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P. Martínez-Gómez, M. Rubio and F. Dicenta

The resistance to a Dideron isolate of Plum pox virus, which causes sharka disease, of four apricot (Prunus armeniaca L.) cultivars from North America (`Harlayne', `Henderson', `Sunglo', and `Veecot') and a Greek cultivar Lito (a cross of American cultivar Stark Early Orange × Greek cultivar Precoce Tirynthos) was evaluated. `Stark Early Orange' and `Canino', previously rated as resistant and susceptible respectively, were included as controls. Resistance, herein, was defined as inability to infect plants by graft-inoculation and negative assays by enzyme-linked immunosorbent assay. Cultivars found to be resistant were: `Harlayne', `Henderson', `Sunglo', `Lito', and `Stark Early Orange'. Cultivars Veecot and Canino were susceptible.

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P. Martínez-Gómez and T.M. Gradziel

The techniques of micrografting provide useful tools for the early propagation of breeding lines in variety improvement programs. In this study, the success of various in vivo micrografts of buds of `Nonpareil' almond (Prunus dulcis) seedlings have been examined. Variables tested include type of microscion, the rootstock genotype, and the growth stage of the rootstock. Microscions tested included small [3 mm (0.12 inches)] microwedges from either unsprouted or recently sprouted buds. Approach micrografts were also evaluated. Three-month-old woody rootstocks tested included `Nemaguard' and `Nemared' peach (P. persica) rootstocks, and the `Hansen' (peach × almond) hybrid rootstocks. `Nemared' rootstocks were also grafted after 1 month of growth when the tissue was still herbaceous. Micrografts onto woody rootstocks and using recently sprouted buds showed the best results. With very young or weak seedlings, where the recovery of microscions was difficult, the use of approach micrografts onto herbaceous rootstock proved a useful technique.

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T.M. Gradziel and P. Martínez-Gómez

California almonds [Prunus dulcis, syn. P. amygdalus, P. communis] possess a moderately lignified `paper' shell rather than the stony, peach-pit type shells common to European and Asian cultivars. At nut maturity, more than 70% of shells of the principal California cultivar Nonpareil can be split. Use of a mechanical shaker to harvest nuts increased the proportion of nuts with split shells by 40% when compared to hand harvest. All shell splitting occurred at the ventral suture with ∼ 80% of the splits occurring at the site of the degenerating funiculus leading to the aborted or secondary ovule. Remaining splits occurred near the site of the funiculus feeding the viable ovule, and only rarely at the suture line. Abortion of one of the two ovules in the almond ovary is often initiated at or shortly after bloom, and so the final site of shell splitting appears to be predetermined early in fruit development. Measurements of the strength of the inner endocarp wall at 50 days after flowering showed distinct weaknesses in the areas of the developing funiculi. Similarly, damage to the developing kernel at 60 days after flowering by the leaffooted bug (Leptoglossus clypealis Heiderman) occurred along the ventral suture, with 80% of the damage located at the point of attachment of the secondary funiculus.

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J. Egea, P. Martínez-Gómez, F. Dicenta and L. Burgos

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P. Martínez-Gómez, T.M. Gradziel, E. Ortega and F. Dicenta

Pollen of the California almond cultivars Nonpareil, Ne Plus Ultra, Sonora, and Peerless was evaluated for viability following storage over 12 months at 4, 0, -20, and -80 °C. The proportion of viable pollen exceeded 80% for all cultivars and for all temperatures evaluated after 2 months of storage. Following 12 months of storage at 4 °C, germination decreased to 8% for `Nonpareil', 10% for `Ne Plus Ultra', 50% for `Sonora', with no germination observed for `Peerless'. Storage at sub-freezing temperatures maintained pollen viability above 70% in `Nonpareil', `Ne Plus Ultra', and `Sonora' and above 40% in `Peerless'. Cultivars differed significantly in their tolerance to low temperature pollen storage. Within cultivars, differences in pollen germination following storage at 0, -20, or -80 °C were nonsignificant.

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P. Martínez-Gómez, T.M. Gradziel, E. Ortega and F. Dicenta

Almond [Prunus dulcis (Mill.) D.A. Webb] breeding programs require successful techniques for pollen storage. We studied the pollen viability of two almond cultivars, `Ramillete' and `Desmayo Largueta', during 8 weeks of storage, in conditions that simulated standard situations including storage at 4, 22, and 4 °C alternating with 22 °C (4 °C/22 °C). Viability remained at 60% or more for 2 weeks under all three conditions. After the second week, germination capacity decreased rapidly at 22 °C, but remained above 50% for as long as 8 weeks at 4 °C or 4 °C/22 °C.

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F. Dicenta, P. Martínez-Gómez, E. Ortega and H. Duval

The effect of pollinizer on sweet or bitter almond flavor was studied by tasting the seeds obtained from 32 crosses between sweet, bitter, and slightly bitter parents. Out of eight female parents, two were homozygous sweet (`Del Cid' and `Aï'); two were heterozygous sweet (`Marcona' and `Nonpareil'); one heterozygous with an almost undetectable slightly bitter flavor (`Ferrastar'); two heterozygous slightly bitter (`Garrigues' and `Marie Dupuy'); and one bitter homozygous (`S3067', self-compatible clone obtained in CEBAS). Each cultivar was hand-pollinated with four male cultivars: one homozygous sweet (`Ramillete'), one heterozygous sweet (`Atocha'), one heterozygous slightly bitter (`Garrigues'), and one homozygous (`S3067'). Since `Garrigues' is self-incompatible, the cross `Garrigues' × `Garrigues' was replaced by `Garrigues' × `S3065' (slightly bitter clone obtained in CEBAS). Tasting of the seeds resulting from each cross resulted in the complete absence of any influence of pollinizer on flavor, which only depended on the female parent.

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P. Martínez-Gómez, M. Rubio, F. Dicenta and T.M. Gradziel

Sharka [(plum pox virus (PPV)] mainly affects Prunus species, including apricot (Prunus armeniaca L.), peach (Prunus persica L.), plum (Prunus salicina Lindl., Prunus domestica L.), and, to a lesser degree, sweet (Prunus avium L.) and sour cherry (Prunus cerasus L.). Level of resistance to a Dideron isolate of PPV in seven California almond [P. dulcis (Miller) D.A. Webb], five processing peach cultivars, and two peach rootstocks was evaluated. In addition, almond and peach selections resulting from interspecific almond × peach hybridization and subsequent gene introgression were tested. Evaluations were conducted in controlled facilities after grafting the test genotypes onto inoculated GF305 peach rootstocks. Leaves were evaluated for PPV symptoms during three consecutive cycles of growth. ELISA-DASI and RT-PCR analysis were also employed to verify the presence or absence of PPV. Peach cultivars and rootstocks showed sharka symptoms and were ELISA-DASI or RT-PCR positive for some growth cycles, indicating their susceptibility to PPV. Almond cultivars and almond × peach hybrids did not show symptoms and were ELISA-DASI and RT-PCR negative, demonstrating resistance to PPV. Two (almond × peach) F2 selections as well as two of three backcrossed peach selections also showed a resistant behavior against the PPV-D isolate. Results demonstrate a high level of resistance in almond and indicate potential for PPV resistance transfer to commercial peach cultivars.

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P. Martínez-Gómez, S. Arulsekar, D. Potter and T.M. Gradziel

The genetic relationships among peach [Prunus persica (L.) Batsch], almond [P. dulcis (Mill.) D.A. Webb or P. amygdalus (L.) Batsch] and 10 related Prunus species within the subgenus Amygdalus were investigated using simple sequence repeat (SSR) markers. P. glandulosa Pall. was included as an outgroup. Polymorphic alleles were scored as present or absent for each accession. The number of alleles revealed by the SSR analysis in peach and almond cultivars ranged from one to three whereas related Prunus species showed a range of one to 10 alleles. Results demonstrated an extensive genetic variability within this readily intercrossed germplasm as well as the value of SSR markers developed in one species of Prunus for the characterization of related species. Mean character difference distances were calculated for all pairwise comparisons and were used to construct an unrooted dendogram depicting the phenetic relationships among species. Four main groups were distinguished. Peach cultivars clustered with accessions of P. davidiana (Carr.) Franch. and P. mira Koehne. The second group contained almond cultivars. A third group included accessions of P. argentea (Lam) Rehd., P. bucharica Korschinsky, P. kuramica Korschinsky, P. pedunculata Pall, P. petunikowii Lits., P. tangutica (Spach) Batal., and P. webbii (Spach) Vieh.. P. glandulosa and P. scoparia Batal. were included in a fourth group.

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F. Dicenta, P. Martínez-Gómez, E. Martínez-Pato and T.M. Gradziel

Aspergillus flavus Link. is a filamentous fungus affecting almond [Prunus dulcis (Mill.) D.A. Webb] kernels in the field and during storage. This fungus can produce afla-toxins (carcinogenic and immunosuppressive mycotoxins), which prevent the marketability of almond kernels. Aspergillus flavus resistance has not been an objective in conventional almond breeding programs. Because the importance of this disease is increasing, evaluations of cultivar susceptibility are being performed. In this study, the screening for A. flavus resistance in 40 almond genotypes has been carried out in controlled inoculation conditions at 26 °C. Eighteen days after the inoculation, kernels of all the almond cultivars assayed showed susceptibility to A. flavus. Nevertheless, differences among cultivars in the percentage of kernel surface colonized by the fungus were observed. The Spanish cultivar Ramillette was the least susceptible. Susceptibility was not related to the geographic origin of the cultivar.