Hybridization Between Evergreen and Deciduous Azaleas
Robert L. Pryor
Research Horticulturist, U. S. National Arboretum
Agricultural Research Service
U. S. Department of Agriculture, Washington, D.C.
The subgenus Anthodendron (azaleas) of the genus Rhododendron contains both evergreen (Sect. Tsutsusi) and deciduous species (Sect. Pentanthera, Sect. Rhodora, and Sect. Sciadorhodion). Wilson and Rehder (1921) have recorded natural (garden) hybrids between species of Pentanthera and Rhodora, as well as hybrids (azaleodendrons between evergreen rhododendrons (subgenus Eurhododendron) and deciduous azaleas of Sect. Pentanthera. More recently, Martin (1970) has produced new azaleodendrons by controlled crosses of evergreen rhododendrons with evergreen azaleas (Sect. Tsutsusi). All of the above natural or controlled crosses have given rise to at least some normal-sized, vigorous plants, However, no really worthwhile plants have been reported from crosses between evergreen and deciduous azaleas. Lee (1958) stated that "attempts to cross between the Obtusum (=Sect. Tsutsusi) and Luteum (=Sect. Pentanthera) subseries have given seed of low germination, the resulting progeny consisting of a high percentage of albinos that die and a few runts that also die without flowering, or at least without setting viable seed." Kehr (1966), however, has produced hybrids between the deciduous tetraploid species R. calendulaceum (Michx.) Torr. and several tetraploid forms of evergreen cultivars. The present study is a report on the successful hybridization of diploid evergreen and deciduous azaleas over the period 1951-60.
These intersectional crosses were undertaken as a part of a large-scale U. S. D. A. azalea breeding program at Beltsville that resulted in the introduction of 67 new cultivars (intra-sectional) between 1950 and 1966. The major goal of intersectional hybridization was the creation of yellow-flowered evergreen azaleas. Although pedigrees and parentages of the crosses were accurately maintained during this work, very little data on seed set, germination, and progeny characteristics were recorded. In retrospect, perhaps it was believed that the desired goal could be achieved rather easily by repeated backcrossing of the first-generation hybrids with the yellow-flowered deciduous parent. Unfortunately, such was not the case, and much potentially valuable crossing data have been lost. However, in view of the recent reactivation of azalea breeding at the U. S. National Arboretum, in which we are using many of the plants derived from this early work, we should present some information on the evergreen x deciduous azaleas.
The evergreen azaleas used in the successful crosses were R. kaempferi Planch., several Kurume cultivars, and the Belgian-Indian 'Pink Macrantha'. The yellow-flowered deciduous parents included many un-named Mollis-type seedlings and the Mollis cultivars 'Hortulanus H. Witte', and 'Anthony Koster'. Many of the parent plants listed in the tables are identified only by pedigree number because an explanation of the often complex parentage would tend to detract from the major intent of the tabular data.
Inheritance of Evergreen Leaves
All but one of the successful intersectional crosses used the evergreen parent as the female-and all progeny (several hundred plants) had persistent leaves. The single successful cross using a deciduous female parent gave six hybrid seedlings, all of which had deciduous leaves and were sterile. Such deciduous plants, although they lose leaves when grown outdoors, may retain their leaves under greenhouse conditions. Thus, at least in the first generation crosses, the inheritance of leaf type appeared to be maternal. When the fertile evergreen x deciduous hybrids were backcrossed to deciduous males, some segregation of leaf type was noted (table 1). The data in the table would seem to indicate that both parents may be contributing genes influencing leaf persistence, but because of small numbers in any progeny group and the lack of suitable test crosses, no hypothesis can be offered at this time.
Table 1. Backcross segregation of evergreen and deciduous leaves - 1960. PARENTAGE NUMBER OF PROGENY (Evergreen x Deciduous) x Deciduous Deciduous Evergreen 1012 x Mollis No. 20 4 9 1012 x Mollis No. 4 4 0 58-5 x Mollis No. 1 4 2 58-5 x 'Hortulanus H. Witte' 3 7 58-2 x Mollis No. 20 2 2 445 x 'Anthony Koster' 1 4
Many of these crosses were repeated in different years, and the results were remarkably similar. The only F2 cross gave only green seedlings.
Evidence of Hybridity
Because of the genetic complexity of most of the parent plants used in the breeding program and the apparent maternal inheritance encountered in first generation hybrids, these hybrids were virtually impossible to distinguish on a morphological basis. However, the segregation of deciduous and evergreen plants in backcrosses is sufficient proof that hybridity was achieved.
Albinos are a common occurrence among the progenies of F1 evergreen x deciduous crosses and in the backcross of these evergreen F1 hybrids to deciduous plants used as male parents. A representative sampling of the segregation of albino vs. green seedlings is given in table 2.
Table 2. Albinism in various hybrid progenies of evergreen x deciduous azaleas. ALBINO GREEN Evergreen x Deciduous (Fl) Number Number 429 x 'Hortulanus H. Witte' 104 10 639 x 'Hortulanus H. Witte' 11 39 770 x 'Hortulanus H. Witte' 28 10 Evergreen x Deciduous (F2) 445 x 448 0 30 (Evergreen x Deciduous) x Deciduous 445 x 'Hortulanus H. Witte' 63 0 445 x Mollis No. 2 118 2 445 x 'Anthony Koster' 191 101
The results of this hybridization program have shown that crosses between evergreen and deciduous azaleas are possible and that some such hybrids are vigorous and fertile. However, up to the present, no hybrids have proved so superior to their parents or to cultivars in the trade that they have been selected and named. The goal of developing a yellow-flowered evergreen azalea has not yet been attained. The amount of yellow pigmentation in the standard and wing petals of otherwise white flowers has been increased, and some hybrids are more cream colored than white. But even the best of the evergreen hybrids do not begin to approach the yellow pigmentation found in the deciduous azaleas.
New research toward creating a yellow-flowered evergreen azalea is being carried on at the U. S. National Arboretum, based on a limited understanding of floral pigments (Santamour and Pryor, 1973) We hope that this new approach will be successful.
Kehr, August E., 1966, Breeding for a purpose. Quart. Bull. Amer. Rhod. Soc., 20(3) 130-141.
Lee, Frederic P., 1958, The Azalea Book, Van Nostrand, 324 pp.
Martin, Alexander C., 1970, Breeding azaleodendrons, Quart. Bull. Amer. Rhod. Soc., 24(1): 39-41.;
Santamour, Frank S., Jr. and Robert L. Pryor, 1973, Flower pigments in Rhododendron: A review for breeders., Quart. Bull. Amer. Rhod. Soc., 27 (4) 214-219.
Wilson, Ernest Henry and Alfred Rehder, 1921, A monograph of azaleas (Rhododendron subgenus Anthodendron), Pub. Arnold Arb., No. 9, 219 pp.