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R.O. Hampton

Cowpea cultivars Early Ramshorn and Dixie Queen, reported to be resistant to cucumber mosaic virus (CMV) by Sinclair and Walker in 1955, were reexamined for the existence of individual CMV-resistant genotypes within seedling populations. CMV-inoculated populations of these two cultivars became CMV-infected at rates of 60% and 80%, respectively, as determined by DAS-ELISA. CMV-free plants were grown to maturity, seeds from these sources were planted, and second-generation seedlings were inoculated again with CMV. Rates at which these seedlings became CMV-infected were 10% and 24%, respectively. The complementary 90% and 76% of these populations remained completely free of ELISA-detectable CMV and were saved to maturity. Evaluations of third-generation populations are being performed. Twenty additional cultivars and breeding lines also are being evaluated. Seedlings of 14 of the 22 cultivars/lines became 95% to 100% infected when inoculated with CMV. Results from eight of the 22 suggested that seedling populations contain CMV-susceptible and -resistant plants. Near isogeneic sublines of susceptible and resistant plants per cultivar/line are being conveyed to cowpea breeders for genetic analyses and breeding purposes.

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A. Smigocki and F. Hammerschlag

Immature `Redhaven' peach (Prunus persica L. Batsch) embryos were infected with Agrobacterium tumefaciens strain tms328::Tn5 carrying the functional cytokinin gene. Shoots were regenerated from callus grown on MS medium without added phytohormones and subsequently rooted on half-strength MS medium with 2.8 -naphthaleneacetic acid. These plants exhibited an increased frequency of branching in vitro. Low levels of cytokinin gene transcripts were detected in these cells by Northern analysis, and using an ELISA assay, the cytokinins zeatin and zeatinriboside were determined to be on the average 30-fold higher. From these results, the expression of the cytokinin gene appears to promote growth of cells in the absence of phytohormones thus serving as a marker for transformation and a promoter of morphogenesis without a 2,4-dichlorophenoxyacetic acid inductive step.

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Clarice J. Coyne and Fred J. Muehlbauer

Aphanomyces root rot of pea (Pisum sativum) in many pea-growing regions. The genetic resistance to this fungal pathogen is quantitatively inherited and confers levels of tolerance to the disease. Genetic gains in selection have been hampered by the difficulty of differentiating the highly tolerant from tolerant lines in segregating populations. Reporter gene systems have been useful in studying genetic resistance to other soil-borne pathogens. We have transformed an isolate of Aphanomyces euteiches, the causal pathogen, with a reporter gene β-glucuronidase (GUS) and a selectable marker gene, hygromycin phosphotransferase or neomycin phosphotransferase. The transformed lines constitutively express GUS as determined fluorimetrically by measuring the conversion of 4-methylumbelliferyl glucuronide to 4-methlyumbelliferone. The efficacy of this GUS enzyme assay will be compared with an indirect enzyme linked immunosorbant assay (ELISA) and visual disease development ratings in inoculated seedlings of three populations recombinant inbred lines of pea segregating for tolerance.

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S.J. Scott, M. Stevens, and R.C. Gergerich

Three methods to inoculate Lycopersicon esculentum 'VF Pink' seedlings with tomato spotted wilt virus (TSWV) were compared. Treatments were 1) two inoculations by hand (rubbing leaves with a sterile cotton swab), 2) a single inoculation using a paint sprayer at 3.56 × 105 N· m-2, and 3) two spray inoculations. All three methods were effective (>95% infection) under moderate temperatures in the spring, but hand inoculation was not effective under hot conditions in the summer. In another experiment, spray inoculation was used to compare effects of light intensity and the leaf inoculated on susceptibility of L.. hirsutum PI 127826, L. pimpinellifoliom LA 1580 and `VF Pink' to TSWV isolate 85-9. All three genotypes were susceptible under full sun and 60% shade cloth in the greenhouse. Inoculation of youngest leaves produced the highest virus titer. Background optical density for noninoculated plants differed between lower and upper leaves in the ELISA assay.

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Oyette L. Chambliss, Arthur G. Hunter, and Richard O. Hampton

Germplasm accessions and advanced lines were evaluated for seed transmission of the seed borne viruses, cucumber mosaic (CMV), cowpea severe mosaic (CSMV), and blackeye cowpea mosaic (B1CMV). Seed samples from 822 field plots (274 out of 300 accessions in 3 replications) which had been evaluated for insect resistance in 1992 were planted in the greenhouse. Mosaic virus symptoms had been apparent throughout the 1992 field planting. Evaluation for mosaic symptoms was done at the seedling stage in the greenhouse and 89 entries exhibited seed borne virus symptoms in one or more plants. Of these, 78 were shown by ELISA (enzyme linked immunosorbant assay, direct antigen coating method) to contain at least one of the seed borne viruses for which we were evaluating.

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Jeff Taylor, Eric Simonne, and John Murphy

Because cucumber mosaic virus (CMV) affects southernpea (Vigna unguiculata) grown in the southeast, plants were preconditioned with different nutrient solutions from germination to the flowering stage, 24 DAT (days after transplanting) and rub inoculated with CMV. Symptoms were observed at a rate of 1/5, 1/5, 4/5, and 4/5 (observed infected plants/plants infected) in the Al, NH4, NO3, and Na treatments, respectively. At 67 DAT, ELISA detected CMV at a rate of 5/5, 5/5, 4/5, and 4/5 (detected infection/plants infected) of the Al, NH4, NO3, and Na treatments, respectively. The interaction of inoculation and preconditioning was nonsignificant for fresh or dry weight (P > 0.10); however, nutritional preconditioning significantly (P < 0.01) affected the fresh and dry weight. These preliminary data suggest that nutritional preconditioning affects southernpea plants' reaction to CMV.

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Clare J. Coyne and Shawn A. Mehlenbacher

U.S. hazelnut production, concentrated in Oregon, is threatened by eastern filbert blight and all currently grown cultivars are susceptible. Resistant cultivars offer the best control method. Field screening for resistance takes 2 years for symptom expression. The goal of this study was to develop a rapid and reliable screen; to confirm that resistance in `Gasaway' is conferred by a single dominant gene: and to investigate inheritance in seedlings of the resistant cultivar `Gem'. Nine controlled crosses made in 1987 and 1988 were screened in the greenhouse in 1992 and 1993. Three trees of each genotype were inoculated and scored for presence or absence of the fungus using either stained tissue sections or ELISA within 6 to 12 months. Progenies of `Vancouver Resistant' parents (resistant progeny of `Gasaway') segregated 1 resistant: 1 susceptible and from resistant × resistant parents segregated 3 resistant:1 susceptible in agreement with the single gene hypothesis. Seedlings of `Gem' were all susceptible.

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Robert K. Stevenson and Karen K. Tanino

Dogwood (Cornus sericea L.) clonal ecotypes from northern latitudes (Northwest Territories “NWT”) and more southern latitudes (Massachusetts, Utah, and Chalk River, Ont.) were allowed to acclimate naturally in a shade house (52°07') beginning in early July and continuing through the middle of October. The NWT ecotype began to attain vegetative maturity by the second week of September, whereas the southern ecotypes did not attain any significant degree of VM before the first lethal frost.

Defoliation tests in controlled environment chambers paralleled shade house results. Under VM-inducing conditions (20/15°C, 8h), NWT ecotype attained VM after 40-50 days. Conversely, after 80 days Utah ecotype had not attained full VM.

Chilling requirement will be compared among ecotypes and ABA levels will be quantified using HPLC and ELISA systems. The results will be compared with date of VM attainment, subsequent freezing tolerance and satisfaction of the chilling requirement.

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G.R. McEachern, Jim Kamas, Doug Cook, and Jerral Johnson

Since 1977, it was believed the grape strain of Xylella fastidiosa, Pierce's Disease (PD), was limited to sites receiving less than 800 hours of winter chilling below 45 °F. Warm winters since 1992–93 resulted in numerous PD-positive vineyards in central Texas which were previously nonaffected. Vine mortality ranges from minimal to over 80% dead vines. A Davis Mountains site receiving over 1,000 hours was also severely infected in 1996. The last severe winter in Texas was 1989–90; therefore, warm winters could be contributing. This climatic change could have affected vines, vectors, bacteria, and/or hosts. Recent work based on a study of 20 PD samples, 11 from Texas and 9 from other states, including California and Florida, indicates that the samples of PD grape strain of Xylella fastidiosa are clonally related. ELISA tests failed to identify PD from 1994–96; therefore, a sensitive REP-PCR test is needed before vine, bacteria, vector, and host management strategies can be developed.

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Setapong Lekawatana and Richard A. Criley

Inflorescence abortion in heliconia contributes to an economic loss to growers. In an effort to determine the cause, we manipulated temperature, daylength and light intensity. Plants of Heliconia stricta cv. Dwarf Jamaican were grown in 4 day/night temperature regimes (15/10, 20/15, 25/20 and 30/25°C) under 14 hr daylength. In a separate experiment, plants were grown in full sun, 60% and 80% shade. Both experiments had been conducted after inflorescences were induced (4 weeks of short days). Apical meristems were dissected weekly to follow inflorescence development. Leaf abscisic acid level was detected by an indirect ELISA. Significantly more inflorescences were aborted in plants grown under high temperature regimes than in plants grown under low temperature regimes and under different light intensity. Abscisic acid concentration increased in heliconia leaves under regimes that induced inflorescence abortion. The results could provide a mean to improve heliconia inflorescence production.