Azalea Root Disease

Root rot can turn beautiful azalea bush into a corpse

Bill Sevier: Ask a Master Gardener  

Saturday, July 4, 2015

Q: I have five azalea bushes next to my house that are dying. They have been losing whole limbs over time, and now most all the plants are dead. The leaves are a bit yellow, but I don’t see any insects. They get plenty of water and afternoon shade. What could this be? J.T., Tulsa.

A: Generally azaleas do pretty well in our area other than some nutritional problems related to soil acidity and infestations with an insect called lacebugs.

However, these conditions will not cause an azalea to go into a death spiral as yours has. When one sees all of a shrub die with no obvious cause, this points to a problem in the plant’s root zone.

There are two main groups of pests that can involve azalea roots leading to dwindling and eventual death. One is a microscopic worm called a nematode, and the other is a root fungus.

Thousands of varieties of nematodes are normally found in the soil, but one variety called the stunt nematode can, as the name suggests, stunt and then kill an azalea. There is no treatment for this.

Far more likely cause is an infection with phytophthora root rot, the most common root fungus of azaleas. This fungus is widespread, infects many different shrubs and trees, and once established is not treatable by the homeowner. The disease may develop slowly, showing smaller yellowish leaves and shortened twigs, which leads to death of individual limbs and then death of the entire plant.

One major factor leading to the disease is overly wet soil, such as areas around the downspout of roof guttering. The disease is also more common in poorly draining heavy clay soils. Planting azaleas too deeply — deeper than the soil line it had in the nursery — causes the plants to be more susceptible to the disease. Genetics also comes into play, and some azaleas are just naturally more susceptible to root rot fungus.

As mentioned, there is no treatment for this; once the diagnosis has been made, the plant should be dug up and sent off in the trash (do not compost as the roots are infectious).

Prevention is key in the management of this disease. One should not replant an azalea in an area previously known to be infected — the fungus is still in the soil. Also, do not plant an azalea in an area where any other plant such as camellia, dogwood, yew, juniper and others have died of root rot.

When selecting a new azalea, inquire about its phytophthora root rot resistance and go for the most resistant ones. Also, plant in an area that drains well; if you have clay soil, plant your azaleas in raised beds.

Garden tips

For all your plants, ornamental or vegetable, mulching and correct watering are keys to surviving the heat of the summer. Mulch conserves water and reduces ground temperature.

Fescue lawns need 2 inches of water per week to survive summer; Bermuda grass needs about half that amount. Watering less frequently and more deeply is better than daily shallow watering.

Brown patch disease of fescue lawns is appearing now, related to excessive rains, heat and high humidity. Wet grass leaves promote the disease. Therefore, if you water in the mornings, allowing the leaves to dry during the day, there will be less likelihood of infections. Fungicides are available, but OSU feels the fungicides available to homeowners are not nearly as effective as those available to professional licensed applicators. None of these chemicals will cure existing infections; they only prevent new disease at best.