QBARS - v34n4 Resistance of Evergreen Hybrid Azaleas to Root Rot

Resistance of Evergreen Hybrid Azaleas to Root Rot Caused by Phytophthora cinnamomi
D. M. Benson, Associate Professor
Department of Plant Pathology
F. D. Cochran, Professor Emeritus
Department of Horticultural Science
North Carolina State University, Raleigh 27607
Reprinted from Plant Disease, Vol. 64 #2, with permission of The American Phytopathological Society

BENSON, D. M., and F. D. COCHRAN. 1980. Resistance of evergreen hybrid azaleas to root rot caused by Phytophthora cinnamomi. Plant Disease 64:214-215.
Of 73 evergreen azalea cultivars in 10 hybrid groups evaluated for resistance to root rot caused by Phytophthora cinnamomi , 20 were rated resistant, 25 were rated moderately resistant, and 28 had severe root rot. The hybrid groups, in order of increasing susceptibility, were Indian, Rutherford, Pericat, Glen Dale, Whitewater, Satsuki, Back Acres, Gable, Kurume, and NCSU.
Root rot caused by Phytophthora cinnamomi is a major disease of evergreen hybrid azaleas (Rhododendron spp.) in nurseries and landscape plantings in North Carolina (1). Symptoms include chlorosis of the foliage, stunting, wilting, general low vigor, and root necrosis. Infected plants in landscape plantings may decline over a period of years or develop a "sudden death" syndrome. At present, no single control measure is available for P. cinnamomi ; rather, disease prevention through sanitary, cultural (5), and chemical measures (1) is recommended. Azalea breeders select cultivars for early season flowering, flower color, growth habit, and winter hardiness and pay little attention to disease resistance (6). Although information is not available on cultivar resistance to Phytophthora root rot, Hoitink and Schmitthenner (4) reported on resistance of hybrid rhododendrons to P. cinnamomi . We evaluated the resistance of hardy-and forcing-type evergreen azaleas in the Back Acres, Gable, Glenn Dale, Indian, Kurume, Pericat, Rutherford, Satsuki, and Whitewater hybrid groups and in the newly released NCSU hybrids.

Azalea cultivars were propagated from specimen plants at the Horticulture Farm, North Carolina State University, Raleigh, and from stock plants maintained in the Department of Plant Pathology. Plants (7-10 mo old) grown in peat-perlite (1:1 v/v) were transplanted into 15-cm diameter pots containing sand-soil-peat (1:1:1, v/v) with 3.8 kg/m3 lime and superphosphate. Final pH was 5.0. Plants were arranged in a randomized complete block design in the greenhouse. A 217-7 (N-P-K) liquid fertilizer (Robert B. Peters, Allentown, PA) at a rate of 1.8 1A g/ml was applied biweekly with a hose-on proportionator.
One isolate of P. cinnamomi (A2 mating type) from rhododendron in Ohio (supplied by H. A. J. Hoitink) and five isolates including 101 (1) from rhododendron and azalea in North Carolina (all AZ mating type) were used as inoculum. Isolates were cultured singly for 30 days on autoclaved oat grains, then combined and mixed by hand. Three weeks after plants were transplanted, 30 gr of inoculum was placed in each of two holes around the edge of the plant root ball at a depth of 2-4 cm below the soil surface.
All pots were placed in saucers containing water to maintain moisture in the potting medium near field capacity. After 4 mo, root rot severity was determined by assigning a rating to each plant: 1 = healthy roots, 2 = fine roots necrotic, 3 = coarse roots necrotic, 4 = crown rot, and 5 = dead plant (3). Cultivars usually were replicated 10 or 20 times in inoculated treatments and five times in non-inoculated treatments. Experiments were repeated three times, but not all cultivars were available for each experiment.

No azalea cultivar tested was immune from infection by P. cinnamomi (Table 1). Hoitink and Schmitthenner (4) reported similar results for hybrid rhododendron. Twenty of the cultivars (27%) were rated resistant (1.832.35). These included Formosa, Fakir, and Corrine Murrah; Formosa, an Indian hybrid, grows quite well in wet planting sites that otherwise would favor root rot development but is not very hardy to cold. The non-inoculated cultivars always had root rot ratings of less than 1.5. Twenty-five of the cultivars (34%) were rated moderately resistant (2.4-2.9); on poorly drained planting sites, these cultivars would probably show decline symptoms if infection occurred. Twenty-eight of the cultivars (38%) had severe root rot, i.e., ratings of 3 or higher; even on well drained planting sites, these cultivars would, probably show decline symptoms.

Table 1. Resistance of azalea cultivars to root rot
caused by Phytophthora cinnamomi
Cultivar Hybrid
groups a
Root rot
rating b
'Formosa' I 40 1.83 a
'Fakir' GD 10 1.90 ab
'Corrine Murrah' BA 20 1.90 ab
'Merlin' GD 20 2.05 ab
'Hampton Beauty' P 20 2.05 ab
'Higasa' S 10 2.10 abc
'Glacier' GD 10 2.10 abc
'Rose Greeley' G 20 2.15 abc
'Polar Seas' GD 20 2.15 abc
'Redwing' I 30 2.20 abcd
'Chimes' I 20 2.20 abcd
'Alaska' R 20 2.20 abcd
'New White' I 15 2.27 abcde
'Shin-ki-gen' S 20 2.30 abcde
'Rachel Cunningham' BA 10 2.30 abcde
'Pink Gumpo' S 10 2.30 abcde
'Eikan' S 10 2.30 abcde
'Sweetheart Supreme' P 20 2.35 abcde
'Pink Supreme' I 20 2.35 abcde
'Morning Glow' K 20 2.35 abcde
'Barbara Gail' P 20 2.40 abcdef
'White Gumpo' S 20 2.40 abcdef
'Rentschler's Rose' W 20 2.45 abcdef
'Dorothy Gish' R 20 2.45 abcdef
'White Gish' R 20 2.50 abcdefg
'ink Hiawatha' P 20 2.50 abcdefg
'Margaret Douglas' BA 2 2.50 abcdefg
'Gaiety' GD 10 2.50 abcdefg
'Gloria' R 20 2.55 abcdefg
'Kingfisher' W 19 2.56 abcdefgh
'White Christmas' W 20 2.60 abcdefgh
'Sensation' P 20 2.60 abcdefgh
'Prince of Orange' I 10 2.60 abcdefgh
'White Jade' BA 20 2.70 bcdefgh
'Copperman' GD 20 2.70 bcdefgh
'Hexe' K 30 2.73 bcdefgh
'Massasoit' K 10 2.80 bcdefghi
'Martha Hitchcock' GD 10 2.80 bcdefghi
'China Seas' G 20 2.80 cdefghi
'Warbler' W 20 2.85 cdefghi
'California Sunset' I 20 2.85 cdefghi
'Amaghasa' S 20 2.85 cdefghi
'Pride of Summerville' I 20 2.90 cdefghi
'Hinodegiri' K 20 2.90 cdefghi
'Flanders Field' P 20 2.90 cdefghi
'Robinhood' GD 10 3.00 cdefghij
'Hershey Red' K 20 3.00 cdefghij
'Herbert' G 10 3.00 cdefghij
'Fortune' P 20 3.00 cdefghij
'Catawba' GD 10 3.00 cdefghij
'Marian Lee' BA 20 3.05 defghij
'Snow' K 40 3.08 defghij
'Royalty' G 10 3.10 defghijk
'Kow-ko-ku' S 20 3.15 efghijk
'Rosebud' G 10 3.20 efghijk
'Mrs. G. G. Gerbing' I 20 3.20 efghijk
'Coral Bells' K 10 3.20 efghijkl
'Treasure' GD 10 3.30 fghijkl
'Pat Kraft' BA 20 3.30 ghijkl
'Saint James' BA 10 3.40 ghijkl
'Carror' N 9 3.44 ghijkl
'Purple Spendour' G 20 3.50 hijkl
'Pinocchio' GD 10 3.50 hijkl
'General MacArthur' K 10 3.50 hijkl
'Pink Pearl' K 10 3.60 ijkl
'Johga' S 10 3.70 ijklm
'Sunglow' N 10 3.80 jklm
'Hino Crimson' K 10 3.90 klm
'Elaine' N 10 4.10 lmn
'Emily' N 8 4.25 lmn
'Pink Cloud' N 4 4.50 imn
'Adelaide Pope' N 10 4.60 mn
'Jane Spaulding' N 7 5.00 n
a BA = Back Acres, G = Gable, GD = Glenn Dale, I = Indian, K = Kurume, N = NCSU, P = Pericat, R = Rutherford, S = Satsuki, W = Whitewater.
b 1 = healthy roots, 2 = fine roots necrotic, 3 = coarse roots necrotic, 4 = crown rot, 5 = dead plant. Ratings followed by the same letter are not significantly different (P = 0.05).

Table 2. Evergreen azalea hybrid group response
to root rot caused by Phytophthora cinnamomi
Hybrid group Cultivars
Root rot rating
Indian 9 195 2.41 a
Rutherford 4 80 2.43 ab
Pericat 7 140 2.54 ab
Glenn Dale 11 140 2.56 ab
Whitewater 4 79 2.62 abc
Satsuki 8 120 2.66 abc
Back Acres 7 120 2.71 be
Gable 6 90 2.91 cd
Kurume 10 180 3.00 d
NCSU 7 58 4.19 c
1 = healthy roots, 2 = fine roots necrotic, 3 = coarse roots necrotic,
4 = crown rot, 5 = dead plant. Ratings followed by the same letter are not significantly different (P=0.05)

When root rot ratings were averaged according to hybrid group, all groups were moderately resistant (2.4-2.9) except the Kurume and NCSU hybrids, which had severe root rot (Table 2). The Kurume hybrids, which include Coral Bells, Hinodegiri, and Snow, are popular among nurserymen because of early-season flowering and winter hardiness. The susceptibility of the Kurume and NCSU hybrids to P. cinnamomi , however, indicates a need to include the more resistant cultivars in other hybrid groups in the nursery trade.
Hoitink and Schmitthenner (4) found that 94% of the rhododendron hybrids inoculated (more than 320 hybrids) with P. cinnamomi developed severe root rot. Although we tested only 73 of the more than 4,000 described azalea cultivars (6), our results suggest that azalea cultivars vary in susceptibility toward P. cinnamomi and that azaleas may be more resistant to Phytophthora root rot than rhododendrons.
We tested two azalea species, Rhododendron poukhanense Leveille (rating = 1-6) and R. mucronatum 'Delaware Valley White' (rating = 2.4). Hoitink and Schmitthenner (4) rated R. poukhanense 1.8, using a different type of inoculum (hemp broth cultures). R. poukhanense is a vigorous growing and very hardy evergreen plant with good flower qualities (2). Attempts to find more resistant germ plasm among azaleas should concentrate at the species level and then incorporate resistance into acceptable horticultural cultivars.

We thank Billy I. Daughtry for technical assistance.

1. Benson, D. M., 1979, Efficacy and in vitro activity of two systemic acylalanines and ethazole for control of Phytophthora cinnamomi root rot of azalea, Phytopathology, 69:174178.
2. Dirr, M. A., Friedhoff, E.and Smith, T., 1978, Plant hardiness evaluations, Am. Nurseryman, 148:12.
3. Hoitink, H. A. J., and Schmitthen-ner, A. F., 1969, Rhododendron wilt caused by Phytophthora citricola , Phytopathology, 59:708-709.
4. Hoitink, H. A. J., and Schmitthen-ner, A. F., 1974, Resistance of rhododendron species and hybrids to Phytophthora root rot, Plant Dis. Rep., 58:650-653.
5. Jones R. K., and Benson, D. M.,. 1978, Spread of Phytophthora cinnamomi in container-grown azaleas, Proc. SNA Res. Conf., 23:100-101.
6. Lee, F. P., 1965,. The Azalea Book, 2nd ed., Van Nostrand, Princeton, N.J., 435 pp.