A Resume of Rhododendron Breeders' Roundtable
August E. Kehr
Staff Scientist, Plant and Entomological Sciences
National Program Staff, Agricultural Research Service
U. S. Department of Agriculture, Beltsville, Maryland
A Rhododendron Breeder's Roundtable, as first proposed by Maurice Sumner at San Francisco, was held at the Pittsburgh meetings on May 27, 1973. It was considered by the attendees to be highly successful and useful. A total of 90 persons attended the opening session. One heard comments about the Roundtable such as: "The best thing yet in ARS," or "The high point of the Pittsburgh meetings." Enthusiasm ran high from the 9:00 a.m. start, to the 9:00 p.m. close. Some of the exciting developments and interesting findings are summarized as follows:
Dr. Harry Hoitink presented evidence that there was definitely a genetic basis for Phytophthora root rot control and demonstrated the feasibility of breeding for root rot-resistant plants. He also found tolerances in selected clones of Rhododendron racemosum, R. williamsianum, R. delavayi, R. ponticum, and in hybrids such as 'Annie Dalton' and 'Caroline'.
Dr. Robert Linderman was introduced as the new leader of the newly completed Federal nursery crop research laboratory at Corvallis, Oregon, starting his work there on June 11, 1973. He outlined the problem of Cylindrocladium on azaleas and rhododendrons.
Mr. Orlando Pride found high levels of mildew resistance in R. arborescens, R. vaseyi, the Ilams, and in some named clones such as 'Toucan', 'Coral Queen', and 'Redder Than'.
Mr. Raymond Miller reported as part of carefully monitored observations that no strawberry weevils fed on leaves of R. yakushimanum, 'Mist Maiden', 'Anna Rose Whitney', R. impeditum, R. mucronulatum, 'Bosutch', or 'Atroflo' in his badly infested garden.
Mr. Tony Shammarello felt that liquid dieldrin formulations gave best controls for strawberry root weevil, but not for the Wood's weevil, which is more serious in the Northwest. (The use of dieldrin on rhododendrons has been cancelled, but this use is under appeals.)
Mr. Orlando Pride, after growing 80,000 seedlings for 15 years, found only 'Nadine' and 'Marjorie' (evergreen azaleas) to be reliably bud-hardy, while Tony Shammarello found 'Corsage' to be most cold-resistant. Tony also reported variability within populations of R. yedoense poukhanense to cold tolerance, and isolated clones of this species that were highly bud-hardy. He stressed that, in breeding for cold-tolerance, it was desirable to select individual clones and to grow large populations in order to select the most cold-resistant.
Mr. George Miller suggested several good parents for cold-tolerance, including R. catawbiense, R. maximum, and R. brachycarpum. For hardy dwarf reds, the Gable hybrid 'Tom Thumb' should be considered, and for hardy yellows 'Maxhaem Yellow' and 'Henry Yates' (formerly 15-56).
Mr. Ross Nelson reported that he could grow most rhododendrons at San Antonio, where temperatures often reach 100 degrees F. or above, provided the plants have afternoon shade. His plants are thrifty, even though calcium deposits accumulate on the leaves, largely from water with a pH of 7.5. He finds the azalea 'Alaska' both heat- and cold-tolerant, and even has a clone of R. occidentale that thrives for him. R. carolinianum and R. chapmanii are unsuccessful. Fred Galle believes the more tolerant species in his area are R. ovatum, R. fortunei, R. maximum, R. chapmanii, R. atlanticum, and R. speciosum.
Captain Warren Berg (along with Lou Baboly and Henry Schannen) led a spirited discussion on naming rhododendrons. A problem being increasingly recognized is that too many unsuccessful clones are being named. To avoid this, a suggestion was made to register new clones under number, rather than name. No conclusions were reached.
Dr. Robert Tichnor and Mr. Ted Van Veen discussed display and test gardens. It was pointed out that if a plant is really good, a garden test will prove it. The distinction was made between test gardens and display gardens. John Ford, who operates the test garden at Wooster, Ohio, indicated some of the problems and costs of a test garden, including the need for weekly evaluation, using a specific check sheet.
Dr. Gustav Mehlquist discussed good parents in azaleas, touching particularly on the size of population needed to obtain the full range of variability. He found the cross of R. arborescens x R. bakeri a very promising one, and reported that R. japonicum was avoided by gypsy moth caterpillars. By way of indicating a need to review all pertinent literature, he remarked that the cheapest plant breeding is done in a library.
Dr. E. A. Hollowell reviewed good parents for rhododendron breeding. After citing several parents, he opened discussion on using heat to overcome self- and cross-incompatibility. Based on van Hoff's rule - every 10 degrees rise in temperatures doubles chemical reaction - he mentioned that crosses should be more likely to succeed at higher temperatures. Self-incompatibility in lilies was overcome by dipping the style (not ovary) in water at 120 degrees F, for 4 minutes. Also, in clover, self-incompatibility was overcome by raising temperatures of the plants to 40 degrees C, (104 degrees F). Dr. Harold Clarke reported that almost no seed set occurred on plants in the field at Long Beach, where temperatures in the 60's prevail during the flowering season. This observation supported Dr. Hollowell's remarks above. Mr. Orlando Pride suggested good parents such as 'Whipped Cream', 'Russell Harmon', and 'Powell Glass', along with crosses of 'Powell Glass' x R. calophytum and 'Atroflo' x R. yakushimanum. Mr. Cecil Smith gave a list of his favorite parents, including the observation that R. griersonianum will produce stronger pinks on "yak" crosses than most other species. Dr. Fred Galle mentioned crosses of R. ovatum with species such as R. hyperythrum and R. oldhamii.
Mr. Maurice Sumner gave a stimulating discussion on breeding Malesians. He feels R. laetum is a perfect yellow, and reported that R. leucogigas produced seed pods nine inches in length. He feels there is need for additional exploration, because only 75 out of 280 species are in cultivation. There is need to attempt more crosses, especially with hardy lepidote species. For example, a cross of R. johnstoneanum x 'Vireya' is reported successful.
Dr. Harold Clarke gave a complete and thorough evaluation of objectives of a breeding program. He touched on weevil damage ('Blue Ensign' is resistant), numbering systems, display gardens, seed exchange, fragrance, time of flowering, lasting quality of flowers, fragrance of leaves, ease of propagating, general adaptability (Syd Burns labels adaptable plants "good doers.") Ted Van Veen indicated he had prepared an evaluation checklist (it will be included in the proceedings). Among the most needed rhododendrons were an early, low-growing red and late-flowering yellows.
Mr. George Ring gave a review of many excellent techniques, including growing seedlings and storing pollen. It was suggested that ARS establish a pollen bank to aid breeders. Other techniques included devices for holding pollen capsules and pit greenhouses by Dr. Hollowell, ultrasonic generators for speeding up seed germination, and many others.
Mrs. Jane Goodrich, Secretary of the Potomac Valley Chapter, took copious notes throughout the meeting and recorded the entire proceedings. She will type these notes and tapes into a form for publication as approved by the Board of Directors. It is probable that the published proceedings will be by offset printing in a booklet the same size as the Bulletin.
A second Rhododendron Breeder's Roundtable was tentatively planned for next year at Portland. A wide range of subjects to be discussed at the next meeting was suggested by the Roundtable participants.