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Phlox is an important genus of herbaceous ornamental plants previously targeted for germplasm development, characterization, and enhancement by the U.S. Department of Agriculture, National Plant Germplasm System. Among Phlox in cultivation, Phlox paniculata is the most widely grown and intensively bred species, but little is known about variation in genome size and ploidy of this species or of related taxa that may be used for germplasm enhancement. The objective of this study was to assess cytotype variation in a diverse collection of cultivars and wild germplasm of P. paniculata (subsection Paniculatae) and of related taxa in subsections Paniculatae and Phlox. The collection included 138 accessions from seven species and two interspecific hybrids. Flow cytometry was used to estimate holoploid (2C) genome sizes and to infer ploidy levels. Chromosome counts were made to calibrate ploidy with genome size for a subset of taxa. Most cultivars were diploid (2n = 2x = 14) and had mean genome sizes that did not vary between subsections Paniculatae (14.33 pg) and Phlox (14.23 pg) although size variation was greater among cultivars within subsection Phlox. Triploid cultivars of P. paniculata, with a mean genome size of 21.36 pg and mitotic chromosome counts of 2n = 3x = 21, were identified. Such triploids suggests previous interploid hybridization within this taxon. Five tetraploid (2n = 4x = 28) cultivars were found in subsection Phlox; all were selections of P. glaberrima ssp. triflora, and had a mean genome size of 25.44 pg; chromosome counts in one of these confirmed they were tetraploid. The putative hybrid Phlox Suffruticosa Group ‘Miss Lingard’ showed an intermediate genome size of 21.21 pg supporting a triploid, hybrid origin of this taxon. Mean 2C genome sizes among wild-collected accessions were similar to values reported for cultivars (Paniculatae = 14.59 pg, Phlox = 14.23 pg), but taxa in subsection Phlox exhibited greater variation that included two tetraploids identified among wild-collected accessions; one, of P. pulchra, had a mean genome size of 26.17 pg, representing the first report of polyploidy in the taxon. This is the first report on genome size for the majority of species in the study. Although genome size could not be used to differentiate taxa in subsections Paniculatae and Phlox, the data provide further insights into cytotype variation of Phlox germplasm useful for plant breeders and systematists.

Foliar spray applications of plant growth regulators (PGRs) benzyladenine (BA) and dikegulac sodium (DS) on herbaceous perennial liners and transplants were evaluated to determine effects on branching and quality. PGRs were applied three times (main plot): liner (24 h after removal from mist), post-transplant (5 to 7 days after transplant), or both (applications at liner and post-transplant). PGRs (subplots) were applied at concentrations of 400, 800, or 1600 mg·L DS; 600 mg·L BA; or a combination of 400 mg·L DS and 600 mg·L BA (DS/BA). All studies included an untreated control. Sedum spectabile ‘Autumn Joy’ treated with PGRs at both application times had a 100% increase in branching compared with either single application time. Applied post-transplant, 800 mg·L DS, BA, or DS/BA increased branching. Applied at both times, all DS concentrations or BA tripled the number of lateral branches per plant, whereas DS/BA resulted in a 4-fold increase in number of branches. Sedum treated with 1600 mg·L DS at liner, post-transplant, or both application times was shorter than controls; plants sprayed with 1600 mg·L DS at both application times were stunted. Phytotoxicity (yellow, narrow leaves) was present in finished plants treated with 800 or 1600 mg·L DS or DS/BA post-transplant or with 1600 mg·L DS applied both times. Gaillardia aristata ‘Gallo Red’ treated with PGRs at both application times had increased branching compared with plants subjected to a single application. Number of branches was increased by liner application of 400 mg·L DS, post-transplant applications of DS/BA, or applications both times of BA or DS/BA, whereas applications both times of 1600 mg·L DS decreased branching and caused stunting and chlorotic foliage. In application as liners or at both times, all PGRs except 400 mg·L DS caused 16to 33day delays in flowering. Branching of Phlox paniculata ‘Bright Eyes’ was unaffected by application time. Only Phlox treated with BA or 1600 mg·L DS at both application times had increased branches, although plants treated with 1600 mg·L DS were shorter than controls and had phytotoxicity in the form of narrow, yellow leaves. Nepeta racemosa ‘Walker’s Low’ treated with PGRs post-transplant or both application times had more branches than plants treated with PGRs once as liners. Number of branches was increased with BA or 800 mg·L DS applied post-transplant or 1600 mg·L DS, BA, or DS/BA applied both times, but the plants treated with 1600 mg·L DS were stunted and had yellow leaves. Neither BA nor DS affected branching in Delosperma ‘Table Mountain’. Application time did not affect branching in Achillea ‘Moonshine’; only both applications of BA increased the number of branches in Achillea, whereas either single application of 1600 mg·L DS or both applications of 800 or 1600 mg·L DS caused phytotoxicity and stunting. Chemical names used: N-(phenylmethyl)-1Hpurine-6-amine [benzyladenine (BA)], sodium 2,3:4,6-bis-O-(1-methylethylidene)-a-Lxylo-2-hexulofuranosonate (DS) To produce uniform plants, growers of herbaceous perennials control plant growth by using both cultural controls, including plant nutrition, light, and temperature, and chemical controls, i.e., PGRs (Albrecht and Tayama, 1992; Whipker et al., 2006). PGRs can increase branching and improve plant architecture in plants by releasing apical dominance and allowing dormant lateral buds to expand, which results in fuller, well-branched plants (Cline, 1991). However, using PGRs can be challenging, because plants respond in different ways to PGRs depending on species, cultivar, growing conditions, and the PGR used (Gent and McAvoy, 2000). BA, a synthetic cytokinin, is a PGR that increases branching when sprayed on ornamental plants (Latimer and Freeborn, 2008; Martin and Singletary, 1999). The number of branches doubled in Echinacea (L.) Moench ‘White Swan’ and ‘Double Decker’ treated with foliar sprays of 300 or 600 mg·L BA at 2 weeks after transplanting to quart-sized (1.1-L) pots (Latimer and Freeborn, 2008). Of 18 juvenile perennials treated with BA as a foliar spray at rates of 1000, 2000, or 4000 mg·L, 89% had an increase in the number of lateral branches, although no additional branching was seen in the higher two rates as compared with 1000 mg·L (Martin and Singletary, 1999). However, in this study, plants treated with higher rates were more compact and had decreased leaf size. DS has been used as a chemical pinch to prevent elongation in woody plants as well as to increase branching in herbaceous plants. Banko and Stefani (1995) assessed the use of DS on container-grown woody ornamentals. Of the nine species they studied, DS effectively controlled growth in five, producing plants that were more compact and appeared to have denser shoot growth. When 3-yearold field-grown Lavandula·intermedia Emeric ex Loisel. ‘Twickle Purple’ plants were treated with 0.5%, 1.0%, or 2.0% Atrinal (PBI Gordon, Kansas City, MO; containing 20% DS), the numbers of vegetative shoots and inflorescences and oil yield increased significantly compared with controls (Porter and Shaw, 1983). Branching of Gaillardia aristata Pursh. ‘Gallo Yellow’ was increased by foliar sprays of 400 or 800 mg·L DS (Latimer and Freeborn, 2010). However, not all species are responsive to treatment with DS; as referenced previously, Banko and Stefani (1995) found five species responded to DS out of nine studied. PGRs are often applied after transplant, when plants are in active growth and have started to expand. Treating liners (rooted cuttings) before transplant can have significant benefits for growers including reduced time of application and reduced chemical cost. Grossman et al. (2012) reported increases in branching in four of five herbaceous perennials treated with BA before transplant; of these, only two had persistent increases in branching after plants grew to finished size. The question of when to apply growth regulators to achieve increased branching in finished plants is an important one for growers who want to grow out liners to finished, marketable plants. The objective of this study is to examine the effects of application time of DS, BA, or a combination of the two on the growth and branching of herbaceous perennial plants during liner production and grow out of finished plants. Materials and Methods Six herbaceous perennials were studied: Sedum spectabile Boreau ‘Autumn Joy’, Gaillardia aristata Pursh ‘Gallo Red’, Phlox paniculata L. ‘Bright Eyes’, Nepeta racemosa Received for publication 26 Dec. 2012. Accepted for publication 7 May 2013. Partial funding for this project was provided by Fine Americas, Inc. and the Virginia Nursery and Landscape Association. We thank Conrad Fafard, Inc., and Aris Horticulture, Inc. for their support of floriculture research at Virginia Tech. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by Virginia Tech and does not imply its approval to the exclusion of other products or vendors that also may be suitable. To whom reprint requests should be addressed; e-mail jlatime@vt.edu. HORTSCIENCE VOL. 48(7) JULY 2013 875 GROWTH REGULATORS

Gaura lindheimeri Engelm. & Gray ‘Siskiyou Pink’ (gaura) and Phlox paniculata L. ‘David’ (garden phlox) were grown for 5 weeks in substrates irrigated at volumetric water contents (Θ) of 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, or 0.45 m3·m−3 using a capacitance sensor-controlled irrigation system. Volumetric water contents of the substrate measured by the capacitance sensors controlling irrigation were correlated with measurements with a separate handheld meter (r 2 = 0.83) and with volumetric water content set points throughout the study (r 2 > 0.98). Only 3.8 (at an irrigation set point of 0.10 m3·m−3) to 53 L (0.45 m3·m−3) of water was used to irrigate gaura and phlox and 0 to 7.74 L of this water leached out of the substrates. Significant leaching occurred only at Θ set points of 0.40, or 0.45 m3·m−3. Gaura had shorter and fewer branches and reduced dry weight when grown at lower volumetric water contents, but plants irrigated at set points above 0.25 m3·m−3 were large enough to be marketable. Gaura may be grown with capacitance sensor-automated irrigation using water efficiently and minimizing or eliminating leachate and thus nonpoint source pollution.

Meristem culture and/or thermotherapy were used for virus elimination from ornamental Phlox paniculata L. (`Blue Boy', `Orange perfection' and `Starfire') mother plants. Shoot tip, leaf, node and flower ovary explants collected from greenhouse-maintained virus free plants were cultured in vitro for shoot initiation. Adventitious shoot initiation was observed on Murashige and Skoog (MS) medium containing the cytokinin BA with or without the auxin NAA. The addition of 0.4 mg·L-1 thiamine, 0.4 mg·L-1 folic acid, and 40 mg·L-1 adenine sulfate to the MS medium did not improve the regeneration rate. Multiplication and rooting were genotype dependent. Blue Boy and Orange Perfection cultivars regenerated the maximum number of shoots from leaf explants. `Blue Boy' leaf explants from in vitro plants had a lower regeneration rate than explants from greenhouse plants. Cultivar `Starfire' had the highest shoot formation with open flower ovary explants and failed to regenerate from leaf explants. In vitro rooting of adventitious shoots in the presence of auxins (IAA, NAA, or IBA) with or without BA was less effective than ex vitro rooting. Chemical names used: 6-benzyladenine (BA); indole-acetic acid (IAA); indole-3-butyric acid (IBA); α-naphthaleneacetic acid (NAA).

Although silicon is not an essential element, it is taken up by plants but is rarely quantified. Therefore, this study quantified the silicon concentration in 10 commonly grown horticultural plants including meadow sage (Salvia ×sylvestris), tickseed (Coreopsis verticillata), garden phlox (Phlox paniculata), New England aster (Symphyotrichum novae-angliae), Chinese astilbe (Astilbe chinensis), coral flower (Heuchera hybrid), garden zinnia (Zinnia elegans), French marigold (Tagetes patula), sweet basil (Basil spp.), and rosemary (Rosmarinus officinalis) using a plant alkaline fusion technique, which involved dry-ashing plant tissue samples and measuring color development with a spectrophotometer. Both zinnia and aster accumulated substantially more silicon from the municipal water source and growing medium (5365 and 4797 mg·kg−1 silicon, respectively) than the other plants evaluated, which had concentrations less than 2500 mg·kg−1 silicon. This study is just one of a few in which the silicon concentration in various horticultural plants has been quantified. Consequently, this may lead to better understanding those plants that will or will not benefit from applications of silicon-based fertilizers to promote cold-hardiness and/or plant resistance to fungal pathogens and insect pests.

In two studies, rooted cuttings of Phlox paniculata L. ‘Ice Cap’ (summer phlox) were cooled for 0, 4, 8, 12, or 16 weeks. Plants were forced in a glasshouse averaging 18 °C nights under extended-day and night-interruption lighting from incandescent lamps providing a minimum of 14 μmol·s−1·m−2 at plant level or continuous lighting from metal halide (HID) lamps providing a minimum of 400 μmol·s−1·m−2 at plant level. The influence of cooling duration on forcing days to flower, flowering stem counts, and flowering stem length was evaluated. Cooling plants promoted longer stems, higher flowering stem yields, and decreased days to flower when forced under long days provided by incandescent lamps, but when forced under HID lamps, days to flower for cooled plants were similar to those of noncooled plants regardless of cooling duration. Phlox forced in extended daylighting flowered in fewer days, had longer stems, and produced more flowering stems than those forced in night-interruption lighting. With continuous HID lighting, stem lengths and stems harvested per plant increased in a linear manner as cooling increased from zero to 16 weeks. Stem lengths ranged from 63.6 cm for noncooled plants to 96.3 cm for those receiving 16 weeks cooling, and flowering stem yields ranged from seven stems per plant for noncooled plants to 13 for those cooled 16 weeks. Phlox forced under HID lights flowered in substantially fewer days and had longer stems than those forced under incandescent lamps.

Received for publication 21 Sept. 2005. Accepted for publication 1 Dec. 2005. Approved for publication by the Director of the Louisiana Agricultural Experiment Station as manuscript 05-38-0593. To whom reprint requests should be addressed; e-mail gholcomb@agcenter.lsu.edu. A Disease of Phlox paniculata Caused by Alfalfa Mosaic Virus Gordon E. Holcomb, Rodrigo A. Valverde, and Dina L. Gutierrez Department of Plant Pathology and Crop Physiology, Louisiana State University AgCenter, Baton Rouge, LA 70803-1720

Phlox paniculata Lyon ex Pursh `Eva Cullum' plants were grown under seven photoperiods following 0 or 15 weeks of 5 °C to determine the effects of photoperiod and cold treatment on flowering. Photoperiods were a 9-hour day extended with incandescent lamps to 10, 12, 13, 14, 16, or 24 hours; an additional treatment was a 9-hour day with a 4-hour night interruption (NI). Noncooled plants remained vegetative under photoperiods ≤13 hours; as the photoperiod increased from 14 to 24 hours, flowering percentage increased from 20 to 89. Flowering of noncooled plants took 73 to 93 days. Flowering percentage was 19, 50, or 100 when cooled plants were held under photoperiods of 10, 12, or ≥13 hours or NI, respectively. Time to flower in cooled plants progressively decreased from 114 to 64 days as the photoperiod increased from 10 to 24 hours. Reproductive cooled plants had at least three times more flowers, were at least 50% taller, were more vigorous, and developed seven or eight more nodes than did noncooled plants. Photoperiod had no effect on height of flowering plants.