Rhododendron Calendulaceum Research Articles

Leaf Discoloration in Rhododendron Species Exposed to Phytophthora Cinnamomi Corresponds with Future Mortality

Abstract Phytophthora cinnamomi, which causes the disease root rot, is an oomycete pathogen that is damaging to woody plants, including many horticulturally important groups, such as Rhododendron. Infecting the root of plants, Phytophthora cinnamomi inhibits water uptake, leading to root damage, wilting, and increased rates of plant mortality. Some observations suggest that P. cinnamomi infection corresponds to changes in leaf coloration, though whether this indicates a plant stress response or plant damage is generally unknown. We used leaf color analysis to test for differences in leaf discoloration between plants inoculated with the pathogen and control plants. We demonstrate a significant link between leaf discoloration in Rhododendron species and Phytophthora cinnamomi inoculation. This method was most useful when mortality was not exceptionally high, and analyzers must consider mortality as well as leaf damage in quantifying effects of the pathogen. Plants with leaf discoloration were 3.3 times more likely to die 2 weeks from our leaf census than plants with no leaf discoloration (P =0.005). This method is particularly inexpensive to implement, making it a valuable alternative to multi-spectral or hyperspectral imaging, especially in contexts such as horticulture and citizen science, where the high speed and low-cost nature of this technique might prove valuable. Species used in this study: root rot disease pathogen (Phytophthora cinnamomi Rands); Rhododendron atlanticum (Ashe) Rehder; Rhododendron brachycarpum D.Don ex G.Don; Rhododendron kiusianum Makino; Rhododendron maximum L.; Rhododendron minus Michx.; Rhododendron calendulaceum (Michx.) Torr.; Rhododendron kaempferi Planch.; Rhododendron keiskei Miq. Chemicals used in this study: Fosal Select Aliette/aluminum phosphite.

First record of Azalea Sawfly (<i>Euura lipovskyi</i>, Hymenoptera: Tenthredinidae) in Canada

Azalea Sawfly (Euura lipovskyi) larvae found feeding on foliage and flowers of cultivated Flame Azalea (Rhododendron calendulaceum) in Ottawa, Ontario, in late May 2021 are the first records of this sawfly in Canada. The native range of Azalea Sawfly includes the eastern United States, but the species has extended its distribution recently to the Pacific northwest of North America and Europe. Recorded foodplants include Rhododendron calendulaceum, Rhododendron luteum, Rhododendron obtrusum, Rhododendron occidentale, Rhododendron molle, Rhododendron viscosum, and possibly Early Azalea (Rhododendron prinophyllum), all in Rhododendron section Pentanthera. The new combinations Euura lipovskyi and Euura azaleae are proposed.

Novel microsatellite markers for evaluation of genetic diversity in the tetraploid flame azalea, Rhododendron calendulaceum (Ericaceae).

Microsatellite markers have become a popular and useful tool for investigating evolutionary processes at shallow taxonomic scales such as within a species or between extremely closely related species. Rhododendron sect. Pentanthera is a closely related group of deciduous azaleas that demonstrate both naturally occurring and horticulturally derived hybridization. Two species, flame azalea and Cumberland azalea, represent a particularly recalcitrant evolutionary problem, which will benefit from the development of rapidly evolving molecular markers. Microsatellite markers were specifically developed for Rhododendron calendulaceum, the flame azalea, for use in studies of genetic structure and potential hybridization with its close relative Rhododendron cumberlandense, the Cumberland azalea. Forty-eight primer pairs designed from paired-end Illumina MiSeq data were screened for robust amplification. Sixteen of these pairs were PCR-amplified in the presence of fluorescently labeled primers and genotyped in 66 flame azalea individuals from three geographically dispersed populations. Fifteen primer pairs were both reliable and polymorphic and exhibit ample variability for use in downstream population-level investigations. Cross-amplification in all other members of Rhododendron sect. Pentanthera was highly successful, suggesting broad utility across the entire clade. The novel microsatellite markers presented here functioned well within the target species and amplified with high success in the remaining members of the clade. They represent a significant improvement to the genetic toolkit available for Rhododendron sect. Pentanthera, and particularly for the flame/Cumberland azalea evolutionary problem.

Reproduction in Flame Azalea (Rhododendron calendulaceum, Ericaceae): A Rare Case of Insect Wing Pollination.

Although many angiosperms are serviced by flying pollinators, reports of wings as pollen vectors are rare. Flame azalea (Rhododendron calendulaceum) is visited by diverse insects, yet previous observations suggested that only butterfly wings may transfer pollen to stigmas. We used an experimental approach to determine whether butterfly wings are the primary vehicle of pollination in flame azalea. Over two seasons of observations, only butterflies (Papilio glaucus and Speyeria cybele) contacted both anthers and stigmas, yet because of differences in wing-flapping behavior, P. glaucus transferred pollen most efficiently. In contrast, bee species specialized either on pollen or nectar but did not contact both anthers and stigmas. A field experiment revealed that flowers excluding butterflies experienced almost complete fruit failure, whereas fruit set in open flowers did not differ from those that were hand pollinated. Additionally, butterflies had 56-fold more azalea pollen on their wings than bodies, while azalea stigmas bore both pollen and wing scales. These results suggest that plants with many visitors contacting reproductive organs may still specialize on a single guild of visitors for pollination and that wing-borne pollen transfer is a key mode of flame azalea pollination.

Host Plant Utilization Within Family Ericaceae by the Andromeda Lace Bug Stephanitis takeyai (Hemiptera: Tingidae)

This study examined host plant utilization by the Andromeda lace bug, Stephanitis takeyai Drake and Maa, within the family Ericaceae. The preferred host of S. takeyai is the Japanese pieris, Pieris japonica (Thunb.) D. Don ex G. Don, but it has been reported to exhibit host alternation and also to occur on other unrelated host plants. We examined the acceptability of ten landscape and fruit plants belonging to the family Ericaceae (Rhododendron calendulaceum, Rhododendron ‘Hampton Beauty’, Rhododendron ‘Autumn Empress’, Vaccinium arboreum, Vaccinium virgatum, Calluna vulgaris, Kalmia latifolia, Pieris floribunda, Pieris phillyreifolia and Pieris japonica ‘Temple Bells’) to S. takeyai. In no-choice tests adult survival did not vary significantly among the taxa. Highest leaf damage was recorded on P. japonica and R. calendulaceum, while slight damage was noted on V. arboreum and Rhododendron ‘Hampton Beauty’. Nymph emergence was observed in P. japonica, R. calendulaceum and Rhododendron ‘Hampton Beauty’. In multi-choice tests adult presence on leaves did not vary significantly except on day 9, when adults were more numerous on ‘Temple Bells’. Maximum leaf damage was recorded on P. japonica, ‘Temple Bells’. Several plants like Rhododendron and Vaccinium spp., which may not be favorable hosts, could serve as reservoirs for the pest.

Post-transplant Irrigation Scheduling for Two Native Deciduous Shrub Taxa

The effect of five irrigation scheduling treatments on shoot growth [growth index (GI)] and stem water potential (SWP) of Itea virginica L. ‘Henry's Garnet’ (‘Henry's Garnet’ sweetspire) and Rhododendron austrinum Rehd. (Florida flame azalea) were studied. Plants were transplanted on 13 Mar. 2008 at soil grade level under shade structures in field plots of sandy loam soil on the Auburn University campus in Auburn, AL. Matric potential was continuously measured 7.6 cm from the stem in the root ball and 20.3 cm from the stem in the soil backfill for three plants per treatment per taxa. Irrigation scheduling treatments included (in order of decreasing irrigation frequency): root ball and surrounding soil matric potential maintained at or above –25 kPa [well-watered (WW)]; root ball and surrounding soil rewatered when root ball matric potential dropped to either –50 kPa (50RB) or –75 kPa (75RB); and root ball and surrounding soil rewatered when surrounding soil matric potential dropped to either –25 kPa (25S) or –50 kPa (50S). In both taxa, GI increased linearly over time in all five irrigation treatments. For I. virginica ‘Henry's Garnet’, GI increased most in WW and 25S treatments followed by 50S, 50RB, and 75RB. Shoot growth of R. austrinum was similar among treatments. Both I. virginica ‘Henry's Garnet’ and R. austrinum had a larger increase in GI during the first growing season (2008). For I. virginica ‘Henry's Garnet’, SWP was higher in 50S and 75RB treatments than in 50RB, WW, and 25S. For R. austrinum, SWP was not different among treatments. Results indicate that although plant growth might be diminished slightly, irrigation frequency can be reduced without compromising plant visual quality or survival if root ball and soil matric potential is monitored. Additionally, until roots grow into the backfill soil, monitoring both backfill soil and root ball matric potential is important for scheduling and reducing post-transplant irrigation applications.

The association between floral longevity and pollen removal, pollen receipt, and fruit production in flame azalea (Rhododendron calendulaceum)

Floral longevity is an important feature of a plant’s reproductive strategy. The goal of this study was to examine flower life span in the shrub flame azalea ( Rhododendron calendulaceum (Michx.) Torr.) to determine if it is fixed or plastic and to evaluate its association with pollen removal and receipt. Unmanipulated flowers remained open for ~10 days, while hand-pollinated flowers closed earlier (~7 days), indicating that these flowers are subject to pollination-induced senescence. In 2002, pollen removal was rapid (~70% of pollen was removed in the first 2 h of anthesis), while female function was a slower process (pollen accumulated on stigmas for up to 4 days). Fruit set was pollen limited, as 80% of hand-outcrossed flowers set fruit versus 35% of naturally pollinated flowers. This is likely the result of the pollen-collecting behavior of a common solitary bee, which did not contact stigmas. In sharp contrast, 2003 was a cooler and wetter spring, few flower visitors were observed, and virtually no pollen was removed from or deposited on stigmas after 48 h. These results suggest that the flower life span of flame azalea is a result of selection for increasing the probability of pollen receipt rather than pollen dispersal.

Frost dehardening and rehardening of floral buds of deciduous azaleas are influenced by genotypic biogeography

Temperate-zone woody perennials may resist cold dehardening and reharden effectively after unseasonably warm winter conditions to avoid frost damage. Few controlled experiments have examined dehardening kinetics or the impact of dehardening on rehardening capacity after cold temperatures return. We used nine genotypes of deciduous azalea from eight known provenances to study the influence of biogeographical origin on floral bud dehardening and rehardening after controlled dehardening. Buds cold acclimated in the field were placed in warm conditions to stimulate dehardening. Visual assays were conducted periodically over 11 days of dehardening to evaluate survival of immature florets at subfreezing temperatures. A rehardening regime was applied to three genotypes after 1, 3, 5, and 8 days of dehardening. Dehydrin abundance after dehardening and rehardening was estimated for selected genotypes. Floral buds from warmer-climate azaleas Rhododendron canescens, Rhododendron prunifolium, and Rhododendron viscosum variety serrulatum exhibited lower mid-winter hardiness than did the colder-climate azaleas Rhododendron calendulaceum, Rhododendron canadense, Rhododendron prinophyllum, and Rhododendron viscosum variety montanum. The dehardening rates of the “low dehardening-resistant” R. canadense, R. canescens, and R. viscosum var. serrulatum were at least twice the rates of “high dehardening resistant” Rhododendron arborescens, Rhododendron atlanticum, R. calendulaceum, R. prinophyllum, R. prunifolium, and R. viscosum var. montanum throughout the time-course. Genotypes originating in colder and warmer climates did not always exhibit high and low dehardening-resistance, respectively. Dehardening was associated with declining levels of dehydrins in R. prinophyllum and the two R. viscosum varieties. All tested genotypes rehardened in response to cold even after 8 days of dehardening. The high-altitude variety of R. viscosum had substantially larger rehardening-capacity than the low-altitude variety. Rehardening was associated with increasing levels of dehydrins in both R. viscosum varieties. Mid-winter hardiness ≥26.0 °C, dehardening rates ≤1.0 °C day −1, a capacity to reharden, and the ability to accumulate dehydrins could all be important winter survival strategies for genotypes originating in colder climates.

Host Range of a Select Isolate of the Ericoid Mycorrhizal Fungus Hymenoscyphus ericae

Studies were conducted to examine the host range of a select isolate of the ericoid mycorrhizal fungus Hymenoscyphus ericae (Read) Korf and Kernan [American Type Culture Collection (ATCC) #32985]. Host status was tested for 15 ericaceous species, including: Calluna vulgaris (L.) Hull, Enkianthus campanulatus (Miq.) Nichols, Gaultheria procumbens L., Kalmia latifolia L., Leucothoe fontanesiana Sleum., Oxydendrum arboreum (L.) DC.,Pierisfloribunda (Pursh) Benth. & Hook.,Rhododendron calendulaceum (Michx.) Torr.,Rhododendron carolinianum Rehd., Rhododendron catawbiense Michx., Rhododendron maximum L., Rhododendron mucronulatumTurcz., Vaccinium corymbosum L., and Vaccinium macrocarpon Ait. Arbutus unedo L., an ericaceous species that forms arbutoid, not ericoid, mycorrhizae, was used as a negative control. All of the species were colonized by the ericoid isolate with the exception of Enkianthus campanulatus and the negative control. Inoculation with this isolate of H. ericae resulted in a significant increase in shoot growth. However, intensity of root colonization was not correlated to amount of shoot growth. In fact, an increase in growth was observed in the two species that lacked fungal colonization.

Host Range of a Select Isolate of the Beneficial Ericoid Mycorrhizal Fungus Hymenoscyphus ericae

Studies were conducted to examine the host range of a select isolate of the ericoid mycorrhizal fungus, Hymenoscyphus ericae (Read) Korf and Kernan [American Type Culture Collection (ATCC) #32985]. Host status was tested for 15 ericaceous species, including Calluna vulgaris, Enkianthus campanulatus, Gaultheria procumbens, Kalmia latifolia, Leucothoe fontanesiana, Oxydendrum arboreum, Pieris floribunda, Rhododendron calendulaceum, Rhododendron carolinianum, Rhododendron catawbiense, Rhododendron maximum, Rhododendron mucronulatum, Vaccinium corymbosum, and Vaccinium macrocarpon. Arbutus unedo, an ericaceous species that forms arbutoid, not ericoid, mycorrhizae was used as a negative control. All of the species were colonized by the ericoid isolate with the exception of Enkianthus campanulatus and the negative control. The benefits of the association and possible commercial applications are discussed.

Reproductive consequences of a gall-inducing fungal pathogen (Exobasidium vaccinii) on Rhododendron calendulaceum (Ericaceae)

We investigated the reproductive consequences of a gall-inducing fungal pathogen (Exobasidium vaccinii) on its host plant, Flame Azalea (Rhododendron calendulaceum (Michx.) Torr.) in southwestern Virginia. The percentage of infected plants ranged from 10 to 90% among the four study sites. There was a high degree of consistency in the pattern of prevalence between years at both the site and individual plant level within sites; plants and sites that showed high gall loads in 1997 were similarly infected in 1998. Infection rates and gall loads were higher in 1997 compared with 1998. All measures of reproduction were negatively affected by galls. Individuals infected in 1997 were less likely to flower the next season. Diseased inflorescences produced fewer flowers, smaller individual flowers, and had a lower probability of producing fruit. At the individual host level, the probability of fruit production was negatively correlated with the gall load on that individual. Finally, branches that were diseased in 1997 were more likely to die the following year, while healthy branches had a high probability of producing leaves and (or) flowers. Our results demonstrate that the fungal galls produced by E. vaccinii have immediate negative consequences to the reproductive process of R. calendulaceum.

A revision of Rhododendron section Pentanthera

Rhododendron sect. Pentanthera G. Don (Ericaceae) comprises a group of closely related, highly ornamental plants which are commonly called ‘azaleas'. Thirteen of the fifteen species recognized in this section are indigenous to North America. One species (R. molle) is native to Japan and China, and one species (R. luteum) is indigenous to the Caucasus region. Phylogenetic analysis of the species within the section indicates that R. molle is the sister to the rest of the section. It is the sole member of R. subsect. Sinensia. The remaining species form a monophyletic group recognized as R. subsect. Pentanthera. Within this subsection the presence of a blotch on the upper corolla lobe defines two primarily orange to red-flowered groups. The first group has a Tertiary Period disjunct distribution and comprises R. luteum, R. austrinum and R. occidentale. The second group is indigenous to eastern North America and comprises R. calendulaceum, R. cumberlandense, R. flammeum, R. prunifolium and R. alabamense. In both groups the cladistically basal species has white flowers with a yellow blotch on the upper corolla lobe (R. occidentale, R. alabamense, respectively). The pink to white early flowering species R. canescens, R. periclymenoides and R. prinophyllum do not form a monophyletic group.Phenetic analyses indicate that the eastern Asian taxon, R. molle, is best recognized as one species with two geographical subspecies; Rhododendron prinophyllum is quite distinct from R. canescens and R. periclymenoides. The latter two species are very similar morphologically, but their similarities are due to the retention of primitive characters and they should be recognized as distinct species. Rhododendron calendulaceum can be distinguished from R. cumberlandense using a combination of morphological and phenological characters. The various taxa previously segregated out of R. viscosum are merely extreme forms of a widespread and variable species and are not given any formal rank. No subspecific taxa are recognized for R. occidentale. Distribution maps, keys to the species, species descriptions and specimen citations are included.

Initial Growth of Seedlings of Flame Azalea in Response to Day/Night Temperature

Seedlings of flame azalea [Rhododendron calendulaceum (Michx.) Torr] were grown for 12 weeks under long-day conditions with days at 18, 22, 26, or 30C for 9 hours in factorial combination with nights at 14, 18, 22, or 26C for 15 hours. Total plant dry weight, top dry weight, leaf area, and dry weights of leaves, stems, and roots were influenced by day and night temperatures and their interactions. Dry matter production was lowest with nights at 14C. Root, leaf, top, and total dry weights were maximized with days at 26C in combination with nights at 18 to 26C. Stem dry weight was maximized with days at 26 to 30C and nights at 22C. Leaf area was largest with days at 18 and 26C in combination with nights at 18 or 26C. Within the optimal, day/night temperature range of 26 C/18-26C for total plant dry weight, there was no evidence that alternating temperatures enhanced growth. Shoot: root ratios (top dry weight: root dry weight) were highest with days at 18 and 30C. Leaf area ratio (total leaf area: total plant dry weight) was highest and specific leaf area (total leaf area: leaf dry weight) was largest when days and nights were at 18C and were lower at higher temperatures. Regardless of day/night temperature, leaf weight ratio (leaf dry weight: total plant dry weight) was higher than either the stem weight ratio (stem dry weight: total plant dry weight) or root weight ratio (root dry weight: total plant dry weight). Net leaf photosynthetic rate increased with day temperatures up to 30C.

Growth Response of Seedlings of Flame Azalea to Manual and Chemical Pinching

Abstract Seedlings of flame azalea [Rhododendron calendulaceum (Michx.) Torr] were subjected to the following manual pinching treatments at the 10-,12-,14-,or 16-leaf stage: no pinching, removal of the terminal two nodes [approximately 1.25 cm (0.5 in)] and removal of terminal growth[approximately 2.5-5.0 cm (1-2 in)] leaving six nodes. The greatest number of lateral shoots (5.3) was produced by removing the terminal two nodes at the 16-leaf stage. Generally, the number of lateral shoots increased with the leaf stage at which manual pinching was imposed. Removal of terminal growth, leaving six nodes, resulted in the lowest leaf, stem and root dry weights at each leaf stage. In a second experiment, dikegulac [Atrimmec;sodium salt of 2,3:4,6-bis-0-(l-methylethylidene)-α-L-xylo-2-hexulofuranosonic acid], at 0, 2000, 4000, 6000 or 8000 ppm, was applied to both nonpinched and manually-pinched plants (removal of the terminal two nodes at the 16-leaf stage). Dikegulac (Atrimmec) treatment significantly affected the nurnber of lateral shoots and dry weights of leaves, stems and roots. The number of lateral shoots increased linearly with concentration up to 4000 ppm. At 6000 and 8000 ppm, the average number of shoots produced (9.4 and 9.6, respectively) was similar to 4000 ppm. Both pinched and nonpinched plants treated with dikegulac (Atrimmec) produced more lateral shoots than manual pinching alone. Dry weights of leaves, stems and roots decreased as the concentration increased. However, the number of lateral shoots and dry weights of leaves, stems and roots of pinched and nonpinched plants were not significantly different when treated with dikegulac (Atrimmec)

Herbicide Tolerance of Selected Ericaceous Species

Abstract This study was conducted to determine the tolerance of seven Ericaceous species to 15 herbicide treatments. Treatment response was species dependent. Pre-M (pendimenthalin) at the 2.2 kg/ha (2 lb ai/A) rate reduced top fresh weight of mountain laurel (Kalmia latifolia), flame azalea (Rhododendron calendulaceum) and Carolina rhododendron (Rhododendron carolinianum) while the 4.5 kg/ha (41b ai/A) rate affected these 3 species plus drooping leucothoe (Leucothoe fontanesiana), sourwood (Oxydendrum arboreum), Catawba rhododendron (Rhododendron catawbiense) and azalea ‘Hinocrimson’ (Rhododendron ‘Hinocrimson’). SWGC (pendimethalin) at 3.4 kg/ha (3 lb ai/A) reduced top fresh weight of sourwood, flame azalea and Carolina rhododendron. Goal (oxyfluorfen) + AG98 reduced fresh weight of every species except Carolina rhododendron. Snapshot DF (isoxaben + oryzalin) at 3.4 kg/ha (3 lb ai/A) reduced top fresh weight of mountain laurel and flame azalea. Mountain laurel showed a reduction in fresh weight for Pennant GR (metolachlor) at the 4.5 kg/ha (4 lb ai/A) rate. At the 2.2 kg/ha (2 lb ai/A) rate of Ronstar WP (oxadiazon), reduced Catawba rhododendron top fresh weight, however, Catawba rhododendron was not affected by the 4.5 kg/ha (4 lb ai/A) rate. The Poast (sethoxydim) treatment at 0.6 kg/ha (0.5 lb ai/A) + crop oil concentrate reduced fresh weight of flame azalea. SWGC at 3.4 kg/ha (3 lb ai/A), OH2 (oxyfluorfen + pendimethalin) at 3.4 kg/ha (3.0 lb ai/A), Gallery (isoxaben) at 1.1 kg/ha (1.0 lb ail A), Snapshot GR (isoxaben + trifluralin) at 4.0 kg/ha (3.75 lb ai/A), Derby (metolachlor + simazine) at 5.7 kg/ha (5.0 lb ai/A), and the ready-to-use formulation of sethoxydim did not reduce top fresh weight of any of the seven species tested.

Whole-Plant Response of Selected Woody Landscape Species to Uniconazole

Abstract Uniconazole was applied as a foliar spray or medium drench to six woody landscape species: ‘Sunglow’ azalea; flame azalea; ‘Spectabilis’ forsythia; ‘Compacta’ holly; ‘Nellie R. Stevens’ holly; and mountain pieris. One hundred days after uniconazole application, leaf, stem, and top dry weight of all species, except flame azalea and mountain pieris, decreased as uniconazole concentration increased. Compared to controls, stem and leaf dry weight were reduced by uniconazole 18 to 60% and 13 to 32%, respectively, depending on species and method of application. Stem dry weight was reduced to a greater degree, compared to leaf dry weight. For all species, drench application was more effective than foliar spray in reducing leaf, stem, and top dry weight. Leaf area of ‘Spectabilis’ forsythia and ‘Nellie R. Stevens’ holly decreased with increasing rates. However, specific leaf weight was not affected. Uniconazole did not significantly affect leaf net photosynthetic rate, stomatal conductance or internal leaf CO2 concentrations in ‘Spectabilis’ forsythia or ‘Nellie R. Stevens’ holly. No phytotoxicity was observed on any species.

Cold Hardiness of Various Provenances of Flame, Roseshell, and Swamp Azaleas

Florets of eight provenances representing three native North American azalea species [Rhododendron calendulaceum (Michx.) Torr., R. prinophyllum (Small) Millais, and R. viscosum (L.) Torr.] being grown in Burlington, Vt., were compared during three seasons for cold hardiness by laboratory freezing during cold acclimation. There was a large variability in the number of florets killed within an inflorescence in response to freezing temperatures. Cold hardiness of florets of the three species ranked, from most to least hardy, were R. viscosum, R. prinophyllum, and R. calendulaceum. Some differences were noted in cold hardiness of florets of provenances, but these were not necessarily related to latitude or elevation of origin. Cold hardiness of most provenances showed a significant linear relationship with the daily mean temperature of the 3 days preceding freezing tests. Ambient temperatures just before subfreezing test temperatures may affect winter injury more than provenance differences for these species.

FLOWER BUD COLD HARDINESS OF NATIVE AZALEA ECOTYPES

Flower buds of eight ecotypes representing three native North American azalea species being grown in Burlington, Vermont were compared for cold hardiness by laboratory freezing during the cold acclimation period for three years. Species were Rhododendron calendulaceum, R. prinophyllum, and R. viscosum. There was a high variation in the number of florets killed within an inflorescens in response to freezing temperatures. There was little difference in the cold hardiness of florets of R. Pinophyllum and R. calendulaceum florets, but R. viscosum florets were hardier. Some differences were noted in cold hardiness of florets of ecotypes, but these were not necessarily related to latitude of origin. Cold hardiness showed a relationship with the daily mean temperature of the three days preceding freezing tests.

Influence of Light and Temperature on Seed Germination of Flame Azalea

Abstract Seeds of flame azalea [Rhododendron calendulaceum (Michx.) Torr.] were germinated at 25°C (77°F) and 25°/15°C (77°/59°F) with daily photoperiods of 0, ½, ½ twice daily, 1, 2, 4, 8, 12 and 24 hr. Seeds exhibited an obligate light requirement. At 25°C (77°F), increasing photoperiods increased germination with maximum germination (85%) occurring by day 12 under continuous illumination. The alternating temperature of 25°/15°C (77°/59°F) enhanced germination when light was limiting. At this temperature, maximum germination of 84 to 91% was reached by day 24 for all photoperiods ≥½ hr although at photoperiods ≥4 hr, comparable germination was noted at day 18.

Micropropagation of Flame Azalea

Shoots tips excised from an actively growing stock plant of flame azalea [Rhododendron calendulaceum (Michx.) Torr.] were surface sterilized, the terminal portions were removed (decapitated) and the shoots placed horizontally on agar-solidified Woody Plant Medium (WPM) supplemented with 15 ppm 6-(γ, γ-dimethylallylamino)-purine (2iP). Within 4 to 6 months multiple shoot formation commenced. After 2 to 3 additional months of growth, axillary shoots were excised from the original explants. The shoots were decapitated and placed on WPM. After 2 subcultures, 8-node axillary shoots were excised, decapitated and cultured on agar-solidified WPM supplemented with 0, 4, 8, 12, 16, 24, and 32 ppm 2iP. The greatest number of shoots (microcuttings) ≥ 5 mm (0.2 in) were produced at 12 ppm 2iP. Microcuttings ≥ 10 mm (0.4 in) were rooted using ex vitro procedures. Enhancement of both axillary shoot multiplication and shoot length was achieved by addition to the medium of 80 ppm adenine sulfate and 200 ppm NaH2PO4.