Garden Detective

End of drought could explain sickly azaleas

Garden Detective: Why do these azaleas look like toast? That’s probably a case of root or crown rot, common this year in Sacramento after so many seasons of drought.
Garden Detective: Why do these azaleas look like toast? That’s probably a case of root or crown rot, common this year in Sacramento after so many seasons of drought.

Experts tackle readers’ garden questions.

Q: Our azaleas have been failing, and I don’t know what to do. At first, the drought seemed to be the cause, but we’ve been watering regularly (twice a week), especially when it rained since these are sheltered by an overhang. I’m not sure if I should be fertilizing them after reading an article that advised against it. The ones at the far end have some new growth, and I’m wondering if I should just cut them down so they might revive in this way. I’m at a loss, and I don’t know what to believe when I read.

Susan Weiss-Shannon, Sacramento

Bee garden writer Debbie Arrington: First, my condolences on your azalea bed. I lost several azaleas, too, this past year.

The likely culprit is azalea root rot caused by the Phytophthora fungus. This fungus is also known as water mold. It attacks during wet winters and infects weakened plants in areas with poor drainage (such as brick planter beds). It can kill a normally healthy plant in a relatively short period of time, especially if that plant was already weakened by drought.

This fungus destroys the plant from the roots up. It rots the roots and the crown, where the plant’s trunk meets its roots. Then, it moves up the branches to kill the foliage and eventually the whole plant.

Your bushes are already showing signs of that fungal disease, particularly the shriveled foliage. The leaves look like the plant is suffering from lack of water or other signs of drought stress, when actually it’s too much water and damp soil that’s led to this problem. The tell tale sign is a change of color on the outside bark near the base of the plant. It darkens and looks brownish red or even black.

If you pull up one of the badly infected bushes, the roots will look black and rotten, and the roots will be much smaller than they should be (the roots should be as big and long as the branches above ground).

There are two other possibilities: The crowns of the plants may be buried under mulch and debris as if the bush is “sinking.” That can slowly suffocate the crown and lead to crown rot, with the same results as the azalea root rot. (The remedy for that is digging up the plant, replacing lost soil and reestablishing the bush at a higher level. Or removing mulch and digging out the crown; in effect, lowering the soil level.)

Or it may be fertilizer burn, either fertilizer applied directly to the plants or weed-and-feed on the lawn area adjacent to the bushes. That can kill the foliage and sometimes the whole plant.

If it is azalea root rot, cut back the infected plants to healthy wood (the bush on the end with the new growth looks like it will pull through). If the wood feels brittle, it’s likely a goner. You want to keep the branches that still have some life.

Remove the mulch, so the crowns can breathe. The branches shouldn’t look like they’re growing out of the mulch but from the base of the plant.

Drench the soil with a fungicide recommended for Phytophthora. That will help eliminate the fungus.

Phytophthora moves quickly through wet soil; that’s how it spreads from one bush to another. So, you need to monitor soil moisture and keep the bed from getting soggy. You don’t want to flood the bed to irrigate; instead, water each plant individually. Twice a week may be too much water.

Don’t fertilize until you stabilize the plants and see a lot of new healthy growth.

If you don’t see positive change, then you’ll need to replace the azaleas – and the soil in those planters. Discard the old soil along with the plants; the fungus survives and will move with the old soil to a new location.

When replacing the soil, add a lot of organic matter such as compost to improve drainage. Treat the planter with fungicide before replanting.

For more information on Phytophthora, consult the University of California’s Integrated Pest Management pest notes, available online at ipm.ucanr.edu.

Bee garden writer Debbie Arrington is a consulting rosarian and lifelong gardener. 916-321-1075, @debarrington

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