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Recent tree scourge poses garden threat

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9:09am, August 6, 2002
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From Milwaukee, Wisc., at a meeting of the American Phytopathological Society

Lab tests suggest that a lethal disease of oak trees in California and Oregon could strike some popular garden shrubs in the rhododendron family. Caused by the funguslike microbe Phytophthora ramorum (SN: 8/5/00, p. 86; SN 8/3/02, p. 70: Bleeding Trees: Microbial suspect named in beech deaths), the disease attacks an unusually wide range of species.

To get a clearer picture of the risks, Paul Tooley of the Agricultural Research Service and his colleagues are testing plants in a well-contained lab at Fort Detrick, Md. There, the pathogen readily attacked a mountain laurel (Kalmia latifolia 'Madeline'), mountain andromeda (Pieris floribunda), dusty zenobia (Zenobia pulverulenta), and the common nursery rhododendron 'Nova Zembla.' In contrast, wild rhododendron (Rhododendron maximum) is "pretty resistant," says Tooley. He adds that it's not clear how well the lab tests predict what will happen outdoors.

Plant pathologist Everett Hansen of Oregon State University in Corvallis points out that in the wild, few species of the rhododendron family have been exposed to the disease. Wild plants of other species just get leaf spots.

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