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Muhammet Tonguç and Phillip D. Griffiths

Black rot, caused by Xanthomonas campestris pv. campestris (Pam.) Dawson (Xcc), is a major bacterial disease of Brassica oleracea L. vegetables. In this study the related species Brassica carinata Braun (ethiopian mustard), which can be used to generate interspecific crosses with B. oleracea was evaluated for resistance to Xcc. Fifty-four accessions and susceptible control plants were wound inoculated with four isolates of Xcc race 4 at the juvenile stage. Of the 54 accessions tested, A 19182 and A 19183 exhibited no symptoms when inoculated with Xcc for all plants tested, and the accessions including PI 199947, PI 199949 and PI 194256 segregated for resistance to Xcc.

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Bruno Lucas and Phillip D. Griffiths

Common bean accessions were evaluated to identify white- or light-seeded beans with resistance to Pythium ultimum Trow. In total, 568 common bean (Phaseolus vulgaris L.) accessions were inoculated with a hyphal suspension of P. ultimum under greenhouse conditions. The bean accessions included represented the Andean-Middle American core collections (406 accessions) and 162 additional white- or cream-seeded accessions. The accessions were categorized into 12 groups according to seedcoat color. Accessions with light seedcoats exhibited higher levels of disease symptoms, with white-seeded bean accessions being the most susceptible class. No symptomless white-seeded accessions were identified. The most resistant white-seeded accessions were PI 430207, PI 527803, PI 290996, PI 299021, PI 194574, and PI 304110. Cream-seeded beans exhibited higher levels of resistance, with nine accessions rated as symptomless out of 188 cream and white accessions tested. Of 568 accessions, 48 tested were symptomless, whereas disease ratings of the other accessions ranged from resistant to highly susceptible.

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Phillip D. Griffiths and Cathy Roe

Eighteen cabbage breeding lines and cultivars were evaluated for resistance to black rot caused by Xanthomonas campestris pv. campestris following wound and spray inoculations at the juvenile and mature stages. Plants were evaluated using four inoculation procedures (juvenile wound, juvenile spray, mature wound, and mature spray) in replicated greenhouse and field experiments. The breeding lines Badger #16, Cornell 101, Cornell 102, NY 4002 and accession PI 426606 exhibited high levels of resistance following all inoculation procedures. `Silver Dynasty' was the most resistant commercial cultivar based on the four tests, yet ranked 12th following the juvenile wound inoculation. The juvenile spray inoculation had a high correlation with both wound and spray inoculations in field experiments (0.89 and 0.86, respectively); however, the juvenile wound inoculation did not correlate well with mature wound and spray inoculations (0.58 and 0.51, respectively). The results indicate that the juvenile wound inoculation is not the most appropriate approach for determining field resistance in Brassica oleracea, and that resistant material could be selected against using this approach. A high correlation between juvenile spray inoculation disease severity ratings and mature plant resistance indicates that plants can be evaluated effectively at the juvenile stage for mature plant resistance to black rot.

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Muhammet Tonguc* and Phillip D. Griffiths

Powdery mildew is a fungal disease of crucifers, caused by Erysiphe polygoni D.C. and it can be problematic during seed increase in green-houses. Crosses were made between Brassica carinata (Ethiopian mustard) accession (PI 360883) and B. oleracea cultivars `Titleist' and `Cecile' to transfer resistance to powdery mildew to B. oleracea germplasm. It was not possible to obtain interspecific hybrids between Ethiopian mustard and B. oleracea through natural seed set. However, interspecific hybrids and backcross one (BC1) progenies were produced via embryo rescue following sexual crosses. Four interspecific hybrid plants were produced with the aid of embryo rescue from cultured pistils with B. carinata as the maternal parent, and their interspecific origin was confirmed through plant morphology and analysis of RAPD polymorphisms. No interspecific hybrids were obtained when `Titleist' was used as a maternal parent. Interspecific hybrid plants were male sterile and they were used as maternal parents to produce BC1 plants. Twenty one BC1 plants were obtained through natural seed set and embryo rescue, although embryo rescue was not necessary to produce first backcross generation plants. When tested in greenhouse with powdery mildew, all interspecific hybrids and eight of the BC1 plants were resistant to the disease. Crosses are being made to produce BC2 plants with 2n = 18 chromosomes for introgression of the resistance in B. oleracea.

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Phillip D. Griffiths, Eric Sandsted, and Donald Halseth

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Katy M. Rainey* and Phillip D. Griffiths

High temperatures (>30°C day and/or >20°C night) in tropical lowlands and production areas in temperate zones reduce yield and quality in common bean (Phaseolus vulgaris L.). Tepary bean (P. acutifolius A. Gray) is a crop adapted to hot arid climates and is grown in the American Southwest and parts of Mexico under temperatures that are too high for pod formation in common bean. Interspecific hybridization may enable transfer of heat tolerance traits from tepary bean to common bean. Twenty-five tepary bean plant introductions (PI) with the ability to set seed under controlled-environment conditions were evaluated under high (35 °C day/32 °C night) and control (27 °C day/24 °C night) temperature treatments during reproductive development. Four accessions (PI 200902, PI 312637, PI 440788, and PI 440789) exhibited normal pod formation and comparatively high yield when exposed to high temperature, while common bean controls displayed zero pod and seed set. These four PIs showed a mean decrease in seed yield of 72.9% from control to high temperature treatment, as compared to 90.3% among all tepary beans. These accessions were hybridized with the dry bean cultivar `ICA Pijao', and the heat-tolerant bean cultivars `Carson' and `CELRK' and breeding line `Cornell 503'. Immature embryos were cultured to obtain interspecific hybrids. Fertility of F1 hybrids and generation of backcrosses are discussed.

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Katy M. Rainey and Phillip D. Griffiths

The genetic basis for heat tolerance during reproductive development in snap bean was investigated in a heat-tolerant × heat-sensitive common bean cross. Parental, F1, F2, and backcross generations of a cross between the heat-tolerant snap bean breeding line `Cornell 503' and the heat-sensitive wax bean cultivar Majestic were grown in a high-temperature controlled environment (32 °C day/28 °C night), initiated prior to anthesis and continued through plant senescence. During flowering, individual plants of all generations were visually rated and scored for extent of abscission of reproductive organs. The distribution of abscission scores in segregating generations (F2 and backcrosses) indicated that a high rate of abscission in response to heat stress was controlled by a single recessive gene from `Majestic'. Abscission of reproductive organs is the primary determinant of yield under heat stress in many annual grain legumes; this is the first known report of single gene control of this reaction in common bean or similar legumes. Generation means analysis indicated that genetic variation among generations for pod number under heat stress was best explained by a six-parameter model that includes nonallelic interaction terms, perhaps the result of the hypothetical abscission gene interacting with other genes for pod number in the populations. A simple additive/dominance model accounted for genetic variance for seeds per pod. Dominance [h] and epistatic dominance × dominance [l] genetic parameters for yield components under high temperatures were the largest in magnitude. Results suggest `Cornell 503' can improve heat tolerance in sensitive cultivars, and heat tolerance in common bean may be influenced by major genes.

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Claudia Cunha, Muhammet Tonguç, and Phillip D. Griffiths

Chloroplast DNA (cpDNA) was used to identify polymorphisms between crucifer species using the polymerase chain reaction-random fragment-length polymorphism (PCR-RFLP) technique. Ten primer pairs based on cpDNA gene sequences were used to amplify cpDNA fragments in Brassica oleracea L., B. rapa L., B. nigra (L.) Koch, B. napus L., B. carinata Braun, B. juncea (L.) Czern, and Raphanus sativus L. accessions. Amplified DNA sequences were then digested using 11 restriction enzymes to identify polymorphisms between the 7 species. Of the 110 combinations, 38 generated polymorphisms that discriminated one or more of the species. Genotyping of these polymorphisms in 10 accessions of each of the diploid species (B. oleracea, B. nigra, B. rapa and R. sativus) did not reveal segregating polymorphisms among accessions within species, indicating that they can be used to help determine species identity. Ten accessions of each of the amphidiploids B. napus, B. carinata and B. juncea were genotyped to infer their maternal ancestry. The diploid source of cpDNA in B. carinata was B. nigra in all accessions tested and B. rapa for nine of ten B. juncea accessions tested. Two B. napus accessions amplified polymorphisms shared with B. rapa, and eight accessions produced unique polymorphisms from neither B. rapa, B. oleracea or B. nigra. The polymorphisms identified in this study can be used to help confirm identity of the diploid crucifer species for taxonomic and conservation studies.

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Phillip D. Griffiths, Laura Fredrick Marek, and Larry D. Robertson

Black rot, caused by Xanthomonas campestris pv. campestris (Pam.) Dawson (Xcc), is a serious disease of vegetable crucifers worldwide. The USDA NC-7 and NE-9 regional PI stations maintain vegetable, mustard, and oilseed crucifers, of which 4084 accessions were available for testing, representing 23 genera and 125 species. These accessions were evaluated for resistance to black rot after wound inoculation with race 1 of the pathogen. Accessions that were symptomless for race 1 of Xcc were replanted and inoculated with race 4 of the pathogen to identify accessions with resistance to both races. Symptomless responses were observed in 362 accessions of the mustard species (Brassica juncea, Brassica carinata, and Brassica nigra), in particular, B. juncea for which 353 of the 389 accessions tested were symptomless. Resistance was identified in five accessions of B. carinata out of 63 evaluated (PI 193460, PI 193959, PI 194254, PI 280230, PI 633077) determined by repeated symptomless responses after inoculation and four accessions of B. nigra from the 83 evaluated (PI 197401, A 25399, A 25401, PI 458981). Five accessions of Brassica rapa (PI 633154, A9285, PI 340208, PI 597831, PI 173847) were identified, which represent promising new sources of resistance to Xcc. Incomplete resistance was identified in an accession of Eruca sativa (PI 633207), an accession of Lepidium spp. (PI 633265), an accession of Sinapis arvensis (PI 296079), and two accessions of B. napus (PI 469733 and PI 469828). These identified accessions represent germplasm that can be used in breeding for resistance to Xcc in vegetable crucifers through interspecific crossing.