Walter J. Walla and Everett Janne
Extension plant pathologist and Extension landscape horticulturist
The Texas A&M University System.
Published originally as TAEX
L-2056, November, 1982
Cotton root rot, caused by the fungus Phymatotrichum omnivorum,
also is known by several other names such as Phymatotrichum
root rot, Texas root rot and Ozonium root rot. It is
one of the most destructive plant diseases and attacks more
than 2,000 species. However, either the fungus infects but does
not kill monocotyledonous plants (grasses, etc.), or these plants
are all highly resistant. In Texas, the disease is economically
important in cotton, alfalfa, ornamental plants, and fruit,
nut and shade trees. The fungus is prevalent in calcareous clay
loam soils with a pH range of 7.0 to 8.5 and in areas with high
summer temperatures. Therefore, the disease is limited to the
southwestern United States.
Phymatotrichum root rot has been reported in Texas
counties from the Red River to the Rio Grande and from Tom Green
County to the Neches River.
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. Affected
areas often appear as circular areas of dead plants in fields
of infected crops. These areas gradually enlarge in subsequent
years as the fungus grows through the soil from plant to plant.
Infested areas as may increase 5 to 30 feet per year.
Phymatotrichum omnivorum exists in the soil in three
distinct forms: (1) hyphae and strands (rhizomorphs), (2) sclerotia
and (3) sporemats and conidia.
Hyphae and strands. The fungus produces root-like strands
(rhizomorphs) that grow through the soil until they contact
the descending plant roots. Strands surround a root and grow
toward the soil surface. Immediately below the surface, the
fungus proliferates around the hypocotyl, producing a cottony,
mycelial growth. Below this mycelium, the bark is destroyed,
and the fungus fills the vascular tissue of the plant. Following
death of the plant, sclerotia form in the strands
Sclerotia. Strand cells divide, grow and enlarge to
form sclerotia. These sclerotia are small (1 to 2 millimeters
in diameter), densely compacted masses of thick-walled cells.
Sclerotia are first white, changing to buff, brown and black
with age. They are irregular shaped, generally taking the shape
of the soil space where they are formed. Sclerotia enable the
fungus to persist in fallow soil or soil planted to resistant
crops for several years. Sclerotia have been found as deep as
12 feet in some soils.
Sporemats and conidia. The fungus often forms sporemats
on the soil surface during warm, rainy weather. These mats vary
from 2 to 16 inches in diameter and are white to tan colored.
They are composed of large-celled, branched fungal strands that
later produce conidia. The conidia appear sterile, and their
role in the spread of the pathogen has not been documented.
Phymatotrichum root rot is one of the most difficult
plant diseases to control. Fungal behavior in different crops
and soils and its activity from year to year in the same field
are so erratic that it is ineffective to rely on one approach.
Use a control program consisting of a systematic course of treatment
involving several recognized control methods.
Organic amendments. Significant control of Phymatotrichum
root rot has been achieved by using various crops as organic
matter amendments. A delay in infection is readily apparent
and has resulted in 90 percent reduction in root rot. Wheat,
oats and other cereal crops are effective in delaying infection
and reducing losses when incorporated in soil in the spring.
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
Fertilizer applications. To reduce root rot, apply
fertilizers high in certain nitrogen forms. When nitrogen is
applied as ammonia in a manner to fumigate as much soil as possible,
research shows a reduced incidence of root rot.
In some cases, valuable ornamental plants and orchard trees
have been treated successfully even after root rot infection
has taken place. First prune the tree (or shrub) back and build
a circular ridge (equal in diameter to the top of the plant)
of soil some distance from the trunk. Work 1 pound of ammonium
sulfate into the soil for each 100 square feet of surface within
this ridge. Fill the area within the ridge with water to a depth
of about 4 inches. Repeat the treatment and watering after 5
to 10 days. Do not apply more than two treatments in the same
season. Following this treatment, water frequently to prevent
drought injury. Acidifying the soil with sulfur around susceptible
trees or shrubs may help delay or prevent root rot infection
in areas where the disease is prevalent.
Resistant varieties. Development of resistant plants
using conventional breeding concepts, has been difficult due
to the pathogen's wide host range. However, the following list
of woody and herbaceous plants has shown resistance or tolerance
to cotton root rot and should be considered by the homeowner
where the disease is prevalent. The hardiness zone is given
for each woody plant listed. Check the map to determine the
zone in which you wish to use the plant. Use any plant with
that zone number or a lower number. Plants with a higher zone
number usually will not be hardy in that area. Check the list
for size and foliage type to aid you in selecting the plants
best suited for your particular purpose.
- PLANTS RESISTANT TO COTTON ROOT ROT
Based on the work of J.J. Taubenhaus and W.N. Ezekiel as well
as the work of H. E. Smith in L-390 Cotton Root Rot.
Click here for the article:
A RATING OF PLANTS WITH REFERENCE TO THEIR
RESISTANCE OR SUSCEPTIBILITY TO THE COTTON ROOT-ROT FUNGUS