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Samir Mhameed, Dror Sharon, Jossi Hillel, Emanuel Lahav, Daniel Kaufman, and Uri Lavi

To estimate heterozygosity level in the avocado (Persea americana Mill.) genome, two types of variable number of tandem repeat (VNTR) markers were used. Multilocus DNA fingerprints (DFPs) were analyzed on avocado progeny resulting from either crosses or selfing of cultivars. In five crosses, heterozygosity was 100%, while in two self-pollinated families, heterozygosity was 90% and 94%. Single locus, simple sequence repeat (SSR) DNA markers were analyzed by typing 59 loci on five avocado cultivars. Average heterozygosity varied from 0.50 to 0.66, while gene diversity varied from 0.42 to 0.66. Heterozygosity varied from 38% to 70%. The percentage of fragments that exhibited Mendelian inheritance was 62.5% to 85% (P < 0.05) for the DFP fragments and 85% for the SSR alleles.

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Uri Lavi, Emanuel Lahav, Chemda Degani, Shmuel Gazit, and Jossi Hillel

Genetic variance components for avocado (Persea americana Mill.) traits were estimated to improve avocado breeding efficiency. The additive and nonadditive genetic variance components were calculated from the variances between and within crosses. In all nine traits examined, i.e.-anise scent, fruit density, flowering intensity, fruit weight, harvest duration, inflorescence length, seed size, softening time, and tree size-a significant nonadditive genetic variance was detected. Additive genetic variance in all traits was lower and nonsignificant. The existence of major nonadditive variance was indicated also by narrow-sense and broad-sense heritability values estimated for each trait. Therefore, parental selection should not be based solely on cultivar performance. Crosses between parents of medium and perhaps even low performance should also be included in the breeding program.

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U. Lavi, J. Hillel, A. Vainstein, E. Lahav, and D. Sharon

Application of four DNA fingerprint probes to avocado (Persea americana Mill.) resulted in identification of various cultivars, characterization of the three avocado races, and a genetic analysis of family structure. Genomic DNA from 14 cultivars was probed with four DNA fingerprint probes. Three of the probes gave well-resolved bands. The individual-specific patterns obtained for each cultivar validate the use of this technique for definitive cultivar characterization, with the probability of obtaining a similar pattern for two different cultivars being 2 × 10-9. DNA mixes representing either Mexican, Guatemalan, or West-Indian avocado races were hybridized with the DNA fingerprint probes, and a band pattern characteristic for each race was obtained. Progeny of a cross between the cultivars Ettinger and Pinkerton were analyzed. Their DNA fingerprints revealed one pair of linked bands and another band allelic to one of them. The application of these observations to identification, evolutionary studies, and breeding is discussed.

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Zeinab A. El-Hamalawi and John A. Menge

The sugary exudate appearing on bark lesions of Persea americana Miller and Persea indica plants after infection with Phytophthora citricola contained viable oospores and hyphal fragments in the field and in the greenhouse. This sugary exudate was a source of inoculum and dispersal of the pathogen within and between avocado plants. Spraying water onto lesions moved inoculum from the sugary exudate to wounds below. Water from sprinkler irrigation washed propagules into the soil around the plants. Viable propagules of Phytophthora citricola were identified in the feces of snails (Helix aspersa) that had fed on infected bark tissues. When these snails were moved to healthy plants, they made wounds on succulent tissue, and the infectious feces induced cankers. Ants (Iridomyrmex humilis) were attracted to the sugary exudate and also transmitted infectious propagules to wounds on avocado stems and to the soil. Control strategy for the avocado stem canker disease should consider control of vectors.

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Chemda Degani, Anat Goldring, Itzhak Adato, Ruth El-Batsri, and Shmuel Gazit

The effects of various pollen parents on outcrossing rates, yield, and fruit and seed weights were studied in a `Fuerte' avocado (Persea americana Mill.). Isozyme analysis was used to identify the pollen parent of mature fruits. Cotyledons were assayed for five polymorphic enzyme systems: alcohol dehydrogenase, leucine aminopeptidase, malate dehydrogenase, phosphoglucomutase, and triosephosphate isomerase. When sampling of fruits was done to a height of 2 m, percent of hybrids produced by `Teague' and `Topa-Topa' pollenizers was in the range of 30% to 40%. With `Teague' as the pollenizer, `Fuerte' yield increased by 30% in trees adjacent to the pollenizer. With `Tops-Tops' as pollenizer, the yield was increased by 40% for trees adjacent to and at a distance from the pollenizer. `Ettinger' trees planted at a distance of 30 to 50 m from `Fuerte' were found to be the pollen parent of 2% to 14% of the progeny, thus supporting our previous conclusion regarding the high potency of `Ettinger' as a pollen parent. `Tops-Tops', `Teague', and `Ettinger' significantly increased fruit and seed weights of crossed compared with selfed `Fuerte' fruits.

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D. M. Eissenstat and J. P. Syvertsen

The effects of elevated levels of ozone on growth, mineral nutrition and freeze resistance were studied using broadleaf-evergreen citrus and avocado trees. `Ruby Red' grapefruit (Citrus paradisi L.) trees on either Volkamer lemon (Citrus volkameriana Ten. & Pasq.) or sour orange (Citrus aurantium L.) rootstock and `Simmonds' or `Pancho' avocado trees (Persea americana Mill.) on the rootstock `Waldin' were exposed to ozone in open-top chambers for 4 mo in 1988 and in a second experiment in 1989 for 8 mo. Citrus tree growth, estimated by total leaf mass, was unaffected by ozone concentrations of 3 times ambient in either year but avocado growth was reduced by ozone concentration at 2 times ambient in 1989. All trees were well-fertilized and ozone had little effect on mineral nutrient concentrations in leaves. Freeze resistance, estimated by electrolyte leakage from leaf disks and survival of leaves, stems, and whole-plants following exposure to freezing temperatures, was often diminished in avocado and citrus at 3 times ambient ozone, but occasionally was increased at 2 times ambient. Thus, ozone can be related to shifts in freeze resistance that can occur prior to discernible growth effects.

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Anthony W. Whiley and Bruce Schaffer

The influence of shoot age on 14C partitioning in potted avocado (Persea americana var. americana Mill.) trees was determined. The oldest leaf of actively growing shoots and the youngest leaf of previously matured shoots were exposed to 14CO2 18 and 34 days after budbreak (DABB) of new shoots. At these times, treated leaves had a positive net CO2 assimilation rate and, therefore, were considered to be net C exporters. Sixteen days after 14C exposure, separate plant tissues were harvested, dried, weighed, and oxidized. The percentage of 14C in each tissue was determined by liquid scintillation spectrometry. Photoassimilates were translocated acropetally and basipetally from all treated leaves. However, at 18 DABB, developing leaves of actively growing shoots seemed to be the strongest sink for C assimilated by the oldest leaf of these shoots, whereas the roots were the strongest sink for C assimilated by the youngest leaf of the previously matured shoots. By 34 DABB, roots were the strongest sink for C assimilated by leaves of new and previously matured shoots. These data are useful in developing improved management strategies for controlling phytophthora root rot (incited by Phytophthora cinnamomi Rands) in avocados by systemic phosphonate fungicides translocated in the photoassimilate pathway. Thus, phosphonates should be applied after shoots have matured and most of the canopy is in a quiescent state for maximum translocation to the roots.

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T.G. Thorp and B. Stowell

Avocado (Persea americana Mill. cv. Hass) trees were pruned over 3 years at either 4 or 6 m in height by removing or heading back selected limbs. Yields were compared with those from control trees with no pruning in the upper canopy. All trees had similar crop loads before pruning. Trees were at 9 × 10-m spacing and were 8 years old when first pruned. Fruit yields were recorded for 2 years before the first pruning and then in each year of pruning. In the final year, trees were harvested in four height zones: 0-2m; 2-4 m; 4-6 m; and >6 m. Cumulative yields over 3 years were similar on 6-m and control trees, but were less on 4-m trees due to the large volume of fruiting canopy removed in the first pruning. The height of the main fruiting zone was lowered on the 4-m trees, with yields in the 2-4-m zone similar to those in the 4-6-m zone of the control trees. Pruning to reduce the number and length of scaffold branches increased fruit yields on the remaining scaffolds without reducing fruit size. Results are discussed in terms of harvest efficiency and the benefits of small tree orchard systems.

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B. Schaffer, A.W. Whiley, and C. Searle

Banana (Musa sp.), mango (Mangifera indica), and avocado (Persea americana) plants were grown in controlled-environment glasshouses in ambient (350 μmol CO2/mol) and enriched (700–1000 (mol CO2/mol) atmospheric CO2 concentrations. At each CO2 concentration, plants were either exposed to sink-limiting (root restriction) or non-sink-limiting conditions (no root restriction). Total carbon assimilation and dry matter accumulation were generally greater for plants in the enriched CO2 environment than for plants grown in ambient CO2. However, plants grown in the enriched CO2 environment were less efficient at assimilating carbon than plants grown in ambient CO2. There was a downward regulation of net CO2 assimilation due to root restriction that resulted in less dry matter accumulation than in non-root-restricted plants. This may explain the lower net CO2 assimilation rates often observed for tropical fruit trees grown in containers compared to those of field-grown trees. Atmospheric CO2 enrichment generally did not compensate for reductions in net CO2 assimilation and dry matter accumulation that resulted from root restriction.

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Etaferahu Takele, Jewell L. Meyer, Mary L. Arpaia, David E. Stottlemyer, and Guy W. Witney

The effect of integrated applications of various irrigation and fertilization rates on productivity (yield and size) and returns of the `Hass' avocado (Persea americana Mill.) have been analyzed from 1987 to 1991 in western Riverside County. Eighteen treatment combinations comprised of three irrigation levels [80%, 100%, and 120% crop water use (ETc)], three N fertilizer levels (0.16, 0.7, and 1.4 kg/tree per year), and Zn (0 and 0.2 kg/tree per year) were included in the analysis. Using a partial budgeting procedure, returns after costs were calculated for each treatment combination. Costs of treatments, harvesting, hauling, and marketing were subtracted from the value of the crop. The value of the crop was calculated as the sum of crop returns in each size category. Three years of data on the relationship between irrigation and N showed 1) irrigating at 80% ETc would be ineffective even at very high water prices; 2) for groves where 100% ETc is sufficient, its application with either low or medium N would be beneficial; and 3) at higher irrigation (120% ETc), N application should be at or beyond the medium level.