Azaleas (Ericales: Ericaceae: Rhododendron L.) are a staple plant in many landscapes of the United States and are largely resistant to predation by insects, with the exception of azalea lace bug [ALB (Heteroptera: Tingidae: Stephanitis pyrioides)]. Within deciduous azalea (Rhododendron: section Pentanthera G. Don) varying levels of resistance to ALB are observed with a continuous distribution from susceptible to highly resistant. In this study, epicuticular leaf wax from two ALB-resistant [R. canescens Michaux and R. periclymenoides (Michaux) Shinners] and two ALB-susceptible (`Buttercup' and `My Mary') deciduous azalea genotypes was extracted and re-applied to fresh azalea foliage. Leaf wax extracted from ALB-resistant genotypes and applied to ALB-susceptible genotypes conferred a high level of resistance to both ALB feeding and oviposition in the treated ALB-susceptible genotypes. Conversely, leaf wax extracted from ALB-susceptible genotypes and applied to ALB-resistant genotypes conferred susceptibility to the treated ALB-resistant genotypes. However, the effect was much less substantial than the effect of resistant wax extracts on susceptible genotypes and confined to ALB oviposition. When applied to the same genotype from which the extract was collected, leaf wax extract from ALB-susceptible genotypes had no effect on susceptibility, whereas resistant wax extract had a moderate effect on ALB oviposition rate. The results indicate that leaf wax serves as a primary mechanism of resistance of deciduous azalea to ALB.
Matthew Chappell and Carol Robacker
Michael C. Long, Stephen L. Krebs, and Stan C. Hokanson
), different disease symptoms are seen on deciduous azaleas (subgenus Pentanthera) and broad-leaved evergreen rhododendrons (subgenus Hymenanthes), and different causal organisms may be involved. Although three fungal genera ( Microsphaera , Sphaerotheca , and
Alexander Q. Susko, Timothy A. Rinehart, James M. Bradeen, and Stan C. Hokanson
Deciduous azalea ( Rhododendron sect. Pentanthera G. Don) cultivars and species are highly ornamental shrubs that are valued in northern landscapes for their colorful and prolific flowers. One important group of deciduous azalea cultivars is the
Scott R. Kalberer, Rajeev Arora, Norma Leyva-Estrada, and Stephen L. Krebs
, dehardening kinetics, and rehardening capacities of nine genotypes of deciduous azaleas were estimated under controlled conditions using floral buds collected in late Dec. 2004. Our data showed that the ability to reharden was still evident in Rhododendron
Matthew Chappell, Carol Robacker, and Tracie M. Jenkins
Deciduous azalea species are indigenous to the eastern United States from southeastern Canada southwestward to eastern Texas and east to the Atlantic coast. The deciduous azalea is an outstanding spring–summer blooming woody ornamental and, along
Yuefang Wang, Carol D. Robacker, and S.K. Braman
Azalea lace bug is the most serious pest of cultivated azalea. Though deciduous azaleas are generally considered to be more resistant to lace bug than are evergreen azaleas, some variation in resistance has been reported. The identification of the genetic and physiological basis of resistance is important to eventual development of resistant cultivars of both the deciduous and evergreen azaleas. The first step in this program is to evaluate a wide range of deciduous azaleas for level of resistance. Laboratory evaluations were conducted on nine species and two hybrid cultivars of deciduous azalea and a known susceptible cultivar of evergreen azalea, `Delaware Valley White'. Oviposition rate, rate of egg hatch, number of nymphs surviving, and percent damaged leaf area were evaluated for each of the tested genotypes. Results indicated a wide range of susceptibility, with R. canescens and R. periclymenoides plants highly resistant to infection, while R. atlanticum and R. viscosum were highly susceptible.
Michael C. Long*, Stephen L. Krebs, and Stan C. Hokanson
Forty deciduous azalea (Rhododendron sp.) cultivars from commercial sources were evaluated for powdery mildew (Microsphaera sp.) resistance. Plants were established in two duplicate field plantings in Ohio and Minnesota and evaluated in 2002 and 2003. Plants were scored using a disease symptom rating based on the percent of leaf area infected, evaluating both ab- and adaxial leaf surfaces. Highly significant differences were found for cultivar, location, year, cultivar × location and cultivar × year for disease severity. Calendulaceum × speciosum, `Fragrant Star', `Garden Party', `Late Lady', `Millennium', `Parade', and `Popsicle' showed no powdery mildew symptoms in both locations. Another group of plants with only minimal symptoms (<25% leaf area affected) included `Jane Abbott', `Magic', `Northern Hi-Lights' and `Snowbird'. The symptom-free cultivars exhibited glaucous foliage, suggesting a potential, common resistance mechanism. The mean scores for the abaxial and adaxial leaf surfaces were 2.34 and 1.64, respectively, although four cultivars had more disease symptoms on the adaxial surface. `Arneson Gem' showed nearly a two-point difference between abaxial and adaxial scores. Evaluations of azalea powdery mildew susceptibility should consider both leaf surfaces and use the highest score as the best estimate of host resistance.
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.
S.M. Scheiber, R. Jarret, and C.D. Robacker
Deciduous azaleas have been gaining popularity because of their showy floral displays and adaptability to adverse environmental conditions. However, an absence of distinguishing morphological characteristics, combined with the wide variability present in most species, has created difficulties in efforts to unambiguously identify the different species. Various DNA isolation protocols were tested in order to determine the most effective methods for isolation of DNA from 22 taxa of Rhododendron for subsequent PCR amplification. DNA yields from the various isolation methods varied widely. A minimum of 50 ng/μL of template DNA was necessary for PCR amplification under standard amplification conditions. Results indicated that the effect of tissue age on the efficiency of DNA isolation was taxa-dependent. For most species, extraction of DNA from freshly harvested young leaf tissue resulted in the highest DNA yields. However, DNA yields from R. serrulatum, R. atlanticum, and R. viscosum `Lemon Drop' were highest when mature leaf tissue was used. Primers designed to amplify the internal transcribed spacer (ITS) region of the nuclear ribosomal genes and the psbD, trnK, and 16S chloroplast genes were tested in various PCR reaction mixes in order to optimize reaction conditions for amplification. Primers to both the ITS and the psbD gene resulted in satisfactory amplification in the presence of 1.5 mM MgCl2 and 50 ng template DNA.
Scott Kalberer, Norma Leyva-Estrada, Stephen Krebs, and Rajeev Arora
Winter survival of temperate-zone woody perennials requires them to resist loss of frost hardiness (deacclimation) during winter and early spring thaws. However, little is known about deacclimation response in woody landscape plants. Moreover, what impact, if any, the degree of deacclimation has on reacclimation capacity has not been systematically studied. We used nine genotypes of deciduous azaleas (Rhododendron subgenus Pentanthera) to investigate effects of deacclimating conditions on bud cold hardiness and reacclimation ability. Dormant floral buds, with 3–5 cm stem attached, were collected in late December from field-grown plants, and placed in constant warm [22 °C 15 °C (D/N)] and humid conditions for increasing durations (0-day to 14-day) to stimulate deacclimation. Bud cold hardiness (lt 50) was determined (using logistic regressions) by evaluating immature flower survival at subfreezing treatment temperatures. Results indicated that azalea genotypes from colder provenances showed greater initial frost hardiness. Typically northern genotypes had slow to intermediate deacclimation rates, while rates of southern genotypes were intermediate to rapid. High initial frost hardiness was frequently associated with slow deacclimation. Buds retained the capacity to reacclimate upon cold exposure [2 °C/–2 °C; (12 h/12 h)] even after 8 days of deacclimation. Distinct differences were observed between the two latitudinal ecotypes of R. viscosum with respect to their initial bud hardiness, deacclimation rates, and reacclimation capacities. We suggest that the three attributes, i.e., high initial hardiness, slow deacclimation, and high reacclimation capacity, together may be important for winter-survival of azalea buds.