Search Results

You are looking at 1 - 10 of 15 items for

  • Author or Editor: Michael A. Dirr x
Clear All Modify Search
Free access

Jeffrey A. Adkins and Michael A. Dirr

Floral induction of 10 Hydrangea macrophylla (Thunb.) Ser. cultivars (`All Summer Beauty', `Dooley', `Endless Summer', `Générale Vicomtesse de Vibraye', `Lilacina', `Mariesii Variegata', `Mme. Emile Mouillère', `Nikko Blue', `Penny Mac', and `Veitchii') was evaluated to determine the remontant flowering potential. Cultivars exhibiting superior flowering potential could facilitate year-round production of florist s hydrangea and expand the geographic adaptation of hydrangeas in the landscape. Plants were tested under either 8-hour inductive short-day (SD) or 24-hour noninductive extended-day (ED) photoperiods at 24 ± 2 °C. Floral induction of H. macrophylla occurred under both SD and ED but was more rapid under SD than under ED. Shoot length and total number of nodes were significantly different for cultivars, photoperiods, and harvest dates. Significant differences in the floral initiation and development were observed among cultivars. `Penny Mac', `Endless Summer', `Lilacina', `Mme. Emile Mouillère', and `Nikko Blue' displayed significantly greater floral meristem development under SD than all other cultivars. Over the nine-week period, only `Dooley', `Endless Summer', and `Penny Mac' floral development was affected significantly greater by SD photoperiods than ED photoperiods. All others cultivars showed similar floral development under SD and ED. Stage of meristem development was highly correlated with number of nodes, but not with shoot length.

Free access

Orville M. Lindstrom and Michael A. Dirr

Cold hardiness levels of six cultivars of Chinese elm (Ulmus parvifolia Jacq.), `Select 380', `Orange Ribbon 740', `Emerald Isle', `Emerald Vase', `Drake', and `King's Choice', were determined over eight sample dates from 31 Aug. 1988 to 16 May 1989 and for `Emerald Vase' and `Drake', over three dates from 14 Feb. 1988 to 25 Apr. 1988. All cultivars tested achieved a maximum cold hardiness in December and January of – 21 to – 24C, except `King's Choice', which survived exposure to at least – 30C. `Emerald Isle' and `Emerald Vase' acclimated earlier (both – 9C on 31 Aug.) and reacclimated later (– 6 and – 9C, respectively, on 16 May) than other cultivars tested. `Emerald Vase' and `Drake' exhibited similar cold hardiness levels over the two years tested.

Free access

Andrea L. Southworth and Michael A. Dirr

Stem cuttings from a prostrate clone of Cephalotaxus harringtonia (Forbes) K. Koch (Japanese plum yew) were taken monthly from Sept. 1994 through Aug. 1995, treated with K-IBA at 0 or 10,000 mg·liter–1, placed in a greenhouse under intermittent mist, and evaluated after 16 weeks. Cuttings taken from December to February and treated with K-IBA averaged 85% rooting, 10 roots per cutting, and a total root length of 35 cm. The next highest rooting percentages were for cuttings taken from March to May; poorest rooting occurred for cuttings taken from June to August and September to November, regardless of K-IBA application. Chemical name used: K-indole-3-butyric acid (K-IBA).

Free access

Donglin Zhang, Michael A. Dirr and Robert A. Price

Cephalotaxus species are needle evergreens offering the aesthetic qualities of Taxus, yew, yet are heat- and drought-tolerant, sun- and shade-adaptable, and resist deer browsing. They are adaptable to nursery and garden cultivation in USDA hardiness zones (5)6–9. Unfortunately, the various species are frequently confused in the American nursery trade due to their extreme similarity in morphology. Recently, molecular data have been widely applied in the taxonomic studies, especially DNA sequencing. The chloroplast gene rbcL of Cephalotaxus has been sequenced for determining species relationships. The preliminary results show that C. oliveri Mast. has 10 base changes from C. drupacea Sieb. et Zucc., while only one base difference occurred between C. drupacea and C. harringtonia (Forbes) Koch. There are between one and 10 base substitutions among C. fortunei Hooker, C. koreana Nakai, and C. sinensis (Rehd. et Wils.) Li. Compared with other closely related conifers, Cephalotaxus has a substantial number of differences among species except between C. drupacea and C. harringtonia, which may not be distinct species. Detailed data relative to gene sequencing, growth morphology, and horticultural characteristics should lead to correct identification of species and great horticultural uses. Furthermore, the method of rbcL sequence can be applied to distinguish other morphologically homogeneous ornamental plants.

Free access

Michael A. Dirr, Jeffrey A. Adkins and Richard T. Olsen

Free access

Donglin Zhang, Michael A. Dirr and Robert A. Price

The correct identification of horticultural taxa becomes more and more important for intellectual property protection and economic reasons. Traditionally, morphological characteristics have been used to differentiate among the horticultural taxa. However, the morphological characteristics may vary with plant age, cultural conditions, and climate. Modern technologies, such as DNA markers, are now employed in the identification of horticultural taxa. Currently, technologies of DNA sequencing (gene sequences) and DNA fingerprinting (RAPD, RFLP, SSR, and AFLP) are available for distinguishing among horticultural taxa. The literature and our personal experience indicate that the application of each technique depends on the taxon and ultimate goal for the research. DNA sequencing of a variety of nuclear or chloroplast encoded genes or intergenic spacers (rbcL, ndhF, matK, ITS) can be applied to distinguish different species. All DNA fingerprinting technologies can be used to classify infraspecies taxa. AFLP (the most modern technique) is the better and more-reliable to identify taxa subordinate to the species, while RAPDs can be employed in clonal or individual identification. Techniques of RFLP and SSR lie between AFLP and RAPD in their effectiveness to delineate taxa. Mechanics, laboratory procedures, and inherent difficulties of each technique will be briefly discussed. Application of the above technologies to the classification of Cephalo taxus will be discussed in concert with the morphological and horticultural characteristics. Future classification and identification of horticultural taxa should combine DNA technology and standard morphological markers.

Free access

Donglin Zhang, Michael A. Dirr and Robert A. Price

Cephalotaxus Sieb. and Zucc. (plum yew) species and cultivars have become popular because of their sun and shade tolerance, resistance to deer browsing, disease and insect tolerance, and cold and heat adaptability. Unfortunately, the nomenclature and classification in the literature and nursery trade are confusing due to their extreme similarity in morphology. In this study, amplified fragment-length polymorphism (AFLP) markers were used to discriminate taxa and evaluate genetic differences among 90 Cephalotaxus accessions. A total of 403 useful markers between 75 and 500 base pairs (bps) was generated from three primer-pair combinations. Cluster analysis showed that the 90 accessions can be classified as four species, C. oliveri Mast., C. fortunei Hooker, C. harringtonia (Forbes) Koch., and C. ×sinensis (a hybrid species); four varieties, C. fortunei var. alpina Li, C. harringtonia var. koreana (Nakai) Rehd., C. harringtonia var. nana (Nakai) Hornibr., and C. harringtonia var. wilsoniana (Hayata) Kitamura; and eight cultivars. Suggested names are provided for mislabeled or misidentified taxa. The Cephalotaxus AFLP data serve as a guide to researchers and growers for identification and genetic differences of a taxon, and a model to establish a cultivar library against which later introductions or problematic collections can be cross-referenced.

Free access

Jeffrey H. Gillman, Michael A. Dirr and S. Kristine Braman

Buddleia taxa were assessed for two-spotted spider mite (Tetranychus urticae Koch) resistance using a leaf disk bioassay, a novel shell vial bioassay and a field trial. Leaf pubescence and chemistry were examined for their role in two-spotted spider mite resistance. Results from bioassays and field sampling identified highly resistant taxa including B. fallowiana Balif. `Alba' and B. davidii × B. fallowiana Franch. `Cornwall Blue' as well as susceptible taxa including B. davidii Franch. `African Queen' and B. lindleyana Fort. ex Lindl. `Gloster'. The shell vial bioassay was an accurate predictor of field resistance to spider mite. Leaf pubescence was quantified by calculating the collective length of trichome branches per square millimeter of leaf surface area [effective branch length (EBL)]. EBL values ranged from 39 to 162 mm·mm-2 of leaf surface area among Buddleia taxa. Resistance was positively correlated with increased pubescence. Removal of pubescence by peeling resulted in increased oviposition of two-spotted spider mites. Exposing female two-spotted spider mites to a methylene chloride extract of B. davidii × B. fallowiana `Cornwall Blue' using a modified shell vial bioassay resulted in reduced oviposition and a methylene chloride extract of B. davidii `African Queen' resulted in no difference in oviposition when compared with a control. While pubescence is the best indicator of resistance to the two-spotted spider mite in Buddleia taxa, it is possible that defensive compounds are involved.

Free access

David R. Sandrock, Jean Williams-Woodward and Michael A. Dirr

Five taxa of Atlantic white cedar [Chamaecyparis thyoides (L.) B.S.P.], `Blue Sport', `Okefenokee', Raulston Form, `Emily', and `Rachel', and one cultivar of Leyland cypress [×Cupressocyparis leylandii (Jacks. and Dallim.) Dallim.], `Haggerston Grey', were screened for resistance to Botryosphaeria and Seiridium cankers. Treatments consisted of Seiridium unicorne (Cke. And Ell.) Sutton, Botryodiplodia Sacc. sp., Fusicoccum Corda. sp. and the non-inoculated control. After 8 weeks, plants were measured for change in caliper at the wound site, change in plant height, and length and width of surface and interior cankers. Seiridium and Botryosphaeria canker development on Atlantic white cedar taxa was not significantly different than that on Leyland cypress. Seiridium unicorne was more pathogenic than Botryodiplodia sp. and Fusicoccum sp. on Atlantic white cedar and Leyland cypress with infection percentages of 100%, 84%, and 80%, respectively. Well-defined, sunken, resinous cankers developed on Leyland cypress plants infected with Seiridium unicorne, whereas Atlantic white cedar showed no visible surface canker.

Free access

David R. Sandrock, Jean Williams-Woodward and Michael A. Dirr

Fifty-four taxa of Atlantic white cedar [Chamaecyparis thyoides (L.) B.S.P.] were assembled and maintained. A protocol for propagation of Atlantic white cedar was established. Plants were grown in containers and in a replicated field plot. Height and width data were recorded from container- and field-grown plants and all taxa were evaluated for growth habit, growth rate, and summer and winter color. Color descriptions of foliage are provided based on the Royal Horticultural Society colour chart. Exceptional taxa were identified based on needle color, texture, growth habit, and growth rate. Superior green forms include Dirr Seedlings 1 and 2, `Emily', `Rachel', and `Okefenokee'. The superior variegated form is `Webb Gold'. Superior blue forms include `Blue Sport', `Glauca Pendula', and `Twombly Blue', and superior slow-growing forms include `Andelyensis', `Meth Dwarf', `Red Star', and `Heatherbun'. These taxa are recommended to growers, landscapers, and gardeners for production and use.