The resistance of Hordeum bulbosum and its hybrids with H. vulgare to common fungal pathogens

Summary Two tetraploid and two diploid clones of Hordeum bulbosum were screened for resistance to five isolates of powdery mildew which are virulent on cultivated barley. All were resistant and this resistance was also expressed in hybrids with H. vulgare. The tetraploid genotypes were also resistant to isolates of yellow rust and brown rust. These results show that H. bulbosum contains useful genes for resistance to these diseases and that there is a potential to transfer these into cultivated barley.     

Qualitative resistance to powdery mildew in hybrid sweet peas

Lathyrus odoratus L. × Lathyrus belinensis L. hybrids were produced using L. belinensis as the pollen parent, with fertile seed produced by the L. odoratus parent. The F1 hybrid plants were completely self-sterile, but produced viable seeds when backcrossed to L. odoratus. The plants produced by backcrossing resembled L. odoratus, the flower colour being purple/magenta, and were self-fertile. Both hybrid plants and those produced by back crossing to L. odoratus were resistant to Erysiphe pisi DC that causes powdery mildew in sweet peas. Continued backcrossing resulted in hybrid plants, that closely resembled the L. odoratus parent, but segregated for complete resistance/susceptibility to E. pisi,with a ratio of 2.46:1 resistant to sensitive plants. This suggests the presence of a single dominant gene that confers resistance. When resistant and sensitive plants were inoculated with E. pisi and their leaf surfaces examined,using a Scanning Electron Microscope, it was found that although spores germinated on the leaves of both resistant and sensitive plants, spores present on resistant plants collapsed shortly after germination. Possible reasons for this are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

The production of multispecific octoploids from Fragaria species and the cultivated strawberry

Summary A method of incorporating genetic material from five species of Fragaria and the cultivated strawberry into fertile octoploids is described. A synthetic octoploid derived from a hexaploid x diploid hybrid was crossed to octoploid cultivar breeding lines until a BC₂ hybrid was produced. A second synthetic octoploid in which two diploid species and a tetraploid species were combined was crossed to a cultivated strawberry to produce a hybrid breeding clone. The two breeding clones were crossed and 222 seedlings were produced. The seedlings were generally vigorous and fruitful, some having commercial potential.     

Inheritance of tree and fruit characters in progenies from crosses of sweet cherry (Prunus avium L.) cultivars

Summary Approximately 1000 seedlings from 20 combinations crossed in 1979, 1980 and 1981 (Theiler-Hedtrich 1985a) were tested for several characters: fruit set (yield), fruit size, fruit colour, formation of abscission layer and bleeding after fruit removal from fruit stalks, bacterial canker resistance, flowering and harvesting time. From progeny of crosses with Stella as pollinator, 56% (Vittoria × Stella) and 46% (Schüttler × Stella) of the seedlings were self-compatible, of which 14 were high yielding with good fruit size and quality. From the data recorded it can be concluded:fruit set is a recessive character; only 5 to 20% of very good yielding seedlings were obtained in different progeny, even if the parental plants were both very good croppers.Fruit juice and skin colour was in most progenies ‘black’ even if they were from combinations with ‘white’ varieties, e.g., Merton Glory or Schüttler. Only from the combination Schiittler (‘white’) × Stella (‘black’), 50% of the seedlings were ‘white’; Stella therefore is heterozygous for the character of fruit juice and skin colour.Fruit size is evenly distributed in progeny with respect to the fruit size of their parent plants.Abscission layer formation and non-bleeding is a genetically complex character. In combinations where both parent plants formed fruits with complete abscission layers and which were not bleeding after fruit removal from the stalk, this character was inherited only to 50% (Vittoria × Schüttler) or 85% (Vittoria × Frühe von der Weid) in the progeny. For the genetical control of this character further studies are necessary.Bacterial canker susceptibility was evenly distributed in seedlings from all combinations even if the highly resistant cv. Vittoria was used as one parent plant, thereby not confirming the expected results of a higher proportion of resistant seedlings from combinations with Vittoria.Flowering and harvest time of the seedlings from different combinations was within the range of the parent plants. Only in the combination of Vittoria × Stella (mid-to late-ripening season) one seedling out of 99 was found to form ripe fruits two weeks earlier than the parental plants. From the seedlings tested 40 have been chosen for further evaluation or genetical studies.     

A tetraploid hybrid plant from 4x × 2x crosses in Vitis and its origin

To study the origin of unreduced (2n) gametes in diploid Vitis cultivars, we surveyed the occurrence of tetraploid hybrid seedlings from 40 interploid crosses with five tetraploid and seven diploid cultivars. A total of 250 seedlings from the interploid crosses were established through embryo culture and by seed sowing. In 20 2x × 4x crosses, no tetraploid hybrid seedlings were derived from 8,902pollinations. In 20 4x × 2x crosses, two tetraploid hybrid seedlings were obtained from 8,057 pollinations. Investigation of isozyme genotypes of the two tetraploid seedlings using three variable enzyme systems indicated that one of the two seedlings resulted from the union of a diploid egg with a 2n male gamete and that failure of second meiotic division resulted in the formation of the 2n male gamete. The percentage of giant pollen grains in `Muscat Bailey A', a pollen parent of the tetraploid seedling, was relatively high(about 5.9%) and 10.9% of the giant pollen grains germinated on agar medium. These results suggested that 2n pollen of diploid cultivars are useful for breeding tetraploid grape. This revised version was published online in August 2006 with corrections to the Cover Date.     

The importance of wild potato species resistant to the potato cyst nematode,Globodera pallida,pathotypes P4A and P5A,in potato breeding. I. Resistance studies

Summary Seven wild diploid potato species, Series Tuberosa, representing 1023 clones were screened for resistance to the potato cyst nematode, Globodera pallida. Over 25% of the clones were resistant to pathotype P₄A and almost 30% were resistant to pathotype P₅A. The resistance in hybrid progenies of these and other resistant species with cultivated potatoes was evaluated, and over 2200 seedlings were screened. High frequencies of resistance (>50%) to P₄A were found in progenies with Solanum leptophyes, S. vernei, S. gourlayi and S. capsicibaccatum, whereas resistance to P₅A was found in these species as well as S. sparsipilum. The importance of nematode resistant wild species for potato breeding is discussed.     

Interspecific hybridisation between Hibiscus syriacus, Hibiscus sinosyriacus and Hibiscus paramutabilis

Interspecific hybrids from crosses between H. syriacus × H. paramutabilis and H. syriacus × H. sinosyriacus were obtained. In both cases unilateral incongruity was observed and reciprocal crosses yielded no fruits. In vitro embryo rescue, 11 weeks after pollination, increased the germination capacity of H. syriacus × H. sinosyriacus embryos, while this was not the case for H. syriacus × H. paramutabilis embryos. However, a lot of the generated H. syriacus × H. sinosyriacus seedlings were lost due to variegated and total albinism. In the progeny of H. syriacus ‘Oiseau Bleu’ × H. paramutabilis about 95% of the seedlings had an intermediate flower and leaf morphology compared to both parent plants. Leaves on the adult F1 hybrids showed a polymorphism. In total four different leaf types could be observed on the same plant. This leaf polymorphism also was seen in the progeny of H. syriacus ‘Melwhite’ × H. sinosyriacus ‘Lilac Queen’. In this progeny about 50% of the seedlings had an intermediate flower and leaf morphology compared with the parent plants. The hybrid nature of the seedlings of both progenies was also confirmed by AFLP analysis. Despite the low germination vigour of the pollen of the hybrids, a small F2 generation was obtained from H. syriacus ‘Oiseau Bleu’ × H. paramutabilis.     

Introgression of crown rust resistance from diploid oat Avena strigosa into hexaploid cultivated oat A. sativa by two methods: direct crosses and through an initial 2x·4x synthetic hexaploid

New sources of resistance to crown rust, Puccinia coronata f. sp. avenae (Eriks.), the major fungal disease of cultivated oat, Avena sativa L. (2n = 6x = 42), are constantly needed due to frequent, rapid shifts in the virulence pattern of the pathogen. Crown rust resistance identified in the diploid oat A. strigosa (Schreb.) (2n = 2x = 14) accession CI6954SP was transferred into cultivated oat using two methods: direct cross of the diploid to a hexaploid cultivar facilitated by embryo rescue, and initial cross of the diploid to a wild tetraploid oat to make a synthetic hexaploid for subsequent crossing to a hexaploid cultivar. Two tetraploids, a crown rust resistant A. murphyi (Ladiz.) accession P12 and a susceptible A. insularis (Ladiz.) accession INS-1, were used in the 2x·4x crosses. Resistant backcross-derived lines were recovered by both methods. Although the 2x·4x synthetic method did not require the laborious discovery and rescue of an infrequent initial hybrid embryo of the direct cross, the direct cross method provided more rapid backcross recovery of plants with high fertility, full transmission of resistance, and desired plant and seed phenotypes. A suppressor effect, present initially but segregating in backcrosses, appeared to come from the CI6954SP donor and is the same as, or analogous to, suppression by an oat line with the crown rust resistance gene Pc38. No resistance from A. murphyi P12 was detected in advanced generations when it was introduced either as a component of a synthetic hexaploid or in direct crosses to A. sativa, indicating suppression of its resistance in interploidy combinations. That the dominant resistance gene transferred from CI6954SP and a gene Pc94 introgressed earlier from a different A. strigosa accession may be the same or quite similar to one another is indicated by their in-common specificity to suppression of resistance expression, susceptibility to a newly recovered rust isolate, and close linkage to the molecular marker SCAR94-2. The introgressed resistance genes from the different sources, even if the same, may have different cultivar genomic introgression sites, which would allow tests of dosage effects on resistance expression.     

Use of ovule culture to cross between Delphinium species of different ploidy

The applicability of ovule culture was examined in reciprocal crosses between the diploid species D. grandiflorum L. and the tetraploid cultivar D. hybridum ‘Galahad’. An interspecific hybrid plant of D. hybridum ‘Galahad’ × D. grandiflorum was obtained,and hybridity was confirmed by chromosome counting. The hybrid was different from the parents in flower color and size but intermediate between the parents in leaf shape. The hybrid showed low pollen fertility and failed to produce viable seeds by either self- or backcross-pollination using fertile pollen grains from its parents. The applicability and efficiency of the ovule culture technique for genetic improvement of the genus Delphinium through interspecific hybridization between different ploidies is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Early selection for partial resistance to powdery mildew,Podosphaera leucotricha (Ell. et Ev) Salm. in apple progenies

Summary Selection of apple for partial resistance to powdery mildew in the nursery before planting trees in the orchard can only be relied upon if genotypic differences in expression do not depend on plant age. Five progenies between susceptible cv. Melrose and five partially resistant genotypes (four CPRO-DLO selections from the cross ‘Elstar × Prima’ and cv. Prima itself) were assessed for mildew resistance following natural infection in the nursery during two successive seasons. Per progeny 60 randomly taken trees were transplanted into the orchard and their mildew resistance levels were assessed for three more years. The mean mildew resistance of the progenies in the nursery was related to that of their resistant parent, but differences in mean resistance levels in the orchard were not consistent with those in the nursery. The individual correlations of resistance between nursery and orchard also varied per progeny. In general the second year's assessment in the nursery gave the best prediction of the orchard performance over three years. In conclusion, nursery selection for partial mildew resistance can be effective, but the efficiency varies between populations.     

Ribes glutinosum and R. Sanguineum as donors of resistance to American gooseberry mildew in black currant breeding

Summary A major dominant resistance gene Sph ₃ has been transferred from Ribes carrierei (R. glutinosum albidum × R. nigrum) to black currants of commercial promise by backcrossing repeatedly to R.nigrum. Probable causes of the almost invariable deficit of resistant plants in segregating progenies are briefly discussed. Response of young seedlings to mildew in the glasshouse was very highly correlated with their field response as mature plants.     

Breeding barriers between the cultivated strawberry, Fragaria × ananassa, and related wild germplasm

Five-hundred interspecific and intergeneric crosses were performed among accessions of the wild strawberries Fragaria vesca(2x), Duchesnea indica (8x), Potentilla tucumanensis (2x) and 9 genotypes of the cultivated strawberry, Fragaria×ananassa (8x), following an incomplete diallele mating design. Crosses between D. indica and F.×ananassa produced many putative hybrids when D. indica was used as female but a few achenes and plants when used as male; therefore, pollen-pistil compatibility relations were analyzed by fluorescence microscopy in this direction of the cross. Of the genotypic combinations, 78.6% were incompatible at the stigma level and 17.2% at the first third of the style. Only 3.6% were pollen-pistil compatible and produced fruits with achenes (seven did not germinate or originated short-lived plants and nine produced normal plants). F.vesca×F.×ananassa crosses produced 35 hybrid achenes but only 14% germinated, yielding short-lived plants; histological analyses revealed that inviable seeds had less developed (or collapsed) endosperms and smaller embryos than control plump F. vesca seeds. P.tucumanensis was only used as male, with negative results. These species and genera are partially isolated by a complex system of pre- and post-zygotic barriers. Knowledge of their nature would allow the breeder to devise strategies to put the genetic variability available in the group into a useful form. This revised version was published online in July 2006 with corrections to the Cover Date.     

Single-gene resistance to powdery mildew in zucchini squash (Cucurbita pepo)

Cucurbita pepo (pumpkin, squash,gourd) is an economically important species that is susceptible to the cucurbit powdery mildew fungus, Podosphaera xanthii(syn. Sphaerotheca fuliginea). ‘True French’, an open-pollinated cultivar of the Zucchini Group of C. pepo, was crossed with an unnamed powdery-mildew resistant straight neck-type accession, the resistance of which was apparently derived from an interspecific cross with a resistant wild species of Cucurbita,and resistant plants were selected in the F₂ generation. This was followed by six cycles of backcross-pedigree selection for resistance, and resulted in the development of an accession true-breeding for resistance to powdery mildew and nearly isogenic to ‘True French’. The resistant and susceptible near-isogenics were crossed and seeds of the filial and backcross generations were produced. Plants of the parental accessions and their progenies were grown together in a controlled-environment chamber, exposed to the pathogenic fungus, and scored as resistant, partially resistant, or susceptible 27–33 days after sowing. The results indicated that resistance is conferred by a single incompletely dominant gene, designated Pm-0. This revised version was published online in August 2006 with corrections to the Cover Date.     

Screening methods of apple trees for resistance toValsa canker

Summary Valsa canker is the most serious wood rot disease of apple in Japan. Dormant, excised twig assay in vitro was used to determine resistance to Valsa canker. ‘Fuji’, ‘Starking Delicious’, ‘M.26 EMLA’, and ‘MM.106’ were considered to be highly susceptible, while Malus sieboldii showed moderate resistance. Several cultivars belonging toM. prunifolia, M. baccata, andM. × domestica showed comparative resistance. The phloridzin content in the bark ofMalus species was studied in relation toValsa canker resistance. Phloridzin content ofM. sieboldii was lower than inM. × domestica. Comparatively resistant cultivars had similar or higher phloridzin content compared to susceptible cultivars. No clear correlations were found between resistance and phloridzin contents in the bark ofMalus species.     

Interspecific hybridization in the genus Arachis through embryo culture

Summary The embryos of a cultivated tetraploid peanut (Arachis hypogaea), a wild diploid species (A. villosa), and their hybrid embryos, which generally abort in nature, were cultured in vitro and the plants have been successfully transferred to the soil. The hybrids showed triploid chromosome number (3x=30). The significance of wide hybridization in peanut-improvement programs is discussed.     

Inheritance of downy mildew resistance in Indian cauliflower (group III)

Summary Inheritance of downy mildew (Peronospora parasitica) resistance in three resistant x susceptible crosses, one susceptible x susceptible and one resistant x resistant cross were studied in Indian cauliflower (Group III) over the two years (1990 and 1991). No significant difference was observed between the years for various estimates and hence pooled data are presented. Downy mildew resistance in crosses cc×HR 5-4 and 3-5-1-1×244 (R×S) is governed by single dominant gene PPA3 but in cross cc×244 (R×S), recessive epistasis was observed. The resistance level was not improved in both the cc×3-5-1-1 (R×R) and 244×267-6-9 (S×S) crosses. Exploitation of downy mildew resistance from cc and 3-5-1-1 in F₁ hybrid is explained in detail.     

Development of a promising interspecific hybrid in black pepper (Piper nigrum L.) for Phytophthora foot rot resistance

For the first time in the history of black pepper cultivation, a partly fertile interspecific hybrid having partial resistance to the dreaded disease Phytophthora foot rot was developed through hybridizing Piper nigrum with the wild species Piper colubrinnum. Hybridity of interspecific progenies was established through morphology, anatomy, cytology, and molecular studies. The hybrid, whose chromosome number is 2n = 39, is a triploid hybrid between a tetraploid and diploid species. The hybrid designated as Culture P5PC-1 exhibited distinct anatomical and morphological feature with a large number of long spikes with reduced setting percentage. The RAPD primers OPE 07 and OPG 08 were identified as hybrid specific molecular markers. Functional evaluation revealed partial introgression of genes—responsible for Phytophthora foot rot resistance—into the hybrid. This hybrid is considered as a successful breakthrough for introgression of resistance to the cultivated species Piper nigrum from the wild species Piper colubrinnum.     

Resistance to stripe rust in Triticum turgidum,T. tauschii and their synthetic hexaploids

Resistance to stripe rust (caused by Puccinia striiformis Westend.) of 34 Triticum turgidum L. var.durum, 278 T. tauschii, and 267 synthetic hexaploid wheats (T. turgidum x T. tauschii) was evaluated at the seedling stage in the greenhouse and at the adult-plant stage at two field locations. Mexican pathotype 14E14 was used in all studies. Seedling resistance, expressed as low infection type, was present in all three species. One hundred and twenty-eight (46%) accessions of T. tauschii, 8 (23%) of T. turgidum and 31 (12%) of synthetic hexaploid wheats were highly resistant as seedlings. In the field tests, resistance was evaluated by estimating area under disease progress curve (AUDPC). Synthetic hexaploid wheats showed a wide range of variability for disease responses in both greenhouse and field tests, indicating the presence of a number of genes for resistance. In general, genotypes with seedling resistance were also found to be resistant as adult plants. Genotypes, which were susceptible or intermediate as seedlings but resistant as adult plants, were present in both T. turgidum and the synthetic hexaploids. Resistances from either T. turgidum or T. tauschii or both were identified in the synthetic hexaploids in this study. These new sources of resistance could be incorporated into cultivated hexaploid wheats to increase the existing gene pool of resistance to stripe rust.     

Transmission of bacterial wilt resistance by First Division Restitution (FDR) 2n pollen via 4x×2x crosses in potatoes

Summary Resistance to bacterial wilt (Pseudomonas solanacearum) found in tuber-bearing Solanum species was transferred into a diploid potato breeding population. Simultaneous selections were made for agronomic characters, production of first division restitution (FDR) 2n pollen, and resistance. Diploid resistant genotypes were identified via inoculation with a virulent isolate (CIP-204) of race 3 of Pseudomonas solanacearum. These resistant diploid genotypes were crossed to susceptible tetraploid potatoes. An investigation was made to assess whether resistant diploid genotypes transmit resistance to bacterial wilt, which is a quantitatively inherited trait, to tetraploid potatoes via FDR 2n pollen. Tetraploid seedlings from 4x×2x crosses were inoculated with the same isolate CIP-204, and the percentage of surviving seedlings was scored. Some 4x×2x families from resistant diploid genotypes demonstrated a high level of survival rate. The transmission of bacterial wilt resistance was achieved by the use of FDR 2n pollen. It was speculated that a female x male interaction effect on the survival rate of the evaluated progeny may exist. Selecting proper 4x and 2x parents would be important for obtaining a higher frequency of transmission of resistance to bacterial wilt in the progeny.     

Utilizing Arachis cardenasii as a source of Cercospora leafspot resistance for peanut improvement

Summary Advanced generation 40-chromosome hybrids between A. hypogaea (2n=4x=40) and a wild diploid species. A. cardenasii were evaluated for early leafspot, Cercospora arachidicola resistance and agronomic potential. The objective of this investigation was to determine if early leafspot resistance derived from a wild species could be incorporated into the A. hypogaea genome. Interspecific hybrid selections were made and then compared in the field and greenhouse to susceptible cultivars and to A. hypogaea lines which are reported to be resistant to early leafspot. Significantly higher levels of resistance were found in five hybrid selections than in cultivated lines based on numbers of lesions per leaf. In a greenhouse study, several hybrid selections also had greatly reduced sporulation from lesions as compared to A. hypogaea. Several mechanisms of resistance are believed to be present. Although hybrid selections had small seeds and low yields as compared to A. hypogaea, a new and valuable source of early leafspot resistance derived from the species A. cardenasii is present.Paper no. 8814 of the journal series of the North Carolina Agricultural Research Service, Raleigh, NC 27695. This research was partially funded by AID-Peanut CRSP grant DAN-4048-G-SS-2065-00.