Understanding Cotton Root Rot: A Comprehensive Guide to Identification and Control in Fruit and Nut Trees

Cotton root rot, also known as Texas root rot or Phymatotrichum root rot, is a serious fungal disease caused by Phymatotrichum omnivorum. This soil-borne fungus is responsible for fatal root rots on over 2,300 plant species, making it a significant threat to fruit and nut trees.

Fortunately, this fungus is limited to specific types of soils, primarily in the southwest US and Mexico, and parts of the south central US. However, in regions where it is prevalent, cotton root rot is extremely destructive and challenging to control.

One of the unique characteristics of cotton root rot is that it does not target specific plant species. An infected cotton root can spread the fungus to the roots of fruit and nut trees, causing widespread damage.

In this article, we will guide you through the process of diagnosing the disease and share effective strategies for controlling cotton root rot in fruit and nut trees.

Understanding Cotton Root Rot

Table of Contents

What to Look For

Symptoms of cotton root rot are most visible from June through September when the soil temperature rises and exceeds 82 F. The initial signs mimic water stress, with tree leaves beginning to die. The upper leaves are usually the first to wither, followed by the lower leaves.

Despite the leaves turning brown, they remain attached to the tree. Infected roots may display a fuzzy, tan-colored mold on the surface, and the bark can be easily peeled off to reveal a reddish-brown stain on the woody tissue.

A unique feature of cotton root rot is the formation of white spore mats on the soil surface near infected plants after they die. These mats turn tan before disappearing, without spreading the disease further.

The Hidden Threat: The Fungus Lurks in the Soil

Cotton root rot can survive in the soil indefinitely by producing tiny resting structures called sclerotia. These seed-like structures can be found as deep as 12 feet in the soil, and they germinate under favorable conditions to infect plants.

Infections can also occur when the roots of an infected plant come into contact with those of an uninfected plant, creating a pathway for the fungus. Native vegetation like mesquite can also harbor the fungus, contributing to its spread.

Control Measures

Controlling cotton root rot is challenging, and while several techniques can help reduce its severity, experts do not consider any of them highly effective.

Strategies for Control

Dramatically Cut Back Your Tree: If detected early, cutting back the top growth of the tree by half and keeping it moist for several weeks can reduce root stress and potentially save the tree.
Treat the Soil: Apply manure, ammonium sulfate, and sulfur to the soil to change its alkalinity levels. This technique may alter the soil conditions to inhibit the fungus, but it requires annual repetition to be effective.
Add Organic Matter: Incorporating organic matter into the soil can promote the growth of beneficial soil microbes that compete with the fungus. Green manure can release chemicals that inhibit the fungal growth and boost root vigor.
Use a Moldboard Plow: Plowing the soil to a depth of 6-8 inches can reduce the formation of sclerotia in the upper soil layer, potentially limiting the disease’s spread.

Dealing With Loss

If a tree succumbs to cotton root rot, it is essential to consider planting resistant species like grasses (monocots) in its place. The soil may remain infested for a prolonged period, making it challenging to replant susceptible trees.

Share Your Experience

We would love to hear from you! Have you encountered cotton root rot in your fruit or nut trees? Were you able to save any trees using the control strategies mentioned above? Share your experiences in the comments to help others learn from your insights.

This comprehensive guide aims to equip you with the knowledge and tools to identify and control cotton root rot in fruit and nut trees effectively. By understanding the symptoms, hidden threats, and control measures, you can safeguard your plants against this destructive fungal disease. Remember to monitor your trees regularly for early signs of infection and take proactive steps to protect them from cotton root rot. Your efforts can make a significant difference in preserving the health and vitality of your orchard.