QBARS - v29n1 Resistance of Rhododendron Species and Hybrids to Phytophthora Root Rot

Resistance of Rhododendron Species and
Hybrids to Phytophthora Root Rot
H. A. J. Hoitink and A. F. Schmitthenner
Associate Professor and Professor, respectively,
Ohio Agricultural Research and Development Center
and The Ohio State University, Wooster, Ohio
Supported by the Cooperative Agreement No. 12-14-100-10,640(34)
from the United States Department of Agriculture

We thank Dr. A. E. Kehr, Dr. D. G. Leach, L. Pride and A. M. Shammerello for help in identification of rhododendron hybrids and species.

Abstract
Rhododendron hybrids 'Caroline', 'Professor Hugo de Vries' and 'Red Head' were most resistant to Phytophthora cinnamomi . Hybrid 'English Roseum' (Grootendorst strain) and 14 other hybrids were moderately resistant. Feeder roots of resistant hybrids were infected but new roots regenerated from the crown in the well-drained superficial soil layers. Most of the hybrids tested were susceptible. Considerable variability in resistance existed among rhododendron species, as well as among individual plants of a species. R. carolinianum was one of the most susceptible, whereas, several azalea species and R. davidsonianum , R. delavayi and R. pseudochrysanthum and some other rhododendron species were resistant. Some selections of R. ponticum and R. racemosum seedlings were resistant, others were susceptible. A total of 336 hybrids and 198 species were tested for resistance to P. cinnamomi .

Root rot is the most serious disease affecting rhododendrons in the United States and other parts of the world. Phytophthora cinnamomi is the most important species involved in this root rot syndrome (2). P. cactorum , P. citricola , P. cryptogea and P. lateralis can be involved but are of minor importance. Resistance of hybrid 'English Roseum' to P. cinnamomi also holds up against the other Phytophthora species (2). Differences in resistance among rhododendron species have been known for several decades, R. carolinianum being one of the most susceptible species and an atypical seedling form of R. ponticum was thought to be most resistant (4).

Rhododendron hybrids, traditionally, were propagated on R. ponticum root stock. During the last 20-40 years, however, hybrids have largely been propagated from cuttings and, therefore, grow on root systems of the same genotype. Many of these hybrids have attractive flowers and foliage characteristics but a poorly developed root system that is susceptible to root rot. In this study we report the resistance of rhododendron hybrids and species, azaleas and of some other ericaceous plants to Phytophthora root rot.

Materials and Methods
Source of plants and method of inoculation: Most species were obtained as seedlings or rooted cuttings from nurseries in the Pacific Northwest and the University of British Columbia, Canada. Rooted cuttings and seedlings were grown in containers in a 1:1 mixture of Canadian peat and muck (pH 6.0-7.0). One flush of growth was forced at 18-25 C in a green house. After roots reached the edge of the container mix, plants were inoculated with a 50 ml hemp broth culture of P. cinnamomi (1). Large seedlings were potted in 4, 6 or 15 liter containers and the inoculum levels for these were 2, 3 and 4 flasks (250 ml), respectively. Just before inoculation the air temperature was raised to 27-32 C. Soil temperatures varied from 22-25 C. P. cinnamomi was re-isolated from all plants that died from root rot to confirm susceptibility. Four months after inoculation, roots were examined and rated on a scale of 1-healthy, 2-moderate root rot (only feeder roots infected), 3-severe root rot (coarse roots infected), 4-crown and root rot, and 5-dead plant. Resistant hybrids and species were tested at least twice when possible. Identity of each plant was verified according to Leach (3) whenever possible.

Results and Discussion
Hybrids with the highest level of resistance are listed in Table 1. Root rot ratings of these plants ranged from 1 to 3. 'Martha Isaacson', an azaleodendron, is resistant. Resistance in this hybrid is similar to that of R. occidentale and some other deciduous azaleas that were tested. 'Caroline', the most resistant true rhododendron hybrid, is also tolerant to heat (Atlanta region) and winter hardy. 'Professor Hugo de Vries' and 'Red Head' also are resistant. The rating of 'Pink Trumpet' is based on a single test of six plants and therefore of limited significance.

Hybrids 'Disca', 'Dr. A. Blok', 'Dr. A. W. Endtz', 'Rocket' (Shamarello) and the azaleodendron, 'Broughtonii Aureum', are moderately resistant (Table 2). Crowns of some of these plants are invaded but root generation in well drained superficial soil layers occurs rapidly. 'English Roseum' (Grootendorst) is the most resistant widely used hardy rhododendron. The crowns of this hybrid may become invaded and under the most severe conditions, succumb to root rot. Resistance however, is distinctly superior to that of 'Roseum Elegans' or the two other strains of 'English Roseum'.

In Table 3, hybrids with various levels of susceptibility are listed. 'Roseum Elegans', 'Jean Marie de Montague', 'Vulcan' and 'Vulcan's Flame' are examples of more tolerant hybrids. 'Boule de Neige', 'Lee's Dark Purple', and 'Purple Splendour' are among the most susceptible hybrids.

Plants that were tested of the species listed in Table 4 are resistant to root rot with ratings ranging from 1 to 3. R. davidsonianum 'serenade', R. delavayi , a seedling of R. lapponicum , R. occidentale , R. pseudochrysanthum , and R. sanctum are the most resistant. R. pseudochrysanthum was obtained from several sources. It has a higher level of resistance than another dwarfed species, R. metternichii and selections thereof, because the crown does not become invaded. R. metternichii types survive long periods of time after inoculation, new roots are regenerated from the crown but under optimum conditions for disease development, inoculated plants succumb to disease.

Plants tested of species listed in Table 5 are moderately resistant to root rot. Some individual plants are quite resistant, others within the same species die. For example, plants of R. racemosum differ considerably in resistance. Similarly a strain of R. ponticum proved susceptible (Table 6) whereas others were moderately resistant. This confirms a previous report by White on resistance of a few atypical R. ponticum plants to P. cinnamomi (4). These resistant plants have reddish-brown stems and thicker, dark green leathery leaves as compared to the typical susceptible species and are similar to R. ponticum I (Table 5).

Plants tested of species listed in Table 6 are susceptible. R. carolinianum is one of the most susceptible species. The variability in resistance that occurs among seedlings of R. racemosum and R. ponticum also may be present in some of the species listed in Table 6. Some plants of species listed in Table 6 may be resistant. The average root rot rating of several seedlings within a species, therefore, is of limited significance.

Variability in the level of resistance found in species presented in Table 4 is small (root rot rating range 1 to 3). Screening of an additional number of seedlings of R. pseudochrysanthum and other resistant species could reveal susceptibility. Resistant plants of the species that were tested therefore, whenever possible were planted in a collection at the Rhododendron Species Foundation, Salem, Oregon.

It is quite evident that resistance to root rot can be introduced into a wide selection of commercial hybrids. However, of most species only a few plants have been tested. Results presented in this paper therefore, are of limited value.

LITERATURE CITED
1. Hoitink, Harry A. J., and A. F. Schmitt-Henner, 1969, Rhododendron wilt caused by Phytophthora citricola, Phytopathology, 59:708-709.
2. Hoitink, Harrry A. J., and A. F. Schmitt-Henner, 1974, Relative prevalence and virulence of Phytophthora species involved in rhododendron root rot, Phytopathology, 64: (In Press).
3. Leach, D. G., 1961, Rhododendrons of the world and how to grow them, Charles Scribner's & Sons, New York, 544 p.
4. White, R. P., 1936, Summary of nine year's experience with rhododendron wilt, Plant Dis. Reptr., 20:204-207.