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What you describe sounds suspiciously like Phytophthora root rot. The symptoms of Phytophthora root rot vary with the cultivar. On Kurume hybrid types, new leaves are smaller than normal with yellowing between the veins (interveinal chlorosis), possibly some purple coloration and defoliation. This chlorosis is often confused with a deficiency of iron or other nutrients. At times light applications of iron and a complete fertilizer can improve the green color of leaves but only for a short time. Older leaves may turn red on these cultivars in late summer. Excessive yellowing and loss of older leaves are the predominant symptoms on these azalea cultivars. On all cultivars, new shoot growth in the early summer is much less and stems are smaller in diameter than shoot growth of healthy plants. Usually, large plants slowly decline in vigor and die, branch by branch, over a period of several months to years, but sometimes they die rapidly. Roots are reddish-brown, brittle and often limited to the upper few inches of the soil. The reddish-brown discoloration advances to the larger roots and eventually to the main stem.
High soil moisture and warm soil temperatures favor development of Phytophthora root rot. The disease is more frequent and severe in heavy clays or poorly drained soils than in well drained or sandy soils. The disease is common and severe in areas where run-off water or rainwater from roofs collects around plant roots. Setting woody plants deeper than the soil level in the nursery or container, over-watering plants, or long periods of heavy rain also favor disease development, especially in shallow soils with underlying rock or compacted hard pans.
The following suggestions may aid in the prevention of root rot:
1. Purchase disease-free plants. Choose plants with normal green color and white or light colored roots. Avoid plants that appear wilted in the morning, have reddish-brown discolored roots or evergreen plants that have excessive winter defoliation.
2. When purchasing new azaleas for the landscape, select disease resistant cultivars.
3. Plant in well-drained areas or establish raised beds. If excessive water from any source collects in the planting site, avoid planting root-rot susceptible plants. If soil is clay, set plants in raised beds and thoroughly mix pine bark mulch (not sawdust or peat) into the bed. The material should be incorporated to a depth of about 6 inches; this also may help reduce excessive soil moisture. For new plantings in poorly-drained soils, set the plant on top of the soil surface, then build up the raised bed by filling in between plants with pine bark mulch.
4. Do not set the new plant any deeper than the soil level in the nursery container. Firm the soil beneath the soil ball so that the plant will not settle deeper into the soil after watering.
5. Avoid use of azalea in areas where root-rot susceptible plants have died. Instead, replant with plants that are not susceptible to root rot; such as Chinese holly, hybrid hollies (eg. Nellie R. Stevens), ligustrum or others.
6. For azaleas in irrigated landscapes, do not overwater. Established landscape plants need an inch of rainfall or irrigation per week in the growing season. Size irrigation equipment for your soil type so that rate of water application does not waterlog the soil.
Hope this information is helpful!
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