What is know as "cotton root rot" often becomes a serious problem because of rollercoaster moisture conditions ... too much, then not enough.
Also called southern root rot or Texas root rot, this condition sometimes manifests itself in trees, but mostly in shrubs that seem healthy one day, then develop a completely dead branch or section the next.
It's called cotton root rot because of its prevalence in cotton fields, but it is also one of the most common landscape plant diseases in the coastal Southwest. It's caused by the fungus Phymatotrichum ominvorum, which attacks over 2,000 species of broadleaf plants. But doesn't affect monocots (grasses).
It shows up, in large part, due to extremes in soil moisture. After a very wet period, the soil is subjected to a very dry period. And in an average landscape, it is usually seen in hawthornes, red tips, ligustrums and a few other evergreen shrubs. The best description I've every received from a GardenLine listener was, "All of a sudden a section of the plant has turned brown. That whole branch looks dead, while the rest of the row of similar plants looks fine."
The first symptom is a slight yellowing or bronzing of the leaves, followed by wilting. The leaves remain firmly attached to the plant, however. The plants die suddenly after wilting, often after excellent periods of growth. Large trees and shrubs may die more slowly. Usually, the fungus has invaded the roots extensively by the time the plants have wilted. When the roots are pulled from the soil, the root bark is decayed and brownish, and wooly strands of the fungus frequently are apparent on the root surface. Affected plants pull up with little effort.
The one sure way to control cotton root rot is to treat the soil with the fungicide captan, then apply a copper-based fungicide like Kocide on the leaves. There has been some recorded success with propaconizol-based (Banner) fungicides as well. And some research shows a deep-root feeding with ammonia-based nitrogen fertilizers helps. It is critical that you use captan fungicide on the root system of the infected plantandon the root systems of adjacent plants as well.
In severe cases of cotton root rot, remove any dead limbs. Plants and shrubs that have some life in them at the time of treatment may come back. However, those that are 80-90 percent dead should probably be removed entirely.
The most interesting control method involves using natural plant barriers. Planting resistant species around an infected area can either exclude or limit the spread of the pathogen. Plant cotton root rot-susceptible species with isolated plants or groups of plants rather than in continuous rows as hedges. If the disease occurs, replace diseased plants with a resistant species. Of course, this technique assumes that the barrier plant does not also harbor the pathogen in its root system. Here is a list of ornamental shrubs from Texas A&M University considered resistant to cotton root rot.