Page Banner

United States Department of Agriculture

Agricultural Research Service

Title: What's New in Ipm for Azaleas?

Authors
item Dutky, Ethel - UNIV. OF MARYLAND
item Shishkoff, Nina

Submitted to: The Azalean
Publication Type: Trade Journal
Publication Acceptance Date: September 2, 2004
Publication Date: November 11, 2004
Citation: The Azalean 26:64-66

Interpretive Summary: The fungus-like organism Phytophthora ramorum has been identified as the cause of foliar blight, shoot blight and cankers on a wide variety of plants. A common name for the organism is 'Sudden Oak Death' or 'SOD' based on the damage caused to oaks in coastal California forests. This disease was observed on nursery plants (rhododendrons, viburnums and some others) in Europe in 1993 and on oaks in California in 1995. In the U.S. the disease was restricted in distribution to the California coastal forest ecosystem until recently. In 2003 the fungus got into several large wholesale nurseries in California. Camellias, viburnums and lilacs potentially infected with P. ramorum were shipped to nurseries and direct to home gardeners in about 35 states during the period from March 2003 through June 2004. P. ramorum usually does not kill the infected plant. Instead it produces leaf spots, small twig cankers and blight of shoot tips. The symptoms seen on azalea are brown leaf spots and dieback of young shoots. If people suspect that they may have azaleas showing odd leaf spot/shoot blight symptoms, and want them tested for P. ramorum they should contact their local state department of agriculture. The most current information on SOD in Maryland for home gardeners can be found at the Home and Garden Information Center (1-800-342-2507) or on their website (www.agnr.umd.edu/users/hgic).

Technical Abstract: The fungus-like organism Phytophthora ramorum has been identified as the cause of foliar blight, shoot blight and cankers on a wide variety of plants. A common name for the organism is 'Sudden Oak Death' or 'SOD' based on the damage caused to oaks in coastal California forests. Other species of Phytophthora cause disease of azalea and rhododendron in nursery production and landscapes, and some produce foliar symptoms that can be confused with this new disease. Expert laboratory help to diagnose P. ramorum will be needed. The state department of agriculture should be consulted for guidance in selecting samples for testing. This disease was observed on nursery plants (rhododendrons, viburnums and some others) in Europe in 1993 and on oaks in California in 1995. The populations of P. ramorum in Europe and California differ. It is not clear if these populations represent separate introductions of the organism from unknown points of origin. The disease has spread throughout the countries of the European Union on nursery stock. P. ramorum has now been found at 339 sites in England and Wales alone. It is being seen causing cankers on trees (oaks, beech) in parks and forests in England and Wales. In the U.S. the disease was restricted in distribution to the California coastal forest ecosystem until recently. In 2003 the fungus got into several large wholesale nurseries in California. Camellias, viburnums and lilacs potentially infected with P. ramorum were shipped to nurseries and direct to home gardeners in about 35 states during the period from March 2003 through June 2004. P. ramorum causes damage by infecting foliage, shoots and the trunks of some trees (oaks, beech). It has been detected on the roots of potted azaleas, but is not considered to cause root disease. It has a very wide host range, which is very unusual for Phytophthora. Most species of Phytophthora studied have a narrow host range. P. ramorum usually does not kill the infected plant. Instead it produces leaf spots, small twig cankers and blight of shoot tips. The fungus produces many microscopic spores on the infected foliage and blighted shoots. These spores can be carried on wind or splashing rain to lodge in the bark of oaks, infect the main trunk, and eventually kill the oak. It usually takes several years to cause wilt and death except for very small seedling oaks. The symptoms seen on azalea are brown leaf spots and dieback of young shoots. If people suspect that they may have azaleas showing odd leaf spot/shoot blight symptoms, and want them tested for P. ramorum they should contact their local state department of agriculture. Only plants purchased during the March 2003 - June 2004 period are considered at increased risk for this disease. Several laboratory tests are required to identify this fungus. It cannot be quickly identified through microscopic examination. For the most current information on SOD in Maryland, home gardeners should contact the Home and Garden Information Center (1-800-342-2507) or visit their web site (www.agnr.umd.edu/users/hgic).

Last Modified: 4/19/2014
Footer Content Back to Top of Page