Azalea Cultivars Research Articles

The Effects of Stomatal Size on Feeding Preference of Azalea Lace Bug, Stephanitis pyrioides (Hemiptera: Tingidae), on Selected Cultivars of Evergreen Azalea

Azalea lace bug (ALB), Stephanitis pyrioides (Scott), is an important economic pest of azaleas in the southeastern United States. In this study, 33 commercially available cultivars of evergreen azalea, Rhododendron spp., were evaluated for S. pyrioides feeding preference in both choice and no-choice feeding bioassays. Mean stomatal length and area, which were hypothesized to affect ALB feeding preference, were also measured for each of 33 cultivars and results were correlated with indices of ALB feeding (mean feces) and fecundity (mean eggs). An azalea cultivar, Fourth of July, was least preferred by ALB in both no-choice and choice tests, whereas ‘Watchet’ was most preferred. Cultivars Fourth of July and Delaware Valley White had the smallest mean stomatal areas despite their disparate susceptibilities to ALB feeding. Although stomates through which ALB insert their proboscides vary in size among azalea cultivars, they confer no obvious resistance to ALB feeding preference. Therefore, the mechanism for lace bug resistance in azalea remains elusive.

A contribution to the classification of evergreen azalea cultivars located in the Lake Maggiore area (Italy) by means of AFLP markers

The amplified fragment length polymorphism (AFLP) technique was employed to assess the genetic conformity within a gene pool of evergreen azaleas. Ninety-three genotypes grown in historical gardens and nurseries of the Lake Maggiore area (Northern Italy), locally classified in the groups Indica, Japonica and Amoena, 44 reference cultivars of the four more or less universally recognized groups Belgian, Hirado, Kurume and Satsuki, and 40 species and species accessions that most concurred in the origin of the reference groups were included. Aims of the study were to define the genetic profiles of the Italian accessions and to provide useful information for their classification and pedigree reconstruction. Ordination analyses were used to evaluate if the AFLP technique can be applied to establish phylogenetic relationships and to investigate the DNA-based relatedness within the Italian gene pool and between the latter and the reference one. The results regarding the Italian accessions were compared with morphological data by means of Mantel’s test. Assignment tests both on the level of the groups and on individual plant level for the reference and Italian genotypes were applied to further evaluate the relatedness of the groups or individual accessions. Results demonstrated that the AFLP technique together with morphological characterization can contribute to depict the phylogeny of the evergreen azalea. Comparing the reference and Italian genotypes, conclusions about the classification of the evergreen azalea cultivars located in the Lake Maggiore area were drawn.

Application of embryo rescue after interspecific crosses in the genus Rhododendron

Interspecific hybridization between evergreen pot azalea (Tsutsusi) cultivars and genotypes of other Rhododendron subgenera or sections (Rhododendron, Hymenanthes, Pentanthera, Vireya) is significantly hampered by many prezygotic and postzygotic barriers. The objective of our work was to overcome spontaneous abortion and lack of endosperm formation and to increase germination rates by establishing an embryo rescue protocol. The optimal germination medium for immature Rhododendron seeds was a basal medium supplemented with 145 μM GA3. This medium induced germination of fertilized ovules after several sexual combinations of subgenera. Its use was clearly more efficient than in vivo sowing. The direction of the cross significantly influenced the occurrence of abortion, germination and albinism. The obtained seedlings were multiplied on Woody Plant Medium + 4.5 μM 2iP, and rooted afterwards. Finally, about 9% of the germinated ovules resulted in vigorously growing seedlings that were successfully acclimatized.

Susceptibility of Evergreen Azalea Cultivars to Anthracnose Caused by Colletotrichum acutatum

Colletotrichum acutatum is the causal agent of anthracnose on azalea (Rhododendron azalea). This disease was observed in Fall 2002 on ‘Palestrina’ azalea in Italy. Severe outbreaks of this disease were recently observed on several cultivars of azalea grown in nurseries located in the Verbano-Cusio-Ossola Province in the Piedmont region of northern Italy. Information on susceptibility to C. acutatum among azalea cultivars currently grown in Italy was unavailable. Artificial inoculations were performed to evaluate the susceptibility of 70 evergreen azalea cultivars produced in the Lake Maggiore area. Sixty of the cultivars evaluated failed to develop disease symptoms and were considered resistant to C. acutatum. Symptomatic cultivars were placed into three susceptibility classes. The cultivars Addy Wery, Fior di Pesco Cavadini, Geisha Orangerot, Kermesina, Orion, Palestrina, and Snow developed severe anthracnose symptoms. The cultivars Conversation Piece, Eikan, and Martha Hitchcock showed moderate susceptibility.

Plant Spacing Effects on Microclimate and Rhizoctonia Web Blight Development in Container-grown Azalea

Rhizoctonia web blight, caused by Rhizoctonia solani and binucleate Rhizoctonia spp., is an annual problem in compact cultivars of container-grown azalea (Rhododendron spp.) in the Gulf Coast states. Increasing the space between plants is commonly recommended for suppression of the disease, but experimental evidence for the effectiveness of this cultural practice in container-grown azalea is lacking. During the summers of 2002 and 2003, disease severity was measured weekly in the inoculated center plant of plots consisting of 49 potted `Gumpo White' azalea plants growing in 3.8-L containers and having a canopy diameter of about 30 cm. Plant spacing within plots was 0, 6, 12, 18, or 24 cm, and plots were arranged in three randomized complete blocks. Evaporation, leaf wetness (LW), relative humidity (RH), and temperature were monitored in each plot. Disease severity increased steadily from mid-July to late August or early September, after which it leveled off or declined. Evaporation increased and the number of hours within the temperature range favorable for disease development (25 to 30 °C) decreased significantly with plant spacing (P < 0.05), but LW and RH were not significantly different among treatments. Plant spacing also had no significant effect on disease severity. Daily overhead irrigation and compact plant form likely contributed to the lack of effect of spacing on disease development.

Susceptibility of Deciduous Azalea Cultivars to Powdery Mildew Disease

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.

OPTIMISATION OF RELATIVE QUANTITATIVE RT-PCR FOR EXPRESSION ANALYSIS IN AZALEA FLOWER COLOUR SPORTS

The fastest way to create new azalea (Rhododendron simsii hybrids) cultivars is by making use of flower colour sports, which appear spontaneously on azalea plants. Unfortunately, there is still very little known on how bud sport induction occurs. Therefore, genes coding for two key enzymes of the azalea flavonoid biosynthesis pathway, chalcon synthase (chs) and dihydroflavonol 4-reductase (dfr) that were reported before to be apt for modification by the action of bud sporting, were isolated and characterized. The expression of these two flower colour genes in the petals of azalea flowers will be compared between all ‘Hellmut Vogel’ flower colour sports. To measure the expression levels of both genes, relative quantitative RT-PCR analysis will be worked out on a real-time PCR machine. The expression of housekeeping genes, which is expected to be the same for all sports, will be used to calculate the relative expression level of the two genes of interest. The optimisation of this technique will be discussed.

Influence of Nitrate:Ammonium Ratios on Growth and Elemental Concentration in Two Azalea Cultivars

Rooted cuttings of Rhododendron canescens “Brook” and Rhododendron austrinum were grown in sand culture with a modified Hoagland's solution under greenhouse conditions. The effect of varying ammonium:nitrate (NO3 −:NH4 +) ratios (100:0, 75:25, 50:50, 25:75, 0:100) on growth, chlorophyll content, plant quality, and elemental tissue concentration were determined. With NO3 − as the nitrogen (N) form, both azalea cultivars exhibited less vegetative growth, lower overall plant quality, with leaves showing visual chlorotic symptoms in comparison to plants receiving NH4 + as the N‐form. Leachate pH was highest with NO3 − as the predominate N‐form and decreased significantly with each increment of NH4 +. With both azalea cultivars, N‐form significantly influenced uptake and utilization of essential plant nutrients. Leaf concentrations of N, potassium (K), calcium (Ca), sulfur (S), boron (B), and molybdenum (Mo) were highest with NO3 −‐N. Leaf elemental concentrations of phosphorous (P), magnesium (Mg), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) increased as NH4 + supplied more of the N‐ratio. Significant differences in Mg, Mn, and Zn were observed between species. Results from this study show that foliar N concentration is not an accurate indicator of plant growth response. Further investigations are needed to determine if foliarchlorosis and low growth rates observed with NO3 − fed plants due to an Fe deficiency, to low nitrate reductase (NR) activity in the leaves, or to a combination of these factors.

Clarification of origin in azalea cultivars by PCR-RFLP analysis of chloroplast DNA.

Clarification of origin in azalea cultivars by PCR-RFLP analysis of chloroplast DNA.

GENETIC DIVERSITY OF JAPANESE WILD EVERGREEN AZALEAS IN KYUSHU (SOUTH MAIN ISLAND OF JAPAN) CHARACTERIZED BY AFLP

ISHS XX International Eucarpia Symposium, Section Ornamentals, Strategies for New Ornamentals - Part II GENETIC DIVERSITY OF JAPANESE WILD EVERGREEN AZALEAS IN KYUSHU (SOUTH MAIN ISLAND OF JAPAN) CHARACTERIZED BY AFLP

Adventitious shoot regeneration and appearance of sports in several azalea cultivars

Shoots can be regenerated from leaf explants of most azalea (Rhododendron simsii) cultivars using a two-step protocol with a callus induction and a regeneration phase. This procedure is unsuccessful for a few cultivars, because of excessive callus production. In most such cases, shoot regeneration could be obtained in one step, starting from petioles as explants. Flower colour variations (the induction of sports) were frequently observed after regeneration. This phenomenon offers prospects for the enlargement of the commercial assortment but can also be used for the study of gene regulation processes in flower colour variegation.

Origin of the Aspartate Aminotransferase Isozymes Detected in Kurume Azalea Cultivars.

Five evergreen azalea species (Rhododendron macrosepalum, R. mucronatum var. ripense, R. scabrum, R. indicum, and R. eriocarpum) and 3 groups of cultivars (Ryukyu, Mochi, and Ookirishima azaleas) were examined for aspartate aminotransferase isozyme bands, "112" and "104", which were detected in Kurume azalea cultivars but not in their known original species, R. kiusianum, R. kiusianum var. sataense, and R. kaempferi. "112" band is peculiar to R. mucronatum var. ripense, whereas "104" band is limited to R. macrosepalum and R. scabrum among the species related to the breeding of the principal domestic evergreen azalea groups. In addition, Ryukyu, Mochi, and Ookirishima azaleas, whose original species are R. macrosepalum, R. mucronatum var. ripense, and R. scabrum, possess either "112" or "104" band. Therefore, "112" and "104" bands detected in Kurume azalea cultivars may be duly derived from these three species.

EVIDENCE FOR INTROGRESSIVE HYBRIDIZATION BASED ON CHLOROPLAST DNA POLYMORPHISMS AND MORPHOLOGICAL VARIATION IN WILD EVERGREEN AZALEA POPULATIONS OF THE KIRISHIMA MOUNTAINS, JAPAN

For the purpose of determining the origin of horticultural evergreen azalea cultivars, this study was focused on the natural populations of azalea in Kyushu (south main island of Japan). The Kirishima mountains, the volcanic mountain mass in Kyushu, are an important centre of diversity for the Japanese evergreen azaleas. Rhododendron kiusianum Makino grows above 1000m alt., whereas R. kaempferi Planch. is distributed below 600m alt. Putative natural hybrid populations of these two species are found in the intermediate region (1000–600m alt.). These two species have been clearly distinguished by their respective morphological features. Rhododendron kiusianum has small pink-purple flowers and small elliptical leaves, whereas R. kaempferi has larger red-orange flowers with dark blotches and large oblong leaves. Interspecific hybrids show phenotypes within the range of the two species, especially with regard to flower colour and leaf shape. A morphological cline of these characteristics corresponding to altitude has been observed between these two species. PCR-RFLP analysis of chloroplast DNA detected specific bands for the two species in the 16S rDNA region when digested with HhaI restriction enzyme. Populations of interspecific hybrids were composed of individuals that had a banding pattern of either R. kiusianum or R. kaempferi. This indicates that R. kiusianum and R. kaempferi are clearly distinct species. Furthermore, natural hybrid populations consist of individuals that have one of two cpDNA. Some individuals in the populations of R. kiusianum (T-1430 and T-1030) possess the cpDNA pattern of R. kaempferi, which suggests that cytoplasmic introgression has occurred in the populations of R. kiusianum from R. kaempferi.

クルメツツジ品種群における葉の背軸側表皮面の形態的特性とリュウキュウツツジの関与

クルメツツジ品種群におけるリュウキュウツツジの遺伝的影響を検討するため, クルメツツジ'裾濃の糸'とモチツツジとの交雑実生58個体, クルメツツジ品種群58品種, リュウキュウ系8品種, モチツツジ系5品種およびオオキリシマ系4品種を供試して, 表皮細胞の形状, 表皮のクチクラの形態的特性, 気孔の特徴および葉脈の毛状突起の有無を調査した.クルメツツジ'裾濃の糸'とモチツツジとの交雑実生では, 87.9%が表皮系に関する3形質中, 1形質以上でモチツツジと同じ形態的特性を示した.モチツツジにみられる葉脈の毛状突起はすべての個体に認められた.クルメツツジ58品種中23品種は, 葉の背軸側表皮面に関する4形質中, 1形質以上でモチツツジおよびキシツツジの種群と同じ形態的特性を示した.リュウキュウ系, モチツツジ系およびオオキリシマ系の17品種は, モチツツジおよびキシツツジの種群と同じ形態的特性を有した.これらの結果から, クルメツツジ品種群の成立にリュウキュウツツジの原種であるモチツツジ, キシツツジの一方, あるいは両種が関与していることが示唆された.

Comparison of Aspartate Aminotransferase Isozymes Between Three Wild Evergreen Azaleas and Kurume Azalea.

The variation on aspartate aminotransferase (AAT) isozymes of flower buds of Rhododendron kiusianum, R. kiusianum var. sataense, and R. kaempferi, which are considered to be the parental species of Kurume azalea (R. obtusum), and Kurume azalea cultivars was studied by polyacrylamide gel electrophoresis. AAT isozymes of Kurume azalea were compared with those of three wild evergreen azaleas to seek information on the origin of the domestic cultivars. Three loci (Aat-1, Aat-2, and Aat-3) were identified for AAT. The Aat-2 locus in the three wild evergreen azaleas had two common presumptive alleles (Aat-2100 and Aat-280), and two species-specific presumptive alleles ; Aat-286 for R. kiusianum var. sataense and Aat-290 for R. kaempferi. Aat-3 locus shared the common presumptive alleles (Aat-367, Aat-358, and Aat-349) with three species. Aat-2 locus of Kurume azalea contained four alleles detected in the above three species and also Aat-2112 and Aat-2104 alleles which were not found in R. kiusianum, R. kiusianum var. sataense, and R. kaempferi. These results indicate that Kurume azalea contains genes from R. kiusianum var. sataense and R. kaempferi, and some other related species than the above three wild ones.

100 A Comparison of Two Media on Growth of Deciduous Azalea Shoots In Vitro

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.

Field Evaluation of Azalea Species and Cultivars for Resistance to Azalea Lace Bug and Cranberry Rootworm

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.

Impact of Pinching Method and Number of Cuttings on Growth of Eight Florist Azalea Cultivars

During Fall 1995–Winter 1996, rooted cuttings of eight Rhododendron Simsii cultivars—Dorothy Gish, Jacinth, Paloma, White Gish, Friedhelm Scherrer, Gloria, Helmut Vogel, and Inga were transplanted July 1995 into 10.5-cm pots. A treatment consisted of one, two or three cuttings per pot. Cuttings were pinched either mechanically using a hedge clipper (control) or chemically with Off-Shoot-O at rates of 63.5 or 111.1 mL·L–1 of water or with Atrimmec at 20 mL·L–1 of water. Pinching treatments were repeated three times during the experimental period. New secondary shoots developed more rapidly after a mechanical pinch than after a chemical pinch. Moreover, greater foliage damage was observed on plants pinched with Off-Shoot-O. Growth measurements (height and diameter of plants, top dry mass, number of days to reach anthesis and visual quality) will be presented.

Use of Flow Cytometric Measurement of Nuclear DNA Content to Assess Ploidy Level in Azalea

We evaluated flow cytometric measurement of nuclear DNA content to determine ploidy level in azalea. If ploidy level correlates with DNA content, ploidy level could be determined more readily than by direct chromosome counts and assist in planning crosses and evaluating progeny. Tested plants included azalea cultivars, materials from the azalea breeding project at the Univ. of Minnesota, and species from the Rhododendron Species Botanic Garden and the North Carolina Arboretum. Data compiled from DNA assays of practically all material analyzed fell into distinct groups consistent with their being either diploid, triploid, or tetraploid. Additionally, a known diploid plant of each of four diploid species, together with a natural or derived tetraploid plant of each of these species was obtained. Results showed that the four diploids had a similar DNA content compared to one another. DNA content of the tetraploids was also similar, and the tetraploid's DNA content was approximately twice that of the diploids, as expected. Unfortunately, success with direct chromosome counts in other material has proven elusive, currently precluding direct correlation of DNA amount with ploidy level across other species and cultivars. Although many cases exist in the literature where DNA content has a direct relationship to ploidy level, this does not always hold. Although the majority of plants tested fell into a diploid, triploid, or tetraploid grouping based on DNA content, further study is required to determine the exact relationship between ploidy level and DNA content in azalea.

Influence of Substrate, Auxin, and Propagation Period on Rooting of Eight Florist Azalea Cultivars

During Spring–Summer 1995, cuttings of eight Rhododendron simsii cultivars `Dorothy Gish', `Jacinth', `Paloma', `White Gish', `Friedhelm Scherrer', `Gloria', `Helmut Vogel', and `Inga' were rooted using two substrates combined with or without (control) four rooting hormone treatments. The rooting substrate was composed of either pure peat moss or 80% peatmoss and 20% perlite. The basal end of cuttings were dipped into one of the four hormone treatments: IBA (Stim-Root powder) at 4000 and 8000 ppm, or IBA (liquid Stim-Root) at 5000 and 10,000 ppm. This experiment was repeated in Fall 1995 and in Winter 1996 to determine the effect of seasonal period on rooting. Spring is the optimal period for rooting the cultivars used in this study. Highest percentage of rooting was obtained with cultivars `Helmut Vogel', `Inga', `Jacinth', and `Paloma'. Dry mass of cultivars propagated using liquid formulation was significantly increased during this period. Substrates did not significantly effect rooting percentage and quality and dry root mass.