Science 93(2411): 260-261 (Mar 14, 1941)
BREEDING A DISEASE-RESISTANT RED CLIMBING ROSE1
H. R. Rosen

UNIVERSITY OF ARKANSAS
COLLEGE OF AGRICULTURE, FAYETTEVILLE

1Published with the approval of the Director of the Arkansas Agricultural Experiment
Station as Research Paper No. 689, Journal Series, University of Arkansas.

NOTWITHSTANDING the great popularity of "everblooming" small, bush roses, climbing roses have a greater range of usefulness in making the roadside, home and its adjoining grounds more attractive. The chief hindrance to a greater utilization of climbing roses has been a lack of disease resistance, although many climbers are far more tolerant to disease than small bush roses as a group.

Two of the most common diseases afflicting climbers over large areas in the United States are powdery mildew and low-temperature injury. (The parasitic disease, black spot, is not so destructive on most climbers as it is on small, bush varieties.) Of the two serious diseases, powdery mildew is frequently so abundant on Dorothy Perkins, Crimson Rambler, Excelsa and other varieties having small blossoms in large clusters (multiflora types), that their usefulness as decorative plants is seriously impaired. Thus, while this type of climber possesses a considerable degree of hardiness and remarkable beauty when free from powdery mildew, the necessity of frequent dusting or spraying with fungicides to control mildew imposes a task which often is not very successful.

Contrasted with the multiflora-type of climbers, those with large blossoms generally show a greater freedom from powdery mildew but are more subject to low-temperature injury. Thus among large-blossomed, red climbers, Paul's Scarlet Climber, Dr. Huey and Climbing American Beauty, three of the hardiest and best known red climbers, frequently suffer considerable injury either from low winter temperatures or from late spring frosts, unless they are partly protected by a building or by covers.

The problem of breeding a red climbing rose to combine disease resistance with good blooming qualities has resolved itself largely in attempts to find disease-resistant parents. Other qualities are not difficult to secure. To find such parents, the writer has investigated many different varieties and species of roses, including our own wild roses. In the latter group, two species are outstanding for vigor, hardiness and tolerance of heat and drought in Arkansas. They are Rosa setigera and R. Eglanteria (R. rubiginosa). In addition, both of these species possess marked abilities to escape from attacks of powdery mildew, although when artificially inoculated, they become subject to infection. Rosa setigera has also shown a fair degree of resistance to black spot in some individuals but not in others, while R. Eglanteria appears to be uniformly susceptible to this disease.

Among the plants under observation for disease resistance were 100-odd hybrids, very kindly sent by the pioneer rose breeder, M. H. Horvath. This group largely represents crosses between R. setigera and various cultivated varieties, and four of them were found to combine resistance to or escape from both powdery mildew and black spot. Although not outstanding in floral qualities, they are very hardy and possess much vigor. The most disease resistant of these is a pink-blossomed climber, which unfortunately was received without designation of parentage or name or number (writer's acc. No. 31). It is probably a hybrid of Rosa setigera and offered, for breeding purposes, a distinct advantage over the original wild parent. Its blossoms possess some 30 attractive petals. Its main disadvantage is that it is not homozygous for resistance to either black spot or to low-temperature injury, since selfings show much variation in these characters.

Utilizing No. 31 as the female parent, and Black Knight, an exceedingly beautiful red Hybrid Tea, "everblooming" small bush variety, as the male parent, a climber has been obtained which combines, under Arkansas conditions, mildew-escaping qualities and hardiness with attractive red blossoms. However, like most large-blossomed climbers, it is not completely immune to mildew. While it has escaped this disease under natural conditions when nearby, highly susceptible varieties were severely infected, this hybrid showed some susceptibility when it was artificially inoculated. It is not resistant to black spot, although the effects of this disease are no more severe than on such varieties as Mary Wallace, Paul's Scarlet Climber, Dr. W. Van Fleet and Dr. Huey. Its fragrant blossoms are of a brilliant red color, approaching Ridgway's Tyrian Rose or Maerz and Paul's Plate 1 D6, the red being replaced by white at the extreme base of the petals. The stamens are very numerous and showy. The average blossom is large, around 3 1/2 to 4 inches in diameter, and is borne singly or in small clusters on stems 4 to 6 inches long. Most of the blooms possess 20 large, recurved petals. The blossom buds are medium large, ovoid acute and of Hybrid Tea form.

The plant has bloomed only in May under Fayetteville, Ark., climatic conditions. In more northern latitudes it may be expected to bloom in June or July.

In addition to showing relative freedom from mildew and from low temperature injury, it possesses considerable vigor and at least a fair degree of tolerance to heat and drought.

During the 1938 and 1939 growing seasons, both characterized by extreme heat and drought at Fayetteville from July on through the summer, this hybrid made satisfactory growth and kept most of its leaves when many other varieties were badly defoliated. It passed through the severe 1939-40 winter with very little injury when Paul's Scarlet Climber, growing close by, lost approximately 50 per cent, of its wood and less hardy varieties were killed to the ground level. Albertine, one of the hardiest of climbers, lost all its canes this winter (lowest temperature -7° F.). Aside from its resistance to low-temperature injury when in a dormant condition, it withstood April freezes, both in 1939 and in 1940, when many varieties, including this hybrid, were full of new, tender growth and when such varieties as Black Boy, Countess of Stradbroke, Souv. Claudius Denoyel, Kitty Kinninmonth and many others were severely injured.

This new hybrid, which is to be made available through responsible nurserymen, is named Stephen Foster, after America's beloved song writer.