nysipm.cornell.edu 2019 Search for this title at the NYSIPM Publications collection: ecommons.cornell.edu/handle/1813/41246 Disease and Insect Resistant Ornamental Plants Mary Thurn, Elizabeth Lamb, and Brian Eshenaur State Integrated Pest Management Program, Cornell University

Rhododendron

Rhodendron pixabay.com

Rhododendron is a large genus, which includes and azaleas, of more than 1,000 species of and small trees in the family Ericaceae. Prized for their glossy foliage and showy clusters of blooms, Rhododendron spp. may be evergreen or deciduous and grow in a wide range of conditions. Hybridization has resulted in thousands of with various foliage and flower charac- teristics, as well as improved hardiness. There are rhododendrons and azaleas for just about every landscape situation. The American Rhododen- dron Society maintains an extensive database of ornamental characteristics and other plant infor- mation at Rhododendron Plant Search Options. Rhododendrons have many potential disease and insect problems, including root rot, powdery mildew, rust, petal blight, lace bugs, Japanese beetle and vine weevil. DISEASES Root Rot, caused by various soil-borne Phytophthora spp., is a water mold disease that affects many ornamentals, particularly in sites with poor drainage. Favored by high soil moisture and warm soil temperatures, the disease kills roots which disrupts movement of water and nutrients eventually re- sulting in wilt and death of the plant. Water and site management are key to prevention–avoid heavy, poorly drained soils and overwatering.

Phytophthora root rot of Rhododendron is usually caused by P. cinnamomi (8, 23). Most commercially available rhododendrons and azaleas are susceptible (17).

PHYTOPHTHORA ROOT ROT Reference Species/Hybrids Resistant Intermediate Susceptible R. aberconwayi 12 R. carolinianum 12 R. charitopes 12 PHYTOPHTHORA ROOT ROT Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible R. ciliatum 12 R. davidsonianum 17 R. davidsonianum Serenade 12 R. delavayi 12 R. glomerulatum 12 R. hemitrichotum 12 R. hyperythrum 12, 15* R. lapponicum 12 R. nitens 12 R. occidentale 12 R. oldhamii 12 R. ponticum 1 12 R. ponticum 11 12 R. poukhanense 12, 17 R. pseudochrysanthum 12, 17 R. quinquefolium 12 R. racemosum 12 R. rigidum 12 R. sanctum 12 R. schlippenbachii 12 R. serpyllifolium 12 R. shweliense 12 R. simiarum 12 R. simsii 12 R. spiciferum 12 R. websterianum 12 R. yunnanense 12 Rhododendron x Adelaide Pope 3 3 Amaghasa 3 Anna H. Hall 16 Arctic Pearl 16 Azurro 16 Bali 16 Barbara Gail 3 Besse Howells 16 Bikini Island 16 Blutopia 16 Bosley Dexter 1020 12 PHYTOPHTHORA ROOT ROT Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible Rhododendron x Brickdust 12 Brittany 16 Broughtonii Aureum 12 Sunset 3 Capistrano 16 Caroline 9, 12, 16, 17 Carror 3 Catawba 3 Chimes 3 China Seas 3 Chionoides 16 Copperman 3 Coral Bells 3 Corrine Murrah 3, 17 Crete 16 Disca 12 Dorothy Gish 3 Dr. Arnold W. Endtz 12 Dr. A. Blok 12 Edith Bosely 16 Eikan 3 Elaine 3 Elviira 16 Emily 3 English Roseum (Groo- 12 tendorst) Fakir 3, 17 Fantastica 16 Firestorm 16 Flanders Field 3 Formosa 3, 17 Fortune 3 Gaiety 3 General MacArthur 3 Ginny Gee 16 Glacier 3 Gloria 3 Goldbukett 16 Golden Gala 16 PHYTOPHTHORA ROOT ROT Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible Rhododendron x Haaga 16 Hachmann’s Charmant 16 Hachmann’s Polaris 16 Hampton Beauty 3 16 Herbert 3 Hershey Red 3 Hexe 3 Higasa 3 Hino Crimson 3 Hinodegiri 3 Hudson Bay 16 Ingrid Mehlquist 16 Jane Spaulding 3 Janet Blair 16 Johga 3 Kalinka 16 Kingfisher 3 Kow-ko-ku 3 Lavender Princess 16 Lavendula 16 Lee’s Dark Purple 16 Lucky Strike 12 Madame Carvalho 12 Manitou 16 Marian Lee 3 Margaret Douglas 3 Martha Hitchcock 3 Martha Isaacson 12 Massasoit 3 Merlin 3 Mikkeli 16 Morning Glow 3 Mrs. A.T. de la Mare 12 Mrs. C.B. Van Nes 12 Mrs. G.G. Gerbing 3 New White 3 Nicoletta 16 Normandy 16 PHYTOPHTHORA ROOT ROT Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible Rhododendron x Northern Starburst 16 Nova Zembla 16 Oudijk’s Sensation 16 Party Pink 16 Pat Kraft 3 Percy Wiseman 16 Peter Tigerstedt 16 Pink Cloud 3 Pink Gumpo 3 Pink Hiawatha 3 Pink Pearl 3 Pink Supreme 3 Pinocchio 3 PJM Compact Form 16 PJM Elite 16 Polar Seas 3 Pride of Summerville 3 Prince of 3 Prize 12 Professor Hugo de 12, 17 Vries Purple Splendor 3 Queen Alice 16 Rachel Cunningham 3 Rangoon 16 Red Head 12, 17 Redwing 3 Rentschler’s 3 Rio 16 Robinhood 3 Rocket (Shammarello) 16 12 Rosebud 3 Rose Greeley 3 Roseum Elegans 16 Royalty 3 Saint James 3 Samoa 16 Scarlet Romance 16 Sensation 3 PHYTOPHTHORA ROOT ROT Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible Rhododendron x Shin-ki-gen 3 Snow 3 Sweetheart Supreme 3 Sumatra 16 Summer Glow 16 Sunglow 3 16 Treasure 3 Trinidad 16 Van Veen 12 Vernus 16 Warbler 3 Weston’s Pink Dia- 16 mond White Christmas 3 White Gish 3 White Gumpo 3 White Jade 3 White Peter 16 Wilbrit 12 Wojnar’s Purple 16

*In this study, R. hyperythrum appeared to be less predisposed to root rot, even under flooded conditions.

Powdery Mildew caused by Microsphaera azalea and several other fungi is a common foliar dis- ease of Rhododendron in the Pacific Northwest and British Columbia, but it is rarely observed in other parts of the country (8). Disease is favored by high humidity and high nitrogen fertility (19). White mycelial growth typical of powdery mildew on many plants is not always present on Rhododen- dron and symptoms can vary among cultivars (8). Some rhododendrons develop pale yellow spots on the upper leaf surface with corresponding purple to brown spots on lower leaf surfaces. Defoliation may occur in severe infections.

POWDERY MILDEW Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible R. macrophyllum 10 R. yakushimanum 10 Rhododendron x Alice 6 Altair 6 Annabelle 6 Antelope 6 Balls of Fire 6 POWDERY MILDEW Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible Rhododendron x Basilisk 6 Beaulieu 6 Berryrose 6 BK 37 6 Brazil 6 Bright Forecast 6 Bryon Mayo 6 Buttercup 6 Cannon 6 Cathye Mayo 6 Cecile 6 Chartreuse 6 Chestnut Hill 6 Choptank River 6 Copper Cloud 6 Coral Queen 6 Corringe 6 Daviesi 6 Exbury White 6 Firefly 6 Fragrant Star 18 Garden Party 18 George Reynolds 6 Gibraltor 6 Ginger 6 Girard Salmon Delight 9 Girard Yellow Pom Pom 9 Girard’s Crimson Tide 6 Girard’s Mount St. Helen 6 Gold Dust 6 Gold Flake 6 Golden Delicious 6 Golden Eagle 6 Golden Flair 6 Golden Horn 6 Golden Sunset 6 Holden Mystery 6 Homebush 19 Hotspur 6 POWDERY MILDEW Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible Rhododendron x Hotspur A.M. 6 Hotspur Red 6 Hugh Wormold 6 Jean Allen 6 Jolie Madame 6 June Flame 18 Lila 6 Maori 6 Mariva 6 Mary Claire 6 Mary Holman 6 Medford Red 6 Millennium 18 Moonlight Rose 6 NA#55181 6 Nancy Buchanan 6 Northern Lights 6 Nova Zembla 10 Orange Beauty 6 Orient 6 Oxydol 6 Palestrina 10 Parade 18 Sunset 6 Persian Melon 6 Persil 6 P.I.#6078 6 Pink Delight 6 Pink Peppermint 6 Pink Plush 6 Pink Puff 6 Popsicle 18 Primrose 6 Redder Than 6 Red King 6 Red Letter 6 Red Pom Pom 6 Red Sissler 6 roseum 6 POWDERY MILDEW Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible Rhododendron x Salmon Joy 6 Sandra Marie 6 Satan 6 Selection #680 6 Silver Slipper 6 Snow Drift 6 Spicy Lights 6 Sulphides 6 Sunrise 6 Sweet Sue 6 Tintoretto 6 vaseyi 6 Verwelda Wendy 6 Visco 6 Visosalum 6 War Paint 6 White Cap 6 White Find 6 White Lights 19 Williams 6 Yellow Giant 6

INSECTS Azalea Lace Bug, Stephanitis pyrioides, is a primary pest of azalea. Accidentally introduced to the US from Japan, it is found from New York and south to and and west to (13). It has recently become established in the Pacific Northwest (7). Although feeding activity takes place on leaf undersides, damage is noticed on the upper surfaces which show yellow or white spots, also known as “stippling”. Stressed plants are much more likely to sustain severe injury, and leaf surface waxes are a key factor in resistance or susceptibility to azalea lace bug (5).

AZALEA LACE BUG Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible R. alabamense 5 24 R. austrinum 5 R. calendulaceum 5 R. canescens 4, 5, 24 R. mucronatum Valley White 4, 14, 24 AZALEA LACE BUG Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible R. oblongifolium* 24 R. periclymenoides 24 R. prunifolium 4, 5 24 R. serrulatum 24 R. viscosum 24 Rhododendron x Encore Autumn™ Series Amethyst 14 Amy 2 BNA Lavender Twist 2 Bollywood 2 Buttercup 24 Encore Autumn™ Series Cheer 14 Deja Bloom Pink Ribbons 2 Encore Autumn™ Series Fire 2 Fourth of July 14 Girard’s Crimson 2 Girard’s Hot Shot 2 Gumpo White 14 Koromo Shikubu Macrantha 22 Mother’s Day 2 My Mary 24 Red Slipper 2 Encore Autumn™ Series Rouge 14 Encore Autumn™ Series Royalty 14 Encore Autumn™ Series Sangria 14 Snow 2 Encore Autumn™ Series Twist 14 Watchet 14

*=R. viscosum, USDA Germplasm Resource Information System

Black Vine Weevil, Otiorhynchus sulcatus, also known as taxus weevil, has a wide range of orna- mental hosts including Rhododendron spp. One of several types of root feeding weevils, the black vine weevil is the most destructive and widespread on woody ornamentals (13). Native to Europe and first reported in 1910 in , it is now found throughout the northern US and southern Canada. In parts of the Pacific Northwest and Midwest, this weevil is often the most important insect pest of nursery crops (1, 7). Although both adults and larvae cause feeding damage, it the larval stage that poses the biggest threat to plants. Larval feeding on roots causes discoloration and wilting of foliage and, eventually, death of the plant. Adult feeding results in characteristic notching along the leaf margin (20). Research at State University has shown some Rhododendron species and hybrids are less susceptible to adult weevil feeding than others. Hybrids are generally less resistant than species rho- dodendrons and dark red flowered hybrid or species rhododendrons are generally susceptible (1).

ADULT ROOT WEEVILS* Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible R. adenophorum 1 R. arizelum 1 R. burnanicum 1 R. campylogynum 1 R. cardiobasis 1 R. carolinianum 1 R. ciliatum 1 R. concatenans 1 R. cuneatum 1 R. dauricum 1 R. davidsonianum 1 R. decorum 1 R. desquanatum 1 R. diaprepes 1 R. discolor 1 R. eurysiphon 1 R. fastigiatum 1 R. ferrugineum 1 R. glaucophyllum 1 R. heliolepis 1 R. hemsleyanum 1 R. hippophaeoides 1 R. impedium 1 R. imperator 1 R. intricatum 1 R. irroratum 1 R. lepidastylum 1 R. minus 1 R. oreodoxa 1 R. oreotrephes 1 R. pemokoense 1 R. praestans 1 R. pubescens 1 R. racemosum 1 R. rubiginosum 1 R. russatum 1 ADULT ROOT WEEVILS* Reference Species/Hybrids Cultivar Resistant Intermediate Susceptible R. scintillans 1 R. ungernii 1 R. vernicosum 1 R. xanthodocon 1 R. yakusimanum 1 R. yunnanense 1 Rhododendron x Candi 1 Cilpimense 1 Cowslip 1 Crest 1 Dora Amateis 1 Exbury Naomi 1 Faggetter’s Favourite 1 Graf Zeppelin 1 Jock 1 Lady Clementine Mitford 1 Letty Edwards 1 Loderi Pink Diamond 1 Lucky Strike 1 Luscombei 1 Moonstone 1 Naomi 1 Oceanlake 1 Odee Wright 1 P.J. Mezzitt (PJM) 1 Pilgrim 1 Point Defiance 1 Rainbow 1 Rose Elf 1 Sapphire 1 Snow Lady 1 Vanessa 1 Richards 1

*Plants were rated on a scale of 70 (moderately resistant) to 100 (highly resistant). For purposes of this table, we have categorized ratings of 70-84 as intermediate and 85-100 as resistant. Cranberry Rootworm, Rhabdopterus picipes, has a broad host range but is not found west of the Rocky Mountains (13). Adults produce conspicuous narrow slits and crescent-shaped holes in the foli- age of rhododendron, camellia, photinia and other shrubs (7). Researchers at the University of evaluated feeding injury on azaleas in field plots for two years. Damage was most severe onR. calen- dulaceum, R. japonicum, R. prinophyllum, and R. ‘Buttercup’, while the evergreen azalea R. ‘Delaware Valley White’ was the most resistant (21). Japanese Beetle, Popillia japonica, is a common foliage feeder of many landscape plants. Fleming (11) reports moderate feeding on R. obtusum japonicum, occasional light feeding on R. catawbiense, R. maximum, R. molle, R. obtusum kaempferi, and R. viscosum and no feeding on R. nudiflorum (=R. pericylmenoides, USDA Germplasm Resource Information System). REFERENCES 1. Antonelli, A.K., and R.L. Campbell. 1984. Root Weevil Control on Rhododendrons. Pullman: Washington State University Extension Bulletin 0970E. cru.cahe.wsu.edu/CEPublications/ eb0970e/eb0970e.pdf 2. “Azalea Cultivar Trial Data Summary.” 2014-2017 Azalea Cultivar Evaluation Program. Center for Applied Nursery Research, Dearing, GA. canr.org/plantevaluations/azalea 3. Benson, D.M., and F.D. Cochran. 1980. Resistance of evergreen hybrid azaleas to root rot caused by Phytophthora cinnamomi. Plant Dis. 64(2):214-215. doi: 10.1094/PD-64-214 4. Braman, S.K., and A.F. Pendley. 1992. Evidence for resistance of deciduous azaleas to azalea lace bug. J. Environ. Hort. 10(1):40-43. 5. Chappell, M.R., C. Robacker, S. Baden, and A. Byous. 2004. The effect of leaf wax extracts from the leaves of eight azalea genotypes (Rhododendron spp.) on feeding and oviposition by azalea lace bug [Stephanitis pyrioides (Scott)]. HortScience 39(4):895. 6. Cochran, K.D., and C.W. Ellett. 1990. Evaluation of powdery mildew severity on deciduous azaleas at the Secrest , 1989. Agr. Res. Dev. Ctr., Special Circular 135:54-58. 7. Cranshaw, W., and D. Shetlar. 2018. Garden Insects of North America. Princeton University Press, Princeton, NJ. 8. Daughtrey, M.L., and D.M. Benson. 2001. Rhododendron diseases, pp. 334-341. In: R.K. Jones and D.M. Benson, editors. Diseases of Woody Ornamentals and Trees in Nurseries. APS Press, St. Paul, MN. 9. Dirr, Michael A. 2009. Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation and Uses. 6th ed. Stipes Publishing L.L.C., Champaign, IL. 10. Dreistadt, S.H., and J.K. Clark. 2016. Pests of Landscape Trees and Shrubs: An Integrated Pest Man- agement Guide. 3rd Ed. University of California, Agriculture and Natural Resources, Oakland, CA. 11. Fleming, W.E. 1972. Biology of the Japanese beetle. Technical Bulletin 1449 of the Agricultural Research Service, USDA, Washington, DC. 12. Hoitink, H.A.J., and A.F. Schmitthenner. 1974. Resistance of Rhododendron species and hybrids to Phytophthora root rot. Plant Dis. Rptr. 58(7):650-653. 13. Johnson, W.T., and H.H. Lyon. 1991. Insects that Feed on Trees and Shrubs. 2nd Ed. Cornell Univ. Press, Ithaca, NY. 14. Kirker, G.T., B.J. Sampson, C.T. Pounders, and J.M. Spiers. 2008. The effects of stomatal size on feeding preference of azalea lace bug Stephanitis pyrioides (Hemiptera: Tingidae), on selected cultivars of evergreen azalea. HortScience 43(7):2098-2103. 15. Krebs, S. 2013. Resistance to Phytophthora root rot varies among Rhododendrons subjected to repeated flooding in the field. Acta Hortic. 990:243-252. doi:10.17660/ActaHortic.2013.990.28 16. Krebs, S.L., and M.D. Wilson. 2002. Resistance to Phytophthora root rot in contemporary rhodo- dendron cultivars. HortScience 37(5):790-792. 17. Linderman, R.G., and D.M. Benson. 2014. Compendium of Rhododendron and Azalea Diseases and Pests. 2nd Ed. APS Press, St. Paul, MN. 18. Long, M.C., S.L. Krebs, and S.C. Hokanson. Field and growth chamber evaluation of powdery mildew disease on deciduous azaleas. HortScience 45(5):784-789. 19. Pscheidt, J.W., and C.M. Ocamb (Senior Eds.). 2018 Pacific Northwest Disease Management Handbook. © State University. 20. “Rhododendron Problems: More Than Just Phytophthora”. 2017, July 21.Branching Out: An Inte- grated Pest Management Newsletter for Trees and Shrubs, Vol 24, No. 8. 21. Robacker, C.D., and S.K. Braman. 1997. Field evaluation of azalea species and cultivars for resis- tance to azalea lace bug and cranberry rootworm. HortScience 32(3):482. 22. Schultz, P.B. 1993. Host-plant acceptance of azalea lace bug (Heteroptera: Tingidae) for selected azalea cultivars. J. Entomol. Sci. 28(2):230-235. doi: 10.18474/0749-8004-28.2.230 23. Sinclair, W., and H.H. Lyon. 2005. Diseases of Trees and Shrubs. 2nd Ed. Cornell Univ. Press, Ithaca, NY. 24. Wang, Y., C.D. Robacker, and S.K. Braman. 1998. Identification of resistance to azalea lace bug among deciduous azalea taxa. J. Amer. Soc. Hort. Sci. 123(4):592-597. OTHER RESOURCES “American Rhododendron Society.” rhododendron.org “Missouri Botanical Garden.” missouribotanicalgarden.org “Woody Plants Database.” Urban Horticulture Institute, Cornell University, woodyplants.cals.cornell. edu/plant/search

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