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An excised twig assay was developed to evaluate cherry genotypes for their tolerance to Pseudomonas syringae pv. syringae. One-year-old wood was collected at monthly intervals from Oct. until Jan. of `Napoleon', `Corum', and a number of cherry rootstock. The rootstock included F/12-1, Giessen (GI), and M×M selections. Twigs were inoculated with one avirulent and three virulent strains. Evaluation of incision browning, callus, and gummosis production were made after incubation for 3 weeks. Based on gummosis is and browning ratings, all the rootstocks tested were found to be more tolerant than `Napoleon' and `Corum' to the 3 strains of Pseudomonas syringae used. No gummosis or browning was observed on twigs inoculated with water or the avirulent strain.

Plant signals extracted from cherry leaves have been shown to control expression of virulence genes in P. syringae. Crude aqueous extracts from `Napoleon' twigs induced the syrB::1acZ fusion in P. syringae strain B3AR132 Other rootstocks are currently being evaluated for their ability to induce virulence in P. syringae pv. syringae,

An excised twig assay was developed to evaluate cherry geno-types for their tolerance to Pseudomonas syringae pv. syringae. One-year-old wood was collected at monthly intervals from October until January of `Royal Ann', `Corum', and a number of cherry rootstock. The rootstock included; F/12–1 and Giessen (GI) and M × M selections. A 2-cm incision (“^”-shaped flap) was made on each twig. A 20-μl droplet of inoculum or water was placed onto each incision. The inoculum was prepared with one avirulent (K4) and three virulent strains (W4N54, AP₂, B₁₅) concentrations (10⁵, 10⁶, or 10⁷ cfu). Inoculated twigs were placed in test tubes and incubated at 15C in high relative humidity for 3 weeks. After incubation, twigs were evaluated for gummosis production (0–3, 0 = no gummosis), incision browning (1–4, 1 = yellow pith), and callus production (0–1, 0 = no callus). The concentration of bacterial suspension had no effect on symptom development. No gummosis or browning was observed on twigs inoculated with water or the avirulent strain. Based on the gummosis and browning ratings, rootstock M × M 2, M × M 39, M × M 60, GI 148-1, GI 154-2, and GI 154-4 were found to be resistant to these three strains of P. syringae in this assay. Root-stock F 12-1, GI 169–15, GI 172–9, and GI 173-9 were found to be tolerant.

Some cultivars of prune (i.e. `Brooks') consistently set good crops while others (i.e. `Italian') are erratic bearers. Fall-applied ethephon increases fruit set. Ovule longevity has been hypothesized to be an important factor in fruit retention. The effects of the cultivar and fall ethephon application on ovule longevity were determined.

Ethephon (0 and 500 mg·l^-1) was applied to `Italian' and `Brooks' prune trees at the 50% leaf drop stage. The following spring, flower buds were emasculated and covered to prevent pollination. Ten flowers were sampled every two days from anthesis until 20 days after anthesis (DAA). Flowers were fixed in FAP and ovule longevity determined using fluorescence microscopy. Ovule longevity was longer in `Brooks' than `Italian'. At 20 DAA, all of the `Brooks' flowers still had viable ovules. Only 40% of the `Italian' flowers had viable ovules. The `Italian' flowers excised from ethephon-treated trees had at least one non-senescent ovule at 17 DAA. Ethephon prolonged ovule longevity in `Italian' prune flowers. No effect of ethephon was observed on the ovule longevity of the `Brooks' prune.

Some cultivars of prune (i.e. `Brooks') consistently set good crops while others (i.e. `Italian') are erratic bearers. Fall-applied ethephon increases fruit set. Ovule longevity has been hypothesized to be an important factor in fruit retention. The effects of the cultivar and fall ethephon application on ovule longevity were determined.

Ethephon (0 and 500 mg·l^-1) was applied to `Italian' and `Brooks' prune trees at the 50% leaf drop stage. The following spring, flower buds were emasculated and covered to prevent pollination. Ten flowers were sampled every two days from anthesis until 20 days after anthesis (DAA). Flowers were fixed in FAP and ovule longevity determined using fluorescence microscopy. Ovule longevity was longer in `Brooks' than `Italian'. At 20 DAA, all of the `Brooks' flowers still had viable ovules. Only 40% of the `Italian' flowers had viable ovules. The `Italian' flowers excised from ethephon-treated trees had at least one non-senescent ovule at 17 DAA. Ethephon prolonged ovule longevity in `Italian' prune flowers. No effect of ethephon was observed on the ovule longevity of the `Brooks' prune.

Au excised twig assay was developed to evaluate cherry rootstocks (hybrids of Prunus avium L.; P. canescens Bois.; P. cerasus L.; P. fruticosa Pall.; P. mahaleb L.; P. pseudocerasus Lindl.) for their tolerance to Pseudomonas syingae pv. syringae van Hall. Twigs of `Napoleon', `Corum', and F12/1 in 1988 and 10 more rootstocks in 1989 were inoculated with water, one avirulent (K4), or one of three virulent strains (W4N54, AP1, and B-15) of bacteria at 10⁵, 10⁶, and 10⁷ colony forming units (cfu)/ml in 1988 and with 10⁷ cfu/ml in 1989. Evaluation of browning and gummosis at the inoculation site after incubation for 4 weeks at 15C and high relative humidity revealed no gummosis or browning on twigs inoculated with water or the avirulent strain. The amount of browning and gummosis induced by concentrations of 10⁶ and 10⁷ cfu/ml of the virulent strains was, in general, not different within genotypes. `Napoleon' and `Corum' had significantly higher browning and gummosis ratings in 1989 than F12/1 and the 10 rootstock selections, most of which did not differ from F12/1. Rootstocks Gisela (Gi.) 172-9 and Gi. 169-15 had higher incision browning than F12/1 in some instances.

Flower bud growth and ovule longevity of plum (Prunus domestics L.) cultivars Italian and Brooks and the effects of fall-applied ethephon and of temperature were studied. Fresh and dry weights of terminal flower buds were measured at l-week intervals from 50 days to 1 day before bloom in 1988. Buds were also analyzed for N, P, K, Ca, and B. After bloom, ovule longevity was determined using a fluorescence method after staining with aniline blue. Ovule longevity was determined in 1990 using shoots excised at full bloom from untreated and ethephon-treated trees of both cultivars and held in growth chambers for 18 days at 5, 10, 15, or 20C. `Brooks' flower buds showed a higher accumulation of fresh and dry weight than `Italian', and ethephon reduced bud weights in both cultivars. Ethephon did not affect mineral content of flower buds of `Brooks', but `Italian' flower buds contained a higher concentration of Ca and a lower concentration of P when treated with ethephon. Boron content was higher in the ethephon-treated buds of `Italian' trees on some sampling dates. Ovule longevity was higher for `Brooks' than for `Italian' in both years. Ethephon treatment delayed ovule senescence in `Italian' flowers, but had little or no effect on `Brooks' flowers. Increasing temperatures induced faster ovule senescence in both cultivars. Chemical name used. 2-chloroethylphosphonic acid (ethephon).

Abstract

During the 1984 growing season, trees of ‘Loring’ peach (Prunus persica L. Batsch) on ‘Halford’ rootstocks were divided into the following components; fruit, leaves, shoots, 1-year wood, 2-year wood, frame, and roots. The sampling dates were coincident with key fruit and tree developmental stages: dormant, 15 days after full bloom, the end of Stages I and II of fruit developement, harvest, and 45 days after harvest (leaf fall). At the beginning of Stage II, four additional trees per remaining sampling date were hand-thinned to 15 cm between fruit. Initially, the mean dry weight of trees was 27.1 kg. Unthinned and thinned trees increased 43% and 39%, respectively, in dry weight by 45 days after harvest. Dormant hand-pruning removed about 4 kg of growth. This amount represented nearly the entire season's dry weight of shoot, 1-year-old, and 2-year-old wood growth. The proportion of total tree weight in fruit was increased at harvest in unthinned trees, whereas leaf and 2-year wood dry weight was reduced. A modified harvest index, the proportion of total annual increase distributed to fruit, was 0.50 for unthinned trees and 0.37 for thinned trees. The ratio of annual dry weight increase per unit leaf area was 0.33 and 0.27 kg·m⁻² leaf area for unthinned and thinned trees, respectively. The maximum mean leaf area was estimated to be 62.9 × 10³ m²·ha⁻¹. The greatest leaf area density, measured at the end of Stage II, was 7.7. Absolute growth rates of vegetative components were highest during Stage II of peach fruit growth, and intermediate during Stage I.

Studies in the use of fall ethephon to delay bloom in peach and prune were carried out during 1985-87. In `Italian' prune, ethephon at 250 and 500 mg·liter<sup>-l</sup> at 10% leaf-drop stage delayed bloom 13 and 16 days, respectively. Only a 5- and 7-day bloom delay occurred when applied at 50% leaf-drop stage. Fruit set and yield were not reduced in `Italian' prune when ethephon was applied at the 50% leaf-drop stage. Early applications, from vegetative maturity to the 10% leaf-drop stage, did not reduce yield in prone when trees had been previously defoliated with 3.0% urea. Early leaf removal, before vegetative maturity, caused reduction in peach flower and fruit number. In several peach cultivars, all the ethephon treatments were detrimental to flower density, fruit set, and yield, in spite of bloom delay. The ethephon-treated prune trees yielded more than the untreated trees in 1987 as a result of frost avoidance.

Abstract

The rate of ethylene evolution of peach fruit (Prunus persica L. Batsch) and ACC content of peach pericarp/mesocarp and seeds was determined during development. Ethylene measurements of whole fruit began 18 days after anthesis (DAA), and ACC quantification was started 32 DAA. ACC levels and ethylene evolution followed similar patterns during stages I and II of fruit growth. At 39 DAA, there was an increase in ethylene evolution and extractable ACC concentration of both pericarp and seeds; however, variability was high at this time. Ethylene evolved by nondeveloping fruit of the “second wave” and “June drop” increased after senescence of the ovule was observed. By 49 DAA, ethylene production and ACC concentration reached a minimum that lasted until a 10-fold increase in ethylene evolution was detected in late stage III. This 10-fold increase in ethylene occurred in four different peach cultivars sampled at “firm-ripe” stage. Seeds excised at 67 DAA, which were incubated for 6 hr in ambient O₂ conditions, evolved 400 nl·g⁻¹·hr⁻¹ ethylene and ACC concentration averaged 54 nmol·g⁻¹ fresh weight. It is suggested that in split-pit fruits, ethylene generated by the seeds may accelerate fruit maturation and ripening. Chemical name used: 1-aminocyclopropane-1-carboxylic acid (ACC).