Wisconsin plant-health officials are advising consumers who bought rhododendrons or azaleas this spring and summer to be on the lookout for signs of a disease that could spread to oaks and kill them.
Plant-health staff with the Wisconsin Department of Agriculture, Trade and Consumer Protection have identified phytophthora ramorum, or P. ramorum, on rhododendrons at a northern Wisconsin nursery. That fungus causes sudden oak death, which has never been found on the landscape in Wisconsin. There's no human health risk.
The find resulted from a July survey of 59 garden centers and nurseries in the state, after it was reported that potentially infected shipments originating from a Washington supplier had gone to those businesses. Potentially infected plants were shipped to at least 28 states, many of which have already found the leaf blight disease at stores receiving the plants. Wisconsin inspectors collected 43 samples; only one was found positive for the fungus.
Consumers should look for leaf and shoot dieback as potential symptoms. Suspect plants should be sent to the University of Wisconsin-Madison Plant Disease Diagnostic Clinic. There's no cost. Visit pddc.wisc.edu for information about how to submit a sample.
“Given the results of our surveys, we would expect that many of the distributed plants are negative for P. ramorum," said Brian Kuhn, director of the Wisconsin Department of Agriculture, Trade and Consumer Protection's Plant Industry Bureau. "However due to the positive detection and because many of the plants had been sold before we received notice, we wanted to enlist citizens in keeping an eye out for symptoms. These plants entered Wisconsin legally, with the proper documentation, and all the businesses involved have cooperated with us.”
Susceptible plants at the supplier and the nursery are being destroyed. Owners are disinfecting soil and equipment.
Although the disease may not necessarily kill rhododendrons and azaleas, it could be transmitted to as many as 100 different plant species, including hardwoods, softwoods and shrubs. Oaks are at the greatest risk because P. ramorum is incurable and has killed 30 to 45 million oaks in California and Oregon. Spores from the disease can travel in plants, soil, gravel and potting mixes, and wind-blown rain and other water sources. Contaminated pots, shovels and other equipment could also transmit the disease unless sterilized with bleach. Visit datcp.wi.gov for more information.