No, this isn’t the neighborhood dog or even the pesky insects in the Southeast. This plant destroyer is Phytopthora root rot.
From the Greek word, which means plant destroyer, Phytopthora spores are numerous in our Southern soils. They lie dormant until the soils are saturated and soil temperatures rise above 65 degrees.
Plants whose roots are stressed from drought, soil compaction, and root collar disorders are especially vulnerable. Once the spores make contact, they germinate and invade. They attack the smaller absorbing roots first and then invade the larger roots and the root collar. The disease grows and causes the death of the infected area, and the decline of the above ground plant.
The plant destroyer is a formidable foe, but it can be defeated with a little planning and proper care. There are four main weapons to use in the battle:
Plant Selection — Species like azalea, rhododendron, boxwood, hemlock, dogwood, and camellia are very susceptible. These plants should not be planted in areas with constant soil moisture. White and red oaks are vulnerable, especially if the root collar is buried with fill soil or mulch, or if the roots have been severed or wounded due to soil compaction common during home construction. Some species are resistant – anise, arborvitae, viburnum, yaupon holly, winterberry holly, butterfly bush, and crape myrtle. River birch, ash, bald cypress, lacebark elm, and willow oak are resistant trees.
Proper Planting and Mulching — When the root collar of a plant is covered, moisture is held against the main stem allowing an easy entry point for Phytopthora. Be sure to expose the root collar before planting. Do not plant too deep and do not over mulch. The area where the roots flare out from the plant should be visible even after mulching.
Soil Moisture Management — Roots that decline in drought are often the first to be attacked when soils become saturated. Try to keep soil moisture consistent. Amend the soil and use drainage systems in poorly drained area. The most common mistake is to overwater, especially in areas with poorly drained soils or where turf irrigation overlaps planting bed irrigation.
Chemical Treatment — All is not lost if your plant becomes infected. Fungicides can suppress the spread of the disease. Repeated applications are often necessary since the disease can return when conditions are favorable. If possible, move susceptible plants to better environments and plant a resistant species in the troubled area.
Austin Proctor works in the Raleigh office of Bartlett Tree Experts. You may contact him at 919-782-7803 or [email protected]