CONTROLLING COTTON ROOT ROT ON ORNAMENTAL
PLANTS
by 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
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.
Causal Organisms
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.
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 species.
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
LARGE SHRUBS OR SMALL TREES: 10’ ‘TO 25’
Texas Persimmon
Japanese Holly
Possum Haw Holly
Yaupon holly
Texas ebony
Mexican plum
Pomegranate
Texas Mountain laurel
TREES: 30’ & ABOVE
Hickory
Pecan
Atlas Cedar
Deodara Cedar
Hackberry
American Holly
Eastern Red Cedar
Osage Orange
Japanese Black Pine
Sycamore
Honey Mesquite
Live Oak
Texas Palmetto
Japanese Pagodatree
Cedar Elm
