By Cat Holmes
University of Georgia
SOD killed entire canopies of oak trees, sometimes in just a few weeks, on the West Coast during the 1990s.
And its potential harm isn't limited to oaks, said Jean Williams-Woodward, a UGA plant pathologist. Some of the South's most beautiful plants, including azaleas, camellias, rhododendrons, viburnums, roses and honeysuckles, are vulnerable to the fungal pathogen.
While SOD isn't fatal to these other plants and usually just kills branches, the plants are the means by which the fungus spreads.
SOD hits non-oak plants with leaf spots and blighting, resembling leaf-scorch symptoms, Williams-Woodward said. "The fungus then produces abundant spores on the infected leaves," she said. "These are usually water-splashed to nearby plants."
When SOD finally spreads to an oak species, it stops.
"Oak trees are a dead end for SOD," she said, "because the fungus doesn't produce spores on infected oaks."
But the disease is fatal for oaks.
"SOD infects through the bark of oak trees and causes girdling cankers that kill the trees," Williams-Woodward said. "The cankers are referred to as 'bleeding cankers' because the sites of infection ooze sap and are often darkly discolored."
So far, all of the infected plants discovered have been camellias from Monrovia Nurseries in Azusa, Calif.
In terms of detection, that's good news.
"Monrovia Nursery plants are 'branded' plants, in that they're grown and sold in specially labeled pots with 'Monrovia' written on the side as well as larger, colorful plant labels," Williams-Woodward said.
"We're only concerned with plants that have been purchased and planted within the past year-- possibly, to be very conservative, two years," she said. "Plants that have been established for many years are unlikely to be infected with SOD."
The first thing homeowners can do is learn to recognize SOD symptoms. One symptom is excessive leaf drop. Another is tan or brown leaf spots that begin at the leaf tip and progress down the leaf.
If you suspect SOD, don't dig up the plants.
"Removing plants could actually spread the disease faster and farther," Williams-Woodward said, "especially if infected plants end up in municipal wastes or are shredded for use as landscape mulch."
Instead, contact your county extension agent. The county agent will submit samples to the Homeowner Plant Disease Clinic in Griffin, Ga. If SOD is detected, the agent will arrange with the appropriate state agency for the destruction of the plant.
"We don't know what the potential threat of SOD is in the eastern United States and, in particular, Georgia," Williams- Woodward said. "The fungus hasn't become established within landscapes or forests."
The goal is to identify all infected plants and destroy them, she said, to keep the fungus from getting a foothold in Georgia. "This recent introduction is just the tip of the iceberg," she said. "We expect to be stopping future introductions and looking for this disease for many years."
Until now, SOD has been confined to the West Coast. But recently infected plants have been found in nurseries in Florida and Maryland, too.
(Cat Holmes was a science writer with the University of Georgia College of Agricultural and Environmental Sciences.)