that well-timed applications can advance flowering, improve uniformity of flowering and fruit ripening, and improve vegetative budbreak. Traditionally, dormant oils have been applied in late December to mid-January, based on experience and calendar date. However, bud development and full bloom dates may differ from year to year, with variable weather cycles and chill accumulation experienced by the plant. In the 2004–05 dormant season, some dormant oil applications timed at intervals calculated by chill portions (defined by the Dynamic Model) advanced and compressed the bloom period. Fruit size (diameter and weight) and total estimated yield were improved by dormant oil treatments applied within a certain range of chill portion accumulation without reduction in total number of fruit per tree, while the percentage of undersized fruit was decreased by 65% to 83% when compared to the untreated control. The use of chill portions and the Dynamic Model to time applications of dormant oil appears to benefit fruit quality. Although chill was not limiting in the trial dormant season, there may be benefit even in years when chill accumulation is adequate.
D.E. Deyton, C.E. Sams, and J.C. Cummins
Treatments of 0, 10, 20, 30, or 40% (v/v) refined (salad) or crude soybean oil or 0, 5, 10, 15, or 20% petroleum (dormant) oil at 0, 5, 10, 15, or 20% were sprayed until drip on `Smoothee' apple trees on 27 February 1991. The internal carbon dioxide concentration was elevated and the oxygen content reduced within one day in buds-twigs treated with oil and remained influenced for up to 12 days. All oil treatments delayed fruit bud development. The lowest tested concentration of soybean oil (either crude or refined) resulted in the greatest delay in bud development and the greatest delay in bloom (approximately 4 days). Crude soybean oil treatment resulted in less damage to flower buds than petroleum oil.
B.R. Bondada, C.E. Sams, and D.E Deyton
Oil sprays increase the phytotoxicity of captan to apple foliage. The purpose of this study was to determine if oils increase the penetration of captan through leaf cuticles. Enzymatically isolated apple leaf cuticles were used as a model system to study captan penetration. A bioassay was developed using the inhibition of growth of Penicillium cyclopium on potato-dextrose agar as a measure of captan penetration through the cuticle. Captan penetrated through both surfaces, but significantly more penetrated through the abaxial cuticles than the adaxial cuticles. Increasing the captan concentration increased the captan penetration through the abaxial cuticle in a linear relationship. Captan penetration through the cuticle was increased by 63% when cuticles were treated with captan plus 1% emulsified soybean oil. Abaxial cuticles treated with captan plus emulsified soybean oil or with captan plus SunSpray Ultra-Fine oil had >125% greater captan penetration than cuticles treated with only captan. Cuticles treated with captan plus dormant oil (petroleum oil) had 220% more captan penetration than the captan only treatment.
Kitren Glozer and Franz J. Niederholzer
Use of rest-breaking chemicals may partially substitute for chill requirement in “French' prune. Many California prune growers use oil in the dormant season to tighten and advance bloom, with application timing judged by experience and calendar date. Other rest-breaking agents have become commonly used in California cherry production and their application is generally timed by chill portion accumulation, calculated by the Dynamic Model. We evaluated the effects of treatments of dormant oil or CAN17 (calcium ammonium nitrate) + Entry on budbreak and bloom progression in `French' prune with applications timed at regular intervals. While most treatments improved fruit set and reduced reproductive bud death, an optimum range for both types of rest-breaking treatments was found for advancement and compression of bloom. All rest-breaking treatments advanced fruit maturity equally, compared to the untreated control, as measured by fruit firmness. Although chill hour (hours ≤7°C) calculations might also be used for timing these treatments, when chill portion and chill hour accumulations are compared for the 2004–05 dormant season at several different sites, differences from site-to-site are small for chill portions, and much greater for chill hours. This fact supports experimental evidence from numerous California trials in sweet cherry in which rest-breaking treatment timings based on the Dynamic Model tend to be more consistent than the timings based on the “chill hour” model.
D.E. Deyton, C.E. Sams, and C.D. Pless
Four-year-old `Gala' and `Widjit' apple trees with significant apple aphid populations were sprayed to runoff on 13 May 1994 with 0%, 0.5%, 1.0%, or 2.0% (v/v) emulsified degummed soybean oil (SO) or with 1.0% petroleum (dormant) oil (PO). Treatments were arranged in a randomized complete-block design with five single-tree replications. Apple aphid populations were determined on 10 tagged shoots per tree. The top fully expanded leaf of two randomly selected shoots per tree were tagged and net photosynthesis (Pn) and transpiration (Tr) measured. Trees treated with SO or PO had <20% as many aphids after treatment as nontreated trees. Trees treated with 2% SO had lower Pn and Tr than the control for 18 days after treatment. Spraying 0.1% or 0.5% SO caused less initial reduction of Pn than 2.0% SO, and the effect was shorter lasting. Four-year-old `Oregon Spur' and `Empire' were sprayed with 0%, 0.1%, 0.5%, 1.0% SO or PO on 26 June. Treatments were arranged in a randomized complete-block design with four single-tree replications. Pn rates of trees treated with 0.1% to 1.0% soybean oil were <40% of nontreated trees the day after treatment, but recovered to >80% of control in 5 days.