Previous studies indicated that the number of shoots formed per nodal explant varied significantly depending upon the type of culture container used. Amount of media per container and amount of time that media was autoclaved were variables that differed among the containers. To determine the cause for the container effect on shoot number, a study was conducted in which autoclave time and amount of media per container were the same for all tested containers. Media was autoclaved in 500-mL batches for 30 min, then poured in 30-mL aliquots into sterile containers. Containers tested were plastic petri plates, GA-7 Magenta vessels and glass jars. Depth of the medium was 5 mm in plates, 8 mm in vessels, and 12 mm in jars. Nodes from in vitro grown shoots of `Triumph', `Regale', and `Fry' were cultured on Murashige and Skoog salts and vitamins with 2 mg·L–1 BA and 8 g·L–1 agar. Results indicated that the greatest number of shoots formed in the jars. In a second study, nodes were cultured on petri plates containing 30 mL of medium (depth 5 mm) or 70 mL (depth 12 mm). Two to three times more shoots formed on the plates with the greater amount of medium. These studies indicate amount and depth of medium are factors influencing shoot number.
Carol D. Robacker
Nodes from in vitro grown shoots of `Regale' and `Triumph muscadine grape (Vitis rotundifolia) were cultured in 25 × 95-mm glass vials, 25 × 150-mm glass culture tubes, 55 × 70-mm glass baby food jars and 100 × 25-mm plastic petri dishes. Culture medium consisted of Murashige and Skoog salts and vitamins, 80 mg/l adenine sulphate, 170 mg/l sodium phosphate, 2 mg/l BA and 8 g/l agar. Amount of medium dispensed was 8 ml per vial, 18 ml per culture tube and jar, and 70 ml per petri dish. Containers were covered with clear plastic lids, sealed with parafilm, and placed under fluorescent lights for six weeks. The number of shoots per explant that formed in petri dishes was three to four times greater than those formed on explants in vials, tubes and jars. However, the number of nodes per shoot were fewer in dishes than in the other containers. Callus formation was excessive in jars to the detriment of shoot production. Vials and tubes had small amounts of callus, while little or no callus was observed in dishes.
Carol D. Robacker and Betty Robicheaux
Micropropagation is a useful technique to propagate species such as deciduous azaleas, which are difficult to root from cuttings. To develop a micropropagation protocol that would be effective with a wide range of species and cultivars of native azalea, two culture media, Woody Plant Medium (WPM) (Lloyd and McCown, 1980) and ER medium (Economou and Read, 1994) were evaluated for ability to support growth of 11 species and four cultivars of deciduous azalea. Shoot tips were obtained from the first flush of growth in the spring on plants growing in the greenhouse or field. Following disinfection, the terminal and basal ends were removed from each explant. The explants were placed in culture tubes containing either WPM or ER medium with 12 mg/L 2iP and solidified with agar. Cultures were transferred to fresh medium every 4 to 6 weeks. Initial evaluations were made in 1996, and the experiment was repeated in 1997. In 1998, six of the taxa were evaluated for a third year. For most of the taxa evaluated, growth was superior on ER medium. On WPM, many of the cultures browned and died. R. canescens, R. viscosum, R. prunifolium, and R. austrinum are examples of species that preferred ER medium. R. alabamense, R. arborescens, and `My Mary' performed similarly on either medium.
Carol D. Robacker and S.K. Braman
Azalea lace bug (Stephanitis pyrioides) is the most serious pest on azalea. Both evergreen and deciduous azaleas are susceptible, though more resistance has been observed in the deciduous. To identify genes for resistance, fourteen deciduous azalea species, three deciduous azalea cultivars derived from complex hybrids, and one evergreen cultivar were planted in a randomized complete-block design under mixed deciduous trees in the fall of 1994. Each block was replicated 12 times. In the spring and summer of 1995, azalea lace bugs were introduced onto branches of six plants of each of the taxa. One month later, and again in the fall of 1996, the percentage of infected shoots per plant was measured. Very little damage from azalea lace bug was observed on the R. canescens, R. periclymenoides, and R. prunifolium plants, while `Buttercup', `My Mary', R. japonicum, and R. oblongifolium had the greatest damage. The cranberry rootworm, Rhadopterus picipes, damages many woody ornamentals, including some azalea species. The injury appears as elongated cuts on the leaves, and is most severe on plants growing under dense canopies. The cranberry rootworm has been observed in this azalea field plot. Plants were evaluated for damage in June 1995 and 1996. Cranberry rootworm damage was most severe on `Buttercup', R. japonicum, R. prinophyllum, and R. calendulaceum, while the evergreen azalea `Delaware Valley White' was the most resistant.
Carol D. Robacker and Sloane M. Scheiber
Abelia ×grandiflora is a drought-tolerant, pest-resistant, flowering shrub that has long been used as a foundation plant. Interspecific hybridization has produced seedlings with an assortment of morphological traits, allowing for development of new cultivars with unique or improved qualities. `Raspberry Profusion' and `Lavender Mist', developed at the University of Georgia, are seedling selections of `Edward Goucher' × Abelia chinensis. `Raspberry Profusion' is a very heavy and very early bloomer. Panicles are large and showy with fragrant pink flowers and raspberry-colored sepals. Flowering begins in early May and becomes very heavy by early June. The bright-colored sepals remain on the plant throughout the summer. Summer foliage is a medium to dark green color. In a pot, `Raspberry Profusion' blooms early and heavily. `Lavender Mist' is a heavy bloomer, with clusters of fragrant lavender flowers beginning in mid-June, and continuing into autumn. Sepals are a straw-green color at the base, becoming rose at the tips. Summer foliage is gray-green. `Lavender Mist' performs well in a pot, forming a gray-green mound contrasting with the lavender blossoms scattered around the plant. Leaves on both cultivars are glossy, particularly from mid-summer through autumn. Both plants tend to be mostly deciduous in the winter. Laboratory evaluations of cold hardiness in Griffin, Ga., during Winter 2003–04 revealed a mid-winter hardiness of –18 °C to –21 °C for `Raspberry Profusion' and –15 °C to –17 °C for `Lavender Mist'. These plants develop into dense compact shrubs following pruning and establishment in the landscape.
Carol D. Robacker and C.J. Chang
Screening of muscadine grape (Vitis rotundifolia) plants in vineyards has revealed that many plants carry Xylella fastidiosa; under suitable conditions, this bacterium causes Pierce's disease which can result in considerable loss. To determine whether propagation of muscadine through shoot tips would eliminate X. fastidiosa, plants were injected with this bacterium. After demonstrating infection, shoot tips were collected and cultured. according to the technique of Barlass and Skene (1978). Plants which were regenerated were found to be free of the bacterium. To determine whether this shoot-tip culture technique would be effective for propagation of a diverse group of muscadines, 19 cultivars were tested. Three of the cultivars failed to produce any plants, and several others reproduced at a low rate of efficiency. In an attempt to improve the rate of regeneration, several modifications to the technique were tested. For most cultivars, better initiation occurred on liquid medium, more shoots were produced with BA than with 2iP, and the addition of adenine sulfate and sodium phosphate improved the regeneration frequency.
Carol D. Robacker and Sloane M. Scheiber
Abelia ×grandiflora is a hardy shrub in the landscape, surviving heat and drought with few pest problems. However, improved cultivars with better form, the ability to retain foliage during drought, and unique flowering and foliage characteristics are in demand. `Plum Surprise' is a new cultivar of Abelia that was developed at the University of Georgia in response to these needs. `Plum Surprise' is a seedling selection from the cross `Edward Goucher' × `Francis Mason'. It forms an unusual weeping, spreading mound with fine-textured foliage. In March and April, foliage is yellow-green with scattered red/purple leaves. In late spring, the foliage becomes emerald green, changing to a lighter green throughout the summer. New stem growth is red. The most striking features of `Plum Surprise' are the fall and winter foliage color and the evergreen habit of the cultivar. As autumn progresses, the outer shoots and leaves transform to red/purple or crimson, while the inner foliage is bright emerald green. Foliage is glossy in the winter, and a deep purple or burgundy color. `Plum Surprise' is a relatively light bloomer, with flowers scattered individually or in pairs. The flowers appear white, but on close examination have a purple blush with a pale yellow throat. `Plum Surprise' is noteworthy for its heat and drought tolerance. In both the summers of 2002 and 2005, when check cultivars had lost 50% to 80% of their foliage, `Plum Surprise' exhibited little leaf drop. `Plum Surprise' performs well in a pot under nursery conditions. The foliage cascades down over the pot, making an attractive appearance in both form and color.
Carol D. Robacker and S.K. Braman
Azalea lace bug, Stephanitis pyrioides (Scott), is a major pest on azalea. Adults and nymphs feed and oviposit on the underside of the leaves, causing a stippled appearance when viewed from above. Previous field and laboratory screenings of 17 taxa of deciduous azalea, including representatives of 11 species, have identified a range of resistance to lace bug. One of the most resistant plants observed was of the species R. canescens. The interveinal region on the underside of the leaves of this plant is highly pubescent. This plant was crossed to a susceptible plant of R. viscosum (formerly R. serrulatum), which was glabrous on the lower leaf surface. The resulting seeds were planted in 1996, and the seedlings were transplanted to the field in 1998. In Sept. 1999, a laboratory bioassay was conducted to determine the resistance levels of these progeny. Five cuttings, each with two leaves, were collected from each plant, including the parental genotypes. Two female lace bugs were transferred onto the leaves of each cutting and the leaves were enclosed in a plastic cup with mesh for ventilation. After 5 days, the number of live bugs and number of eggs per cutting were counted. The percent damage from feeding was estimated. To determine whether pubescence was correlated with lace bug resistance, two terminal leaves were collected from each plant, and interveinal leaf hair density was calculated. Results from the laboratory bioassays revealed a high degree of susceptiblity to lace bug among these seedlings. Most of the progeny were pubescent, indicating no relationship between leaf hair density and resistance.
Susan M. Hawkins and Carol D. Robacker
Native grasses are increasingly used in the landscape. Little bluestem (Schizachyrium scoparium L.), a perennial bunchgrass native to most of the United States, has ornamental traits, such as variation in leaf color, differences in growth morphology, and attractive seed heads. Traditionally, cultivars of little bluestem are propagated by division, which limits the production of new plants. Our objective in this study was to develop an improved micropropagation protocol for little bluestem that would produce true-to-type plants. In 2016, we cultured immature inflorescences of eight genotypes of little bluestem on Murashige and Skoog (MS) medium with four combinations of kinetin (1.0 or 2.0 mg·L−1) and 2,4-D (0.5 or 1.0 mg·L−1) under three levels of light (dark, semilight, full light) to initiate callus. Cultures were evaluated 30 days after initiation and those that had initiated callus were subcultured. Media for subculturing and rooting contained either 0.1 mg·L−1 or no 1-Naphthaleneacetic acid (NAA). Light level had no effect on callus initiation. Initiation media with 1.0 mg·L−1 kinetin and either level of 2,4-D induced callus at almost twice the rate of media with 2.0 mg·L−1 kinetin, and cultures initiated on those media also produced almost twice the number of rooted plants over all genotypes. Genotype affected the number of rooted plants produced. The addition of NAA to medium for subculturing and rooting did not increase the number of rooted plants. In 2017, we cultured immature inflorescences of four genotypes of little bluestem on MS medium with 0.5 mg·L−1 2,4-D and either 1.0 mg·L−1 kinetin or 6-benzylaminopurine (BAP) under full light. Cultures were evaluated 30 days after initiation. Cultures that had initiated callus were subcultured onto MS medium with the same growth regulators as the initiation medium but without 2,4-D. Cultures were cycled between subculture medium with growth regulator and subculture medium with no additional growth regulator until rooted. Cultures initiated and subcultured on medium with BAP initiated two to three times more callus than those on kinetin and produced twice as many rooted plants. Our recommendation for rapid micropropagation of little bluestem is to initiate cultures on MS medium with 1.0 mg·L−1 BAP and 0.5 mg·L−1 2,4-D. After callus initiation, cultures should be subcultured to medium with BAP but no 2,4-D, alternating with medium with no additional growth regulators, until rooted.
Susan M. Hawkins, John M. Ruter and Carol D. Robacker
Four species of Dissotis and three species of Tibouchina, two genera of the Melastomataceae family, were crossed in an attempt to create interspecific and intergeneric hybrids. Intergeneric crosses set seed at a rate of 18.1% and interspecific crosses had a 32.3% rate of seed set. Germination was extremely poor, with only four crosses having germinated seed. Crosses produced 31 seedlings. Three of the seedlings were from intergeneric crosses between Dissotis canescens and Tibouchina lepidota. Interspecific crosses produced 25 seedlings from crosses between Dissotis princeps and Dissotis rotundifolia and three seedlings from crosses between D. canescens and D. princeps. The prognosis for conventional breeding for species in Dissotis and Tibouchina is poor due to low seed set, poor germination, and slow growth of progeny.