As promised last weekend, this week's email tip will deal with the ugliness of Cotton Root Rot/Southern Root Rot/Texas Root Rot.
Trees and shrubs that are seemingly healthy one day and completely dead the next have most likely died from a disease called Texas Root Rot. Also known as Cotton Root Rot because of it's prevalence in cotton fields, it is by far one of the most common disease of landscape plants in the Southewest. Cotton root rot and/or Texas root rot, is caused by the fungus Phymatotrichum ominvorum. The fungus attacks over 2,000 species of broadleaf plants, but does not affect monocots (grasses). Before I get into more technical information on what it looks like, the simple description on how it manifests itself is due in large part to extremities in soil moisture conditions. Or put another way - after a period where it was extremely wet, then it became extremely dry. And in the average landscape it usually is seen in Hawthornes, Red Tips, Ligustrums and a few other evergreen shrubs.
On the GardenLine Radio Program, and in the various emails I get, the best way anyone has described the onslaught of CRR/TRR goes something like this - "Randy, I have a ligustrum that all of a sudden a section of the plant has just turned brown and that whole branch looks dead, while the rest of the row of ligustrums looks fine."
Disease symptoms are most likely to occur from June through September when soil temperatures reach 28oC (82oF). The first symptoms are slight yellowing or bronzing of leaves followed by wilting. Plants die suddenly after the first symptoms of wilting. Leaves remain firmly attached to the plant. Affected plants die suddenly, often after excellent growth. Large trees and shrubs may die more slowly.
Usually roots are invaded extensively by the fungus by the time plants have wilted. When roots are pulled from the soil, root bark is decayed and brownish, and wooly strands of the fungus frequently are apparent on the root surface. Affected plants pull from the soil with little effort.
Under moist conditions, sporemats sometimes appear on the soil surface. These mats, 2 to 16 inches in diameter, are first snow-white and cottony and later tan and powdery. On large roots and tubers, there are numerous small, cushion-like sclerotia or resting bodies about the size of a pinhead. At first they are light tan but later appear dark and warty.
The fungus generally invades new areas by continually slow growth through the soil from plant to plant. Occasionally, it spreads more rapidly on the roots of infected transplanted plants. The fungus can survive in the soil for many years, and often it is found as deep in the soil as roots penetrate.
The one sure way to control Cotton Root Rot in the soil is with the Fungicide Captan. There has been some recorded success with any of the Propaconizol (Banner-based) fungicides as well. And there is research that shows a deep root feeding with ammonia-based nitrogen fertilizers. Obviously, in severe cases of Cotton Root Rot, you will need to remove any limbs that are dead from the disease. Plants/shrubs that have some life in them at the time they are treated can come back. However, for those that are 80-90% dead at the time of treatment should probably be removed entirely.
The most interesting control method has to do with Natural Plant Barriers. This technique consists of planting resistant species around an infected area. These barriers either exclude or limit the spread of the pathogen. This technique assumes that the barrier plant does not harbor the pathogen in its root system. Make ornamental plantings of cotton root rot-susceptible species with isolated plants or groups of plants rather than in continuous rows as hedges. When the disease occurs in an ornamental planting, replace diseased plants with resistant species.
Here is a list of ornamental shrubs from Texas A&M University that are considered "resistant" to Cotton Root Rot.
In case you have not ordered your copy of it yet, here's the link for ordering my new book "Gulf Coast Gardening with Randy Lemmon."
Until next issue, here's to
Great Gardening from the GardenLine, heard
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