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Historical information on
Phymatotrichopsis omnivora (Duggar)
Hennebert, the root rot fungus
Discovery - 120 years ago
• In 1888, Dr. Gulley, director of the TAES commissioned L. H.
Pammel, graduate student at Washington University, St. Louis, MO
to explore the root worm believed to be the culprit.
– He examined cotton roots in TX and saw mycelial strands on the
surface, dispelling the worm theory as well as many other theories
• He published his findings in 2 TAES bulletins in 1888 and 1889.
• Used findings for his Master’s degree at the Univ. of WI.
• Samples were submitted to
Professor Farlow at Harvard who
identified the fungus as Ozonium
auricomum Link.
• In 1907 the fungus was renamed as
O. omnivorum Shear based on a
comparison to an type culture of O.
auricomum.
• Duggar in 1916 discovered the
conidial state of the life cycle and
the fungus was renamed
Phymatotrichum omnivorum
(Shear) Duggar.
– JJ Tabenhaus joined the faculty of
Texas A&M with a mission to study
P. omnivorum
Taxonomy
• Phymatotrichum omnivorum (Shear) Duggar is a
filamentous fungus in the subdivision Dueteromycotinia,
Order moniliales and Family moniliceae.
– Genus Phymatotrichum was created in 1851. The name is Greek in
origin: phymato = tumor or swelling, tricho = hair, omnivora = all
devouring
• In 1973, P. omnivorum was studied by Hennebert and he
erected a new genus: Phymatotrichopsis omnivora
(Duggar) Hennebert
– No conclusive evidence of a telemophic stage
Geographic Distribution –
Southwestern United States & Mexico
– Texas – 1888
– Arizona – 1898
– Mexico States:
Sonora, Coahuila,
Chihuahua, Sinaloa, Nuevo Leon, Durango,
Baja California & Tamaulipas
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–
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– by 1922
New Mexico – 1923
Southern CA – 1928
Arkansas – 1929
Southern Utah – 1933
Nevada – 1936
Oklahoma – 1939
Louisiana - 1947
The Fungus:
In 1923, Tabenhaus and Killough identified the survival stage of P. omnivora, but
described the fungus as an obligate parasite surviving as mycelium on plant tissue.
Vegetative (Mycelial) Stage: P. omnivora produces two
types of hyphae.
•Large celled hyphae with right-angle branching and
multinucleated cells with septa 60-120 µm apart with
sepetal pores and Worin bodies
•Smaller hyphae, branched in pairs that give rise to
acicular hyphae with characteristic cruciform branches.
•Young hyphae are hyaline and at maturation are brown
in color
•Strands are the infection network that connect one host
to another. Also serve as the translocation vehicle of
acquired nutrients deep in to the soil to the over
seasoning propagules.
The Fungus:
Six years later King and Loomis observed mycelial growth and sclerotia
development in the absence of a host; this proving the fungus was not an
obligate parasite.
•Sclerotial Stage: Primary survival propagules and inoculum for the disease.
– Multinucleated parenchymatous cells, surrounded by a layers of thick walled
cells and a thick outer melanin rind.
– Variable in size and shape, often taking the shape of the air pockets in the
soil.
– Considerable carbon source for a growing mycelium.
•Conidial Stage: Buff colored spore mats are occasionally produced that
consist of hyphae and blastocondia.
– The mats are observed under rainy conditions or irrigation.
– Seldom produced in vitro and role in the life cycle not known.
– It is not possible to produce sclerotia or to produce disease symptoms from
spore inoculations.
Life Cycle: Simple, but Powerful !
1.
2.
3.
4.
Vegetative mycelium grows through
the soil profile until it contacts a root.
The mycelium grows up the root to
right below the soil surface where
infection occurs
Mycelium translocates nutrients from
the plant and stimulates sclerotia
production, deep below the soil
surface.
Sclerotia over seasons until
environmental conditions are
favorable for germination of new
mycelium. Starting the process again
Means of Parasitism –
Indiscriminate Pathogen with Little Host Specificity
• Commences with the flowering of the host.
• At the root-soil surface; the fungus invades through wounds,
lenticels or by mechanical force and kills the periderm tissue.
• In addition to the invasion, there is evidence of an unknown toxin
produced by the vegetative mycelium that is capable of
weakening or killing cells allowing for aggressive invasion
between all of the cells, cortex, endodermis, pericycle, etc.
• The vascular system of the root is invaded blocking the flow of
water and phytosynthates.
Pathogenesis
• Parasitizes over 2000 hosts, dicotyledonous
plants species
– Most tap-rooted agronomic and horticultural
plants are highly susceptible to P. omnivora.
• First symptom is slight yellowing of leaves,
followed by wilting of the top most leaves with
48 hrs. Wilting of the lower leaves within the
next 24 hrs.
• Dead occurs quickly, brown leaves remain
attached to the plant
• A reddish-brown discoloration is visible at the
point of invasion beneath the periderm or bark.
– Reports of monocotyledonous plants being
resistant to P. omnivora are based more on
the root architecture than any other factor.
• P. omnivora can be sustained on fibrous roots.
Lesions can be observed as well as reduced
root systems.
• Crop rotations with monocot plants seldom
reduce disease incidence when ground is
replanted to susceptible host
Environmental Influences
• Chemical composition of the soil affects the growth and survival of the
fungus. Indigenous to calcareous, montmorillonitic clay soils.
– Alkaline, pH ~ 8.0
• Does not form sclerotia in acidic soils
– Extensive expanding and shrinking capacity
• Soils with poor gas exchange favor the sclerotial producing state
– Moisture content is vital for fungal growth and disease development (45%
holding capacity)
• Mycelium growth and sclerotia production from 15-35 C
– Northern boundary of PRR was influenced by temperature
– P. omnivora grows most rapidly in a laboratory at 28 C
– Indigenous to regions where annual mean temperature is 16.6 C or higher
• Root Rot Belt is not uniformly infested
– Many individual agronomic acres are infested with P. omnivora
– Investigations at the Blackland Research Center in Temple, TX show 1% or
less of the total soil to be infested
Why are the heavy clay soils conducive for
Phymatotrichum Root Rot?
• Grows best under soil conditions of poor aeration P. omnivora is
tolerant of high concentrations of CO2.
• Sclerotia formation correlated with the accumulation of CO2.
– Sclerotia have been found as deep as 8 ft, but the majorities are located 20
– 60 centimeters below the soil surface.
• A 3 year study showed at 30-60 cm below the soil surface CO2
was highest in June and July. A rise in CO2 correlates to rainfall
when the soil expands and closes cracks in the soil. Soil with 23% accumulation of CO2 enhance the life cycle of the fungus.
• The elevated CO2 environment gives the fungus a competitive
advantage over many of the indigenous microorganisms.
Epidemiology
•
The incidence of PRR varied significantly from
one year to the next, even in soils with known
infestation
•
P. omnivora forms circular areas of infestations
that gradually enlarge over time
– In cotton fields rate of movement ranged from 5-30
ft/year. (Faster in alfalfa fields and slower in
orchards)
– Fungus moves both parallel and perpendicular to
rows
•
P. omnivora has shown to be spread by river
water. Possible explanation of movement into LA
by Red River from TX & AR.
•
No vectors are known for the disease, but
transplanted trees, either infected or by
associated infested soils can move the disease
into previously not infested environments.
What can be done?
The Blackland Experiment Station is
dedicated to studying and dealing
with the problems of the Texas black
waxy lands. Experimental control
plats are in the foreground.
http://waterhome.brc.tamus.edu/present
ation/experiment/index.html
“Root rot seems to be one evil that the cotton farmer in the Blacklands must
learn to live with. The fungus has been found as deep as eight feet in the
ground. One field on Temple Station in which there had been heavy loss from
root rot has laid fallow since 1928, with no crop planting of any kind. The land
has been plowed at regular intervals to simulate a cultivated state but no
treatment has been used on it. This year (1940), for the first time since 1928,
four rows of cotton were planted on this ground. Root rot has developed with the
usual intensity in these four rows of cotton, very good evidence that letting the
land lie idle will not get rid of this disease.”
Control Measures – Environmental Manipulation
• Deep Tillage ~Earliest recorded studies were documented
in 1907; results for disease control were highly variable
– Movement of soil causes disruption of the formation of new sclerotia
– Deep plowing causes a marked reduction in disease when compared
to conventional practices
• Most effective when sclerotia population is the upper 30-45 cm of the
soil profile. Sclerotia can be found down to 90 cm.
– Deep plowing encourages erosion and moisture depletion
– Very costly
• Dynamite Subsoiling ~ A three year study in TX investigated
aerating soils by dynamiting. Increased yields for both corn
and cotton was observed. Yield differences would not pay
for cost.
Control Measures – Environmental Manipulation
• Flooding ~ Root rot does not occur on land prone to
occasional flooding
– Studies in AZ showed flooding over 60 -90 days reduced disease
but did not eradicate the fungus.
– A study in TX flooded a field to a depth of 15 cm for 120 days – no
reduction in disease observed.
• Root Rot barriers ~ Trenches were dug around infested
soils to prevent the spread of the fungus.
– A 30 cm ditch kept the fungus from spreading for 40 days; a 60 cm
ditch was required for an entire season.
– Both galvanized sheet metal and concrete barriers were placed in
the soil profile to prevent movement of the fungus.
– Chemical barriers were utilized. Sulfur and copper sulfate were
placed in trenches.
Calcium versus Sodium
•
As early as 1891, common table salt was
utilized to inhibit the disease in cotton fields.
– Over wintering sclerotia were reduced in the
presences of NaCl in large container studies.
– Lyda and Kissel analyzed total exchangeable
sodium in infested a and non-infested areas.
There was 4-10 less exchangeable sodium in the
infested areas.
•
Replacing calcium with sodium in infested soils
correlated with a reduced potential for
producing sclerotia.
– The decline was proportional to the level of
sodium
•
Soils conducive to infestation of P. omnivora
have sodium contents ranging from 100 to
2000 ppm within a single field
– Viable explanation for the localized distribution of
the disease
Control Measures – Biological Control
• Soil fungi ~ Many fungi are found on root tissue of roots
dead from PRR
– P. omnivora is a poor saprophyte: many other fungi will out grow
and out compete for nutrients
– Work in the 1980’s isolated several fungi from the environment of
sclerotia and investigated mycoparasitism success. Consistent
control not achieved.
• Green manure ~ Onset of disease is delayed
– In 1937 Presley reported the effect of manure was not just the
influence of added nutrients, N, P, K, but also from the increase
population of soil microorganisms.
– Several bacteria were able to stop the growth of P. omnivora
under laboratory conditions. Erratic results dependent on
climatic and geographic environment.
Control Measures – Chemical Control
• Soil fumigation ~ In the 1960’s, Lyda et al compared the
efficacy of several fumigants to control PRR.
– All of the fumigants, Telone, Trizone, Dowfume MC-33 and methyl
bromide all showed efficacy.
– Telone was effective with 38 L/ha when applied at 50-60 cm deep
and with 115 L/ha when applied at 15-20 cm. Yield increases
ranged from 140 – 150% compared to cotton yield on untreated soil
– Anhydrous ammonia and ammonium salts are toxic to P. omnivora.
The minimal movement in soil makes this technique less promising.
– Very expensive due to application depth needed to achieve control.
Control Measures – Chemical Control
• Soil fungicides ~ Success is variable
– Large amounts required to treat an acre especially at the depth
sclerotia are found
• Many fungicides do not have the ability to move sufficiently in soil
• Application timing late in the season is difficult
– Introduction of systemic fungicides brought renewed interest
• Benzimidazole fungicides worked well in the greenhouse, but the
inability to control soil moisture in the field made success less than
desired.
• Triazole fungicides are very toxic to P. omnivora. Timing of application
and application depth in row-crops is often not achievable in cotton
growing dry land regions.
– On irrigated lands the reduction in number of dead plants shows
successful progress.
What will the next 100 years bring?
Thanks to and my apologies to anyone that I have copy wrote pictures/ text
Texas A&M University, Extension Service and Experiment Station
University of Arizona
University of New Mexico
University of Arkansas
Louisiana State University
Texas Tech University
And to my PO mentor:
Dr. Stuart D. Lyda
Working on this fungus help me understand that all things are not easily
done, and helped me to learn perseverance and the importance of not
giving up….
The characteristics of this truly unique plant pathogen