Phymatotrichum Root Rot

February 2017


Phymatotrichum root rot, also known as cotton root rot and Texas root rot, is caused by the fungus Phymatotrichopsis ominvorum.  The fungus attacks more than 2,000 species of broadleaf plants, but does not affect monocots (grasses).  The disease is economically important to cotton, alfalfa, peanut, ornamental shrubs, and fruit, nut and shade trees.

The fungus is capable of persisting for a long period of time in the soil.  It is most destructive in soils that are of limestone origin (calcareous), highly alkaline (high pH), and are exposed to high summer temperatures.  Therefore, soil type and cold temperatures limit the geographic distribution of this disease to areas in the southwestern and south central United States.  In Oklahoma, Phymatotrichum root rot is mostly restricted to the southern tier of counties in the Red River Valley.  The disease has been reported in the following counties: Atoka, Bryan, Carter, Choctaw, Comanche, Cotton, Garvin, Jefferson, Johnson, Kiowa, Love, Marshall, McCurtain, Muskogee, Pushmataha, Stephens, and Tillman.

Symptoms

Symptoms of Phymatotrichum root rot occur  most often from June through September after soil temperatures reach 82 F.  In row crops, symptoms appear as patches of wilted plants, which rapidly die.  Trees and shrubs may die more slowly.  The leaves of affected plants dry, turn brown, and remain attached to the plant.  Affected areas expand to form circular areas of dead plants (Figures 1 and 2).  Newly wilted plants develop at the leading edge of the affected areas as the infection centers enlarge during warm, moist periods.  Grassy weeds may invade areas killed by the fungus.  Affected areas continue to enlarge in subsequent years as the fungus grows through the soil from plant to plant.  Reported rates of spread for alfalfa may be as much as 2 to 8 feet per month in mid summer and 5 to 30 feet per year in cotton.

Taproots of newly wilted plants are severely decayed.  The surface of decayed roots may be partly covered by a fuzzy, tan-colored mold (mycelium).  The bark strips readily from the surface of decayed roots, revealing a reddish brown stain along the white woody tissue.

A unique feature of this disease is the production of spore mats that may form on the soil surface near dead plants.  The spore mats are circular  patches of white moldy growth of varying size.  The spore mats turn tan in color before they dissipate after a few days.

Figure 1. Circular area of alfalfa killed by Phymatotrichum root rot.

Disease Cycle

The fungus survives almost indefinitely in the soil as small (1 to 5 mm in diameter), resistant, seed-like structures called sclerotia.  Sclerotia have been recovered from soil at depths of up to 8 feet.  In the summer, mycelium from germinating sclerotia or from fungal fragments overwintering on roots of perennial plants infects nearby roots. The fungus enters the roots of infected plants, decaying the bark and penetrating the woody portions of the root.  Nutrients derived from infected plants support the growth of the fungus through the soil to infect new plants.  The function of spores produced on spore mats in the disease cycle is unknown.

The fungus appears to be indigenous to certain fields and there is little known on how the fungus spreads to new fields.  However, disease may develop following the transplanting of infected plants.

Control

Although there are several management practices available that help reduce the occurrence and severity of this disease, none are highly effective.  Attempts to control the disease with fungicides and soil fumigation have not been successful.  The most effective strategy is to define areas infested with the fungus and plant crops and plant species that are not susceptible to the disease.

Use of a moldboard plow (6 to 8 inches deep) has reduced the disease.  Deep plowing immediately after harvest is thought to disrupt the formation of sclerotia in the upper soil profile.

Crop rotation for three to four years with a grass crop such as grain sorghum has been beneficial in some instances.  Rotation may not reduce levels of sclerotia in the soil.  The effects of crop rotation are thought to result from the addition of organic matter to the soil, which increases the activity of soil microorganisms that compete with or inhibit the fungus.  The direct addition of organic matter to soil and the incorporation of green manure into soil are thought to produce the same  beneficial effect.  The decomposition of green manure also may liberate chemicals that inhibit the fungus.

Barriers of resistant crops can be used to limit the spread of the fungus.  Grain sorghum is typically planted in border rows surrounding infested areas.

Planting early and the use of early maturing varieties have been used in some crops to escape the disease.  The goal is to harvest before the disease becomes active in the summer.  In Oklahoma, vegetable production systems in the spring may be useful.

Addition of fertilizers and other amendments such as sulfur have been used in attempts to change the alkalinity of the soil, but results have been variable.  Nitrogen applied as anhydrous ammonia may be beneficial for cotton production.  However, one of the more effective treatments was devised in Arizona in 1939 for infected trees and shrubs and for replanting valuable plants into infested sites.  A ridge of soil is formed around each plant at the drip line.  The entire basin within the ridge is covered with cow manure or similar organic matter to a depth of 2 inches.  Ammonium sulfate and sulfur, each at a rate of 1 lb. per 10 sq. ft., is broadcast over the manure.  The basin is then flooded with sufficient water to soak the soil to a depth of 3 feet.

Trees or shrubs showing slight symptoms of the disease also can be cut back to one-half the top growth to compensate for roots lost to the disease.  The soil should be kept moist for several weeks after treatment.  Recovery is usually complete within the season.  This technique is most effective when applied before the root rot is extensive.

Resistant varieties have not been developed for crops that are normally susceptible to the disease.  Therefore, crop and plant species that are naturally resistant must be selected.  The following list is intended to aid in selecting resistant plants and avoiding susceptible ones for areas known to be infested.

Shade and Ornamental Trees

Shade and Flowering Trees

Susceptible and Intermediate

Alder

Ash

Beech

Black Locust

Catalpa

Chestnut

Chinese Tallow Tree

Chinquapin oak

Chittanwood

*Cottonwood

Cypress

*Dogwood

Elm (all but cedar-elm)

Filbert

Hawthorn

Hazelnut

Honeylocust

Hoptree

Hop-hornbeam

Locust

*Silver Maple

*Mulberry (all but weeping)

Oak (all except live oak)

*Pistachio

Poplar

Redbud

Sassafras

Soapberry

Sweet Gum

Tree of Heaven

Willow

Witch Hazel

*Texas Umbrella

Resistant

Cedar-elm

Live Oak

Southern Magnolia

Weeping Mulberry

Conifers

Susceptible and Intermediate

Arborvitae

Cypress

Pine (all but Japanese Red Pine)

Spruce

Resistant

Cedar (Juniper)

Japanese Red Pine

Shrubs

Susceptible

Amur and Chinese Privet

Cotoneaster

Euonymus

Lilac

*Loquat

*Roses

Spirea(s)

*Waxleaf Ligustrum

(Ligustrum lucidum)

Intermediate

Arborvitae

Butterfly Bush (Buddleia)

Boxwood

Cactus

California Privet

Japanese Barberry

Jasmine

Japanese Privet

(Lingustrum japonicum)

Laurel

Pyracantha

Rhododendron

Trumpet Creeper (Campsis)

Virginia Creeper

Ivy (Hedera)

Rhus copallina lanceloata

Resistant

Bamboo

Crape myrtle

Holly

Evergreen honeysuckle

Eucalyptus

Oleander

Pomegranate

Fruit and Nut Crops

Susceptible

Apple

Apricot

Avocado

Cherry

Grape

Japanese persimmon

Peach

Pear

Quince

Walnut

Intermediate

Blackberry

Common persimmon

Gooseberry

Pecan

Plum

Raspberry

Red Currant

Resistant

Black Currant

Strawberry

Vegetable Crops

Susceptible

Beans

Beets

Carrots

Chard

Endive

Horseradish

Jerusalem artichoke

Lima beans

Okra

Parsnip

Pepper

Rhubarb

Salsify

Sweet potato

Intermediate

Eggplant

Globe artichoke

Irish potato

Lettuce

Radish

Rutabaga

Tomato

Turnip

Watermelon

Resistant

Asparagus

Cabbage

Cantaloupe

Celery

Cauliflower

Cucumber

English pea

Garlic

Kale

Mint

Onion

Spinach

Squashes (most)

Flowers

Susceptible

Chrysanthemum

Dahlia

Hollyhock

Peony

Poinsettia

Intermediate

China Aster

Cosmos

Morningglory

Sunflower

Resistant

Calendula

Calla Lily

Candytuft

Carnation

Coleus

Columbine

Crocus

Ferns

Gaillardia

Geranium

Gladiolus

Gypsophila

Hibiscus

Iris

Lily

Larkspur

Mignonette (Reseda)

Narcissus

Nasturtium

Pansy

Petunia

Phlox

Poppy

Sage

Snapdragon

Stock

Sweet pea

Sweet William

Tube-Rose

Tulip

Verbena

Vinca (Periwinkle)

Violet

Zinnia

Field Crops

Susceptible

Alfalfa

Cotton

Cowpeas

Peanuts

Soybeans

Velvet beans

Intermediate

Broad beans

Flax

Mung beans

Vetch

Resistant

All grasses

Barley

Corn

Millet

Oats

Rye

Sorghum

Wheat

*Highly susceptible

 

John P. Damicone
State Extension Plant Pathology Specialist

 

DASNR Extension Research CASNR
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Oklahoma State University
Stillwater, OK 74078
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