(Texas) Root Rot
Extension, College of Agriculture & Life Sciences, The University of Arizona
Mary Olsen, Plant Pathology Specialist
At a Glance:
- Cotton Root Rot commonly causes a sudden wilt and death of susceptible
plants in summer months but may also cause a slow decline, especially
at cooler temperatures.
- Positive identification of disease by an experienced person is essential.
- Replant infested soils only with tolerant or immune plants.
Soil borne fungus, Phymatotrichopsis omnivora (= Phymatotrichum omnivorum)
Many dicotyledonous trees and shrubs.
Cotton (Texas) Root Rot often causes a rapid wilt and death
of the host in the late spring, summer and early fall when temperatures
are warm. Dead and dying leaves remain attached to the plant. However,
infected plants also may decline more slowly, especially at cooler temperatures
and when plants are well cared for. The roots of dying or declining plants
are rotted. With careful examination under at least 10X magnification,
light brown strands or hyphal webs of the fungus can be observed on the
root surface. With sufficient moisture, the fungus may occasionally produce
a white to light tan sterile spore mat on the surface of the soil near
the host, but these mats are not common.
Rapid wilting and death occurs in hot weather due to the inability of
the host plant to take up enough water through its rotted roots. Thus,
Cotton Root Rot is usually considered a warm weather disease. However,
host plants, especially mature trees, may have been infected for some
time and die rapidly in warm weather as transpirational demand for water
Cotton Root Rot occurs throughout the southwestern United States and
Mexico. It is easily recognized in infested cotton fields in late summer
by large areas of dead plants, hence its common name. It is most common
in the low desert areas where winters are mild, but also occurs at higher
elevations, up to at least 5000 ft, where susceptible plants are introduced.
Disease occurs in different soil types and in areas as diverse as the
low lying flood plains of rivers and washes of central and western Arizona
and the higher grassland hills of southern Arizona. The pathogen, Phymatotrichopsis
omnivora (also called Phymatotrichum omnivorum), is an indigenous soil
borne fungus that is found deep in soils. P. omnivora produces hyphal
strands that colonize the roots and cause rot of the entire root system.
A dense web of hyphae covers the root once the fungus has penetrated and
caused decay. The strands grow through the soil and infect healthy roots
nearby. The fungus also survives for long periods of time in the soil
by producing hyphal structures called sclerotia that have been found as
deep as 12 ft in soils. Since P. omnivora produces no airborne spores
or other reproductive structures, it spreads only by growth of the strands
in soil. It has an extremely wide host range and has been reported as
a pathogen of over 2000 dicotyledonous plants. Monocots are immune.
Unfortunately, there is no way to test soils for presence of the fungus
other than planting a susceptible plant. Since other pathogens can cause
root rots and other factors could result in similar symptoms, it is very
important that a positive identification of the pathogen be made by an
experienced person. Hy-phae and strands of the fungus used for diagnosis
are easiest to find on fresh tissue but can also be found on older, decayed
How to sample
If the plant is dead or dying, remove as much of the root system as possible
when taking it out. Take several samples of rotting and discolored roots
on which the outer or cortical tissue still remains attached. The samples
should be pencil size or slightly larger and at least 6 inches long.
Leave soil attached and keep the roots cool (refrigeration is fine) in
a plastic bag. Do not add water or wet paper towels. Submit the sample
to your County Extension Office or The Department of Plant Pathology,
Forbes 204,The University of Arizona, Tucson, AZ 85721. Please send samples
early in the week to avoid delays in transit.
Susceptible plants should not be planted in areas where Cotton Root Rot
is known to occur. Trees such as fruit and nut trees, ash, cottonwood,
elms, figs, sycamore, bottle tree, silk oak, pepper tree and African sumac
are considered very susceptible. Many shrubs including pomegranate, xylosma,
cassia, Mexican bird of paradise, oleander, and roses also are very susceptible.
Annuals usually escape disease since they are in the ground such a short
time or are planted in winter months when the fungus is apparently less
Tolerant and immune plants
Although many dicotyledons have been found to be susceptible to some
degree, some are very tolerant. Mesquites, palo verde, Atriplex, hackberry,
jojoba, and cacti are tolerant and remain healthy in landscapes where
other plants have died from disease. All monocots, such as palms, yuccas
and grasses are immune and are good choices to plant anywhere that Cotton
Root Rot has been diagnosed. Citrus, eucalyptus, tamarisk, and pine are
considered tolerant, but Cotton Root Rot has been confirmed on all of
these trees. Check the list of tolerant or immune plants before planting
in any area where Cotton Root Rot has been identified and before replanting
a site in which a plant has died from this disease.
Treatments with soil additives, such as manures and fertilizers, are
rarely successful and are not recommended for control. Chemical controls
have been successful in some cases, but are expensive, must be applied
by a licensed applicator, and should be repeated every year or two in
order to control disease.
EXAMPLES OF PLANTS IMMUNE TO COTTON ROOT ROT (MONOCOTS)
||Mediterranean fan palm
||Canary Island date palm
||Mexican fan palm
||California fan palm
PLANTS TOLERANT TO COTTON ROOT ROT (MONOCOTS)
|Celosia argentea var. cristata
|Condalia lycioides var. canescens
||Western Australian floodedgum
| Marrubium vulgare
| Pelargonium spp.
||Southern Live oak
|Rorippa nasturtium aquaticum
|Sambucus caerulea var. arizonica
||European wild pansy
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Document located http://cals.arizona.edu/pubs/diseases/az1150.html
Published February 2000
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