Q: One branch on our azalea bush is dying. It’s getting plenty of water. Any idea what is causing this?
Ellen Burmester, Fair Oaks
According to University of California master gardener Rachel Tooker, the solution to this branch problem likely is at the roots.
In your note, you state that the bush is getting plenty of water and that fact, along with a photograph you submitted, provides a good clue. It is possible that the bush is getting too much water, and that the soil is not able to drain sufficiently. This may lead to a condition known as root or crown rot, caused by a pathogen species in the genus Phytophthora.
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Phytophthora kills the roots and root crown area of infected plants and sometimes spreads upward into the stem. This causes plants to wilt and leaves to discolor, stunt and drop prematurely.
Almost all fruit and nut trees, as well as most ornamental trees and shrubs, can develop Phytophthora rot if soil around the base of the plant remains wet for prolonged periods, or when planted too deeply. Mulch mounded around the trunk also can lead to this problem. Root rot flourishes when soils are compacted, drain poorly, or are usually damp.
The leaves of plants affected by Phytophthora rot can appear drought stressed, so sometimes home gardeners respond to these symptoms by adding more water, instead of allowing the plant to dry a little between watering. Once infected, trees or plants often wilt and die rapidly with the first warm weather of the season.
Infected mature plants grow slowly and may gradually decline. Twigs and branches die back and the entire plant can be killed. Depending on the species of Phytophthora, the pathogen may affect only small feeder roots or rootlets, major roots, or all roots and the crown.
In many, but not all plants, a vertical streak, stain, or canker becomes visible on infected trunk wood under the bark. Black or reddish sap may ooze from darkened areas of infected bark.
To confirm the presence of disease, cut away the outer bark around the stain streaks or canker. The concentric margins between the healthy whitish or yellowish wood and the reddish or brown infected wood in the trunk look like they are soaked with water.
Confirmation of the particular species of the disease requires a laboratory test, although usually diagnosis of the disease in general can be pretty clear based on the condition of the plant and information about cultural practices (watering, mulch, fertilizer, etc.).
The most effective way of preventing Phytophthora rot diseases is to provide good drainage and practice good water management. Avoid prolonged saturation of the soil or standing water around the base of plants. Irrigate only as much and as often as necessary; most shrubs do best with infrequent (weekly or less) deep irrigation.
Once it resides in the soil, the pathogen can survive for many years – as long as the soil remains moist – and will then enter susceptible plants through the crown or roots. If replanting in an area known to contain Phytophthora, prepare the ground by breaking through any soil compaction and hardpan to improve drainage. Consider planting trees and shrubs on mounds 8 to 10 inches high to promote better drainage.
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