QBARS - v31n4 Azaleodendron Breeding

Azaleodendron Breeding
August E. Kehr
From a paper given at the 1977 Breeder's Roundtable Eugene, Oregon

The first successful hybrid ever recorded in rhododendrons is said to have been an azaleodendron. This hybrid was a natural cross between Rhododendron nudiflorum x R. ponticum . Although it has little to recommend it as a plant to include in most gardens, this cross still persists in the trade as 'Azaleoides.'
Of the approximately 7,200 hybrids named, only about 20 or a very small fraction of 1 percent are azaleodendrons. >From these statistics one may conclude that this method of breeding has an extremely low probability of successful results. In my opinion, a serious hybridizer should avoid such wide crosses, and confine his efforts to hybridizing within one of the five classes of rhododendrons, i.e., deciduous azaleas, evergreen azaleas, non-scaly rhododendrons, Malesians, and scaly rhododendrons (except Malesians).

Fig. 1.  Crossability of five classes of rhododendron and other genera. Solid lines indicate successful crosses can be made with difficulty, broken lines indicate rare successes. Arrows point to female or seed parent in all cases.

In figure 1 is shown the crossability of the five classes of rhododendrons, with the arrows indicating the direction of pollen movement-arrows point to female or seed parent in all cases. The solid lines indicate some hybrids have been successfully made, but with difficulty, while broken lines indicate the successful hybrid is an exceptional case and instances of successful hybridizations are rare. With few exceptions, unless the parents used are both tetraploids, azaleodendrons are completely sterile. I will mention the exceptions later in this talk.
To my knowledge, no successful crosses have been made to date between the Malesian rhododendrons and any of the other four classes. Many attempts have been made; all of which have proven failures. Some hybrids involving Malesians have been reported prior to 1900, but none of these reports have been verified, and almost certainly were not inter-class hybrids.
In searching the available literature, I could find few instances of true interclass hybrids in which an evergreen azalea was the seed or female parent. In the chart following you will find one example of a cross in which an evergreen azalea was a successful seed parent, but this one exception is at least a partial backcross involving some evergreen germ plasm.
As a matter of interest, figure 1 also shows some reported crosses of rhododendrons to plants outside the genus. Like azaleodendrons, time expended in such hybridization is counter productive. Despite this, there are some bona fide reasons in special cases to attempt azaleodendron breeding. I would like to list these reasons as follows:
1) To transfer desirable genetic characteristics from one class to another class of rhododendrons,
2) To study taxonomic and evolutionary relationships and associations,
3) For the fun of doing it.
Of these reasons, the first two represent serious and worthy research, while the latter is exactly the reason most of us get involved in hybridization in the first place.
I should now at this point like to describe my own attempts to transfer desirable genetic characteristics from deciduous azaleas to evergreen azaleas.
As you know there are no true yellow evergreen azaleas. Those approaching yellow are really ivory whites. These ivory whites contain a water soluble pigment, probably a flavone. Attempts to develop higher levels of yellow color by intensifying these water soluble pigments have failed, and this method is therefore not a promising one.
On the other hand bright yellow, but water insoluble, pigments are known to occur in the flowers of species found in each of the other four classes of rhododendrons. For several years I have worked at transferring the water insoluble pigments from deciduous azaleas to evergreen azaleas. As parental stock I am using all tetraploids, including R. calendulaceum 'Colossus' as the source of the bright yellow pigments; and 'Tahei' and 'Gettsu-toku' as the evergreen azalea parents.
Crosses between R. calendulaceum 'Colossus' (seed parent) and Tahei or Gettsu-toku are easy to make and one obtains large quantities of viable seed. The seedlings appear normal, but are very weak. Because of their lack of vigor, seedling mortality is high and I have succeeded in flowering very few hybrids.
Despite this, all first generation (F _l ) hybrids obtained to date have been highly fertile, and can be used as either pollen parent or seed parent. The F _l hybrids cross readily to the deciduous parent, but to date I have not succeeded in crossing the F _l back to the evergreen parent. However, in the last year I have obtained many seedlings with the following pedigrees:
'Tahei' x [ calendulaceum x ( calendulaceum x 'Tahei')]
'Gettsu toku' x [ calendulaceum x ( calendulaceum x 'Tahei')]
These seedlings (now just forming true leaves) offer my best prospects to date, to transfer the yellow water insoluble pigments from deciduous azaleas to evergreen azaleas - a program in which I have worked since about 1965.
If anyone is interested in pursuing the project of developing a yellow flowered evergreen azalea, I can suggest other approaches than the one I have outlined. As mentioned above, yellow-flowered species occur in all the other four classes. Consequently, these yellow characteristics could potentially be transferred. My own program deals only with the transfer from deciduous azaleas. There still remains the possibility of transfer from 1) the Malesians, 2) the scaly rhododendrons, and 3) the non-scaly rhododendrons. Of these, the most exciting is the transfer from the Malesian class. In addition, a cross between R. keiskei and a Satsuki evergreen azalea has been reported.
In preparing for this talk, I have prepared 12 tables showing details on all the azaleodendrons which I could locate. These 12 tables are attached. Undoubtedly I have overlooked many additional azaleodendrons, and I will welcome additions to the list at any time, or information in instances of unknown parentages or unknown developers.
In studying these tables there are several interesting observations. First is that R. occidentale appears to accept pollen quite readily from both evergreen azaleas and non-scaly rhododendrons. One such hybrid named 'Martha Isaacson' has been quite successful and has been distributed quite widely by nurserymen.
A second observation is that crosses between non-scaly rhododendrons and deciduous azaleas succeed rather frequently, although such hybrids most commonly are lacking in vigor. Surprisingly these are exceptions which are vigorous and have proven quite successful to the point of being handled by commercial nurserymen in the United States. Chief, among these are 'Broughtonii aureum' and 'Glory of Littleworth.' Without exception all are completely sterile to the best of my knowledge.
Another observation is that R. racemosum crosses rather readily with evergreen azaleas. Several of these crosses have been named and are exceedingly successful plants.
As a general rule, azaleodendrons are completely sterile. The one exception I know of are those in which both parents are tetraploid, the other exception being the hybrid of R. carolinianum x R. roseum , named 'Carolina Rose.' The latter is fully fertile. If one considers the chromosome numbers of the two parents to be diploid as reported in the literature, the resultant F _l hybrid would be expected to have one single set of chromosomes from carolinianum and one single set from roseum . If so, I would certainly have anticipated that the hybrid would have been sterile in exactly the same manner as are all other azaleodendrons involving diploid parents. As a geneticist, the fertility of this azaleodendron is difficult to explain.
Finally, the cross between Ledum and 'Elizabeth' which resulted in the clone now sold under the name of 'Brilliant, appears to resemble more a miniature 'Elizabeth' It is suggested that 'Brilliant' may be a haploid plant which arose from the functioning of the pollen grain from 'Elizabeth' to form a plant known as an androgenetic haploid-a plant with the chromosome set from only the male parent. Such haploids are rare, but have been reported occasionally in other plants. This hypothesis can be resolved only by a count of the chromosomes found in 'Brilliant,' and would represent a nice study for a graduate student.