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James E. Simon, W. Dennis Scott, and Gerald Wilcox

A study was initiated at the Southwest Purdue Ag Center to demonstrate the effect of transplant age and transplanting date on the scheduling of melon harvests. Muskmelon (Cucumis melo cv. Superstar) was seeded into #38 growing trays with Jiffy-mix media. Seeding dates were such that 14 and 21 day old seedlings were transplanted April 25, May 9, 17 and 24. The plants were grown on black plastic with trickle irrigation Marketable fruit were harvested starting on June 28 and continuing through August 12. Neither transplant age or date had a significant effect on the number of fruit harvested or on total yield. However, each transplant date showed a distinctive harvest peak beginning June 30 for the April 26 transplant then June 7, 14 and 21 for each successive transplant date.

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Zongrang Liu, Ralph Scorza, Jean-Michel Hily, Simon W. Scott, and Delano James

Prunus L. fruit production is seriously affected by several predominant viruses. The development of new cultivars resistant to these viruses is challenging but highly desired by breeders and growers. We report a posttranscriptional gene silencing-based approach for engineering multivirus resistance in plants. A single chimeric transgene, PTRAP6, was created by the fusion of 400 to 500-base pair (bp) gene fragments from six major Prunus fruit viruses, including american plum line pattern virus, peach mosaic virus, plum pox virus (PPV), prune dwarf virus (PDV), prunus necrotic ringspot virus, and tomato ringspot virus (ToRSV). Both strands of PTRAP6 were found being transcribed as an ≈2.5-kilobp transcript in planta without splicing interruption. To induce gene silencing/virus resistance, we placed two copies of PTRAP6 in an inverted repeat under the control of the cauliflower mosaic virus 35S promoter and separated by an intron spacer fragment to create PTRAP6i. Inoculation of the resulting transgenic Nicotiana benthamiana Domin. plants revealed that 12 of 28 R0 PTRAP6i transgenic lines (43%) were resistant to ToRSV ranging from mild symptoms to symptom-free phenotypes. Detailed analysis of two of three highly resistant homozygous R3 generation lines demonstrated that they were resistant to all three viruses tested, including PDV, PPV, and ToRSV. The remaining three viruses targeted by PTRAP6i were either unavailable for this study or were unable to systemically infect N. benthamiana. Transgene-wide and -specific small interfering RNA species were detected along with disappearance of transgene transcript in the resistant lines, indicating that posttranscriptional gene silencing underlies the mechanism of resistance. This work presents evidence that PTRAP6i is able to confer gene silencing-based resistance to multiple Prunus fruit viruses.

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Philip G. Gibson, Gregory L. Reighard, Simon W. Scott, and David R. Ouellette

Delaying bloom to reduce spring frost risk and reducing labor costs by increasing orchard efficiency are important goals of peach producers. At the Musser Fruit Research Center near Clemson, S.C., `Coronet' peach trees were inoculated with Peach Latent Mosaic Viroid (PLMVd) to induce bloom delay and reduce shoot vigor. Trees were grown in a high-density, Y-trained orchard system to determine the potential benefit of reduced shoot vigor on labor efficiency in summer pruning operations. In Aug. 1997, `Ta Tao 5' buds were grafted onto the scaffolds of 2-year-old `Coronet' peach trees to transmit PLMVd. Transmission was confirmed by dot-blot on N+ nylon membranes using cRNA probes. Bloom was not delayed in the following year, Spring 1998, but bloom was delayed 7 days in Spring 1999. Yields were unaffected in 1998, but the total fruit weight produced on PLMVd-treated trees was significantly less in the 1999 harvest. Individual fruit weight, firmness, and color were improved on the treated trees. Shoot vigor was reduced on the PLMVd treated trees in the summers of 1998 and 1999, resulting in a significant reduction in pruning time and pruning weights. Light penetration was significantly increased in the treated trees when compared to the untreated controls in the summers of 1998 and 1999. Fruiting shoot length and the number of fruit per shoot was unaffected by the PLMVd presence. The trunk cross-sectional area was significantly less on the treated trees when compared to the untreated controls after each year of growth. Autumn defoliation occurred earlier on the treated trees in Fall 1998 and Fall 1999. The manipulation of peach tree growth and development was accomplished using graft transmissible agents as PLMVd induced several beneficial growth and developmental modifications in established peach trees.