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M.L. Arpaia, G.S. Bender, and G.W. Witney

A project evaluating the performance of cv. Hass on eight clonal avocado rootstocks—G755A, G755B, G755C, Duke 7 (D7), Borchard (BR), D9, Toro Canyon, and Topa Topa was established in southern California in 1986. Two additional rootstocks, Thomas and G1033, were added in 1987. Of the trees planted in 1986, the BR and D7 rootstocks have consistently had the highest total yields for all rootstocks, whereas the three G755 selections have had the lowest productivity. No differences in productivity between the two rootstocks planted in 1987 have been detected. The influence of rootstock on the magnitude of alternate bearing will be discussed, although the oscillation in yield is greater for the higher-yielding rootstocks. Tree size has been measured throughout the study. The BR selection has consistently produced a larger tree, even though it has continued to have high productivity. There are no consistent differences between the other rootstocks. Yield efficiency, measured as the kg fruit/m3 of canopy volume has been calculated. In selections that are prone to severe alternate bearing, the swing in yield efficiency is also the greatest. The data thus far suggests that a yield efficiency of ≈2.5 kg fruit/m3 canopy volume is the maximum yield possible for California `Hass' avocado.

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Olfa Zarrouk, Yolanda Gogorcena, Maria Angeles Moreno, and Jorge Pinochet

Trials were established at Aula Dei Experimental Station (EEAD-CSIC, Zaragoza, Spain) to assess graft compatibility between peach cultivars [Prunus persica (L.) Batsch] and new Prunus spp. rootstocks or selections. Peach cvs. `Catherina' and `Tebana' and nectarine cvs. `Big Top' and `Summergrand' were grafted on peach seedlings, plum rootstocks, almond × peach hybrids, and other interspecific rootstocks. Part of the evaluated material belongs to the EEAD-CSIC selection program, which has showed good adaptation to Mediterranean growing conditions. Other rootstocks such as Bruce, Evrica, Hiawatha, Ishtara, Tetra, and Krymsk-1 have been recently introduced in Spain. A peach and a plum source, GF 677 and Adesoto 101, respectively, were used as compatible reference rootstocks. Both are widely used for peach and nectarine production in the Mediterranean area.

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D. Struss, M. Boritzki, R. Karle, and A.F. Iezzoni

Two rootstocks from the Giessen (GiSelA) series of dwarfing cherry (Prunus sp.) rootstocks, GiSelA (GI) 5 (syn. 148/2) and GI 6 (syn. 148/1), are becoming commercially important and five other Giessen cherry rootstocks are being evaluated for horticultural traits. Since GI 5 and GI 6 are morphologically similar, a DNA fingerprinting project was undertaken to identify molecular markers that could be used by the nursery industry to differentiate these two rootstocks. The project was extended to include six additional Giessen rootstocks of varying pedigrees. Fourteen DNA primer pairs were tested for their ability to differentiate among the eight rootstocks. None of the primer pairs could differentiate all eight rootstock selections; however, three primer pairs could differentiate all but two selections. Two primer pairs, PMS 15 and PceGA59, were identified as the most suitable for high throughput screening of GI 5 and GI 6 due to the simplicity and the size of the base pair differences among the polymorphic fragments. These results demonstrate the utility of molecular markers to differentiate the Giessen cherry rootstocks.

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Duane W. Greene

increasingly important role in modern tree fruit culture. This action is particularly true as trees are planted at much higher densities and they are managed more intensively. Control of vegetative growth and productivity through rootstock selection plays an

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R.K. Striegler and D.R. Wineman

Selected rootstocks were evaluated for four seasons in a Zinfandel vineyard located in the northern San Joaquin Valley of California. The vineyard was drip-irrigated and vineyard spacing was 2.1m × 3.3m (vine × row). A two-wire vertical trellis system was used and row orientation was east to west. Vines were trained to a bilateral cordon and spur-pruned. Rootstocks included in the study were: AxR #1, Freedom, Harmony, Kober 5BB, and Teleki 5C. Vines grafted on AxR #1 rootstock were considered to be the control treatment.

A randomized complete block experimental design was used. There were six blocks and plots consisted of five vines. Data collected included yield, components of yield, fruit composition, bloom petiole nutrient content, shoot number, and pruning weight.

After the initial season, yields were consistently highest for vines grafted on Freedom rootstock, with yields only slightly lower on AxR #1 rootstock. Significantly lower yields were recorded for vines grafted on Kober 5BB and Teleki 5C. Rootstock did not have a consistent effect on fruit composition. Shoots/vine and shoots/meter of canopy were not significantly effected by rootstock. Dormant pruning weight was highest for Freedom and lowest for Kober 5BB.

These results suggest that rootstock selection can influence vineyard productivity in the northern San Joaquin Valley. Continued research is needed to determine long term effects of the rootstocks used in this study.

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Kim D. Bowman

Phytophthora parasitica Dast. causes several root and trunk diseases of citrus, including damping-off, root rot, foot rot, and gummosis. Phytophthora resistance is needed in Citrus rootstocks and is available in Poncirus trifoliata (L.) Raf. and some hybrids between Citrus and P. trifoliata. Field or greenhouse tests of rootstocks require large amounts of space and time. To provide a preliminary indication of rootstock resistance to P. parasitica, nucellar seedlings of P. trifoliata selections, and Citrus × P. trifoliata hybrids were tested for response to P. parasitica by in vitro inoculation. Seeds of individual selections were germinated in sterile culture and 3-week-old shoots were excised and inoculated with a cultured isolate of Phytophthora. After 1 week of incubation, response to the disease organism was measured by length of stem discoloration. Progression of Phytophthora in the stem also was measured by plating sequential 5-mm segments of the shoot and determining presence or absence of Phytophthora in individual segments. Stem discoloration length corresponded with location of Phytophthora in the stem. Relative resistance, as measured by this technique, approximated field resistance for several common rootstock cultivars. Resistant, intermediate, and susceptible selections were found in populations of Citrus × P. trifoliata rootstock hybrids using in vitro inoculation.

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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.

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Robert E. Rouse, David P. H. Tucker, and Edgar D. Holcomb Jr.

Evaluation ratings of cold injury following a freeze on December 24 & 25, 1989, showed differences among scion cultivars and rootstock. `Star Ruby' grapefruit (Citrus paradisi Macf.) and `Fallglo' citrus hybrid, a cross of Bower mandarin citrus hybrid × Temple tangor (C. temple Hort. ex Y. Tanaka) were the most severely damaged scion cultivars. `Rohde Red' valencia orange selection 472-11-43. [C. sinensis (L.) Osbeck]. was the least damaged scion cultivar. Scions budded to Cleopatra mandarin (C. reshni Hort. ex Tan.) and FL 80-18 citrumelo [C. paradisi × Poncirus trifoliata (L.) Raf.] rootstock were damaged more than on other rootstock. Scions budded to smooth flat seville (C. aurantium?) and P. trifoliata × Ridge pineapple sweet orange selection 1573-26 [C. sinensis (L.) Osbeck] had the least injury. Analysis comparing replications showed the greatest damage to be in the north side of the planting.

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Raúl Leonel Grijalva-Contreras, Arturo López-Carbajal, Adán Fimbres-Fontes, Cristobal Navarro-Ainza, Rogelio Juárez-González, and Fabián Robles-Contreras

Apricot production in México is limited; actually, the area devoted to this crop is ≈880 ha, from which 230 ha are established in Sonora State. The main cultivar is `Canino'. The fruit yield ranges from 15 top 20 t·ha-1. The present study tested 20 low-chilling (300-400 chill hours) requirements of apricot selections; `Nemaguard' was the rootstock used. On the fourth production year, from the 20 apricot selections tested, 7-23, 1-81, and 15-1 yielded 31.8, 20.2, and 15.5 t·ha-1, respectively. all of these selections showed higher yields than `Canino' (14.6 t·ha-1). The fruit of these apricot selections ripened by mid-May, exhibiting a similar fruit quality (size, flavor, color, and °Brix) in all the tested selections. We have not recorded any important insect pests or diseases during this trial.

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Amelia Camprubí and Cinta Calvet

The selection of the most effective arbuscular mycorrhizal (AM) fungi for growth enhancement of citrus cultivars used as rootstocks was the first step toward development of an AM inoculation system in citrus nurseries in Spain. AM fungi were isolated from citrus nurseries and orchards in the major citrus-growing areas of eastern Spain. The most common AM fungi found in citrus soils belonged to Glomus species, and G. mosseae (Nicol. & Gerd.) Gerdemann & Trappe and G. intraradices Schenck & Smith were the AM fungi most frequently associated with citrus roots. The most effective fungus for growth enhancement of citrus rootstocks was G. intraradices. Significant differences in mycorrhizal dependency among rootstocks were confirmed. Sour orange (Citrus aurantium L.) and Cleopatra mandarin (C. reshni L.) were more dependent than Troyer citrange [C. sinensis (L.) Obs. × Poncirus trifoliata (L.) Raf.] and Swingle citrumelo (C. paradisi Macf. × P. trifoliata). Moreover, several inoculation systems for plant production were evaluated for their effectiveness in promoting root colonization of the rootstock cultivars.