Texas root rot (also known as Phymatotrichopsis root rot, Phymatotrichum root rot, cotton root rot, or, in the older literature, Ozonium root rot) is a
disease
A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
that is fairly common in
Mexico
Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...
and the southwestern
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
resulting in sudden
wilt and
death
Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including brainstem, and brain ...
of affected
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
s, usually during the warmer months. It is caused by a soil-borne
fungus
A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from th ...
named ''Phymatotrichopsis omnivora'' that attacks the
root
In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the sur ...
s of susceptible plants. It was first discovered in 1888 by
Pammel and later named by
Duggar in 1916.
A monograph of this disease, which includes a historical review, was written by R.B. Streets and H.E. Bloss in 1973.
Host and symptoms
''Phymatotrichopsis omnivora'' is a necrotic fungal pathogen that has a very broad host range, attacking almost 2000 dicotyledonous species. It inhabits alkaline, calcareous soils in southwestern United States.
It particularly targets dicots as most monocots are immune.
Economically important plant hosts affected by the pathogen include: peanuts, cotton, alfalfa, apple, pecans, and ornamental trees.
First symptoms of disease is often
chlorosis
In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...
of the leaves then followed by browning and wilting. Two weeks of the first symptoms appear, the plant dies.
In the field, infected cotton plants exhibit wilting in the mid to late summer form large circular patches and later die. Upon closer examination, the host plant's vascular system show extensive discoloration.
Beneath the soil surface, other observable signs are present. Distinctive cruciform branched hyphae develop on infected root tissue which are observable with compound microscope (Figure 1). In addition, taproots of the infect plant are covered in mycelial strands.
![Cuciform Hyphae of P](https://upload.wikimedia.org/wikipedia/commons/c/c1/Cuciform_Hyphae_of_P._Omnivera.jpg)
Another macroscopic sign are tan and white spore mats developed on the soil surface near the infected plants during favorable high moisture environmental conditions.
Despite the name, these spore mats are not known to aid in dispersal. Although presence of the conidial phase on the spore mats is known, the function of the conidia remains unknown since conidial germination is rarely observed
Environment
The highest concentrations of Texas root rot disease are found in the southwestern United States and northern Mexico, with cases also reported in
India
India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
and
Pakistan
Pakistan ( ur, ), officially the Islamic Republic of Pakistan ( ur, , label=none), is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a population of almost 24 ...
.
Texas root rot is caused by a pathogen that prefers
alkali
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
ne and
calcareous
Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.
In zoology
''Calcareous'' is used as an adje ...
soils (pH between 7-8.5), affording the potential for control via soil amendment.
Scientific research has discovered that both high precipitation and high temperatures (below 93 degrees
Fahrenheit
The Fahrenheit scale () is a temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit (symbol: °F) as the unit. Several accounts of how he originally defined his ...
) are further environmental enhancers for Texas root rot by increasing its
virulence
Virulence is a pathogen's or microorganism's ability to cause damage to a host.
In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ca ...
.
The pathogen earned its common name because of the geographical area where it is most prevalent, the cotton-growing region of
Texas
Texas (, ; Spanish language, Spanish: ''Texas'', ''Tejas'') is a state in the South Central United States, South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2 ...
.
Texas, an area located in the southern half of the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
, receives an annual rainfall between 127 cms and less than 25 cms.
Researchers collected data on Texas root rot virulence over 13 years at the
Blackland Research Center using cotton plants infected with Texas root rot and found that virulence correlates somewhat with precipitation in the range of 36–100 cm of rainfall.
Higher virulence of Texas root rot was observed after large precipitation events.
These experimental findings support the conclusion that Texas root rot preferentially develops in wet over dry soil conditions.
The same study also found that virulence of Texas root rot develops inversely to air temperatures over 93 degrees Fahrenheit.
The symptoms of Texas root rot are most severe during hot summers when air temperature increases the average soil temperature over 80 degrees Fahrenheit. The
elevation
The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface (see Geodetic datum § Vert ...
in temperatures produces Texas root rot’s most severe symptoms:
wilting
Wilting is the loss of rigidity of non-woody parts of plants. This occurs when the turgor pressure in non- lignified plant cells falls towards zero, as a result of diminished water in the cells. Wilting also serves to reduce water loss, as it ...
and bronze colored leaves.
The pathogen is only active in the summer months when the high average temperatures can heat the soil microclimate one foot deep over 80 degrees Fahrenheit.
Disease cycle
![Disease Lifecycle of P](https://upload.wikimedia.org/wikipedia/commons/7/7c/Disease_Lifecycle_of_P._Omnivera.jpg)
The disease overwinters as sclerotia or as mycelium on dead plant tissue. In spring to early summer, the germination phase begins with hyphae growth and root colonization. In middle and late summer, the disease reaches its infectious stage when associated symptoms are observed.
The pathogen penetrates the host and colonize plant root tissue causing a root rot. This results in the first symptoms of the disease manifested by chlorotic leaves and eventually wilting.
Root rot disease rings enlarge, and the field can be categorized into three zones based upon plant status: asymptomatic, disease front and survivor. The pathogen disseminates by infecting neighboring plants, with infected plant tissue serving as a secondary inoculum and further spreading the disease. In situations of high moisture, conidia are produced on spore mats but their role in dispersal is unknown since conidia rarely germinate.
''Phymatotrichopsis omnivora'' form several kinds of differentiated hyphae. Initially, hyphae emerge from sclerotia overwintering in soil.
Sclerotia are the primary inoculum in affected fields.
The emerging hyphae either infects the host root or form mycelial strands with a differentiated rind. Upon contact with host roots, ''P. omnivora'' forms a mycelial mantle on the root's surface.
This leads to necrosis of epidermis and underlying cortical tissue, leading to root lesions. As the disease progresses, the roots are covered by the characteristic cinnamon-colored mycelial strands covered with acircular sterile hyphae, a diagnostic sign of Texas Root Rot.
The roots at later stages of infection show extensive vascular discoloration due to root necrosis. The mycelial strands and symptom development in field-infected roots are especially conspicuous on cotton.
During the late summer and fall, mycelial strands formed on the root surfaces or in the soil form sclerotia to survive the winter, thus completing the life cycle.
Pathogenesis
As a soil borne pathogen, ''P. omnivora'' enters the plant host via the roots.
It penetrates the host by growing infectious hyphae that cover the host plant root's epidermis and eventually infects epidermis and cortical cell junctions of plant host instead of having specialized penetration organs like an appressoria.
From there the fungal pathogen infects root vascular system and begin cause cortical root lesions, which is most pronounced in cotton. Microarray analysis and gene expression profiling revealed that certain pathways related to plant defense such as jasmonic acid, ethylene, and flavonoid production were reduced at later infectious stages.
This suggests that ''P. omnivora'' is able to suppress the production of these phytochemical defenses to ensure disease success.
Control
The most common management strategy to limit the spread and damage produced by Texas root rot is soil manipulation. The goal of soil manipulation is to create a soil environment that is not favorable to Texas root rot to limit its activity. A common approach is to change the soil acidity/soil
pH because the pathogen prefers alkaline soils. A recommended method to decrease soil pH is through the application of
ammonium sulfate
Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen a ...
or
ammonium phosphate
Ammonium phosphate is the inorganic compound with the formula (NH4)3PO4. It is the ammonium salt of orthophosphoric acid. A related "double salt", (NH4)3PO4.(NH4)2HPO4 is also recognized but is impractical use. Both triammonium salts evolve ammoni ...
fertilizer at around 4.5 kg per 9.3 m^2. Applications of fertilizer in this manner causes the soil pH to decrease making it less favorable to Texas root rot disease which prefers a more basic soil environment.
This management method is impracticable for any moderate or large scale agricultural operation because of the cost required to amend large areas of soil. However, it is well-suited for hobby gardens or other small scale operations.
Texas root displays a varied infection pattern so the most effective treatment plans contain a variety of management strategies. Other control methods for Texas root rot are deep plowing of the soil post-harvest, the establishment of protective plant barriers, and the incorporation of
organic crop residues into the soil.
Deep plowing after harvest breaks up potentially infected soil 6 to 10 inches deep and has been shown to combat Texas root rot by disrupting the pathogen’s ability to form
sclerotia
A sclerotium (; (), is a compact mass of hardened fungal mycelium containing food reserves. One role of sclerotia is to survive environmental extremes. In some higher fungi such as ergot, sclerotia become detached and remain dormant until favor ...
.
The planting of resistant grass crops, such as
sorghum
''Sorghum'' () is a genus of about 25 species of flowering plants in the grass family (Poaceae). Some of these species are grown as cereals for human consumption and some in pastures for animals. One species is grown for grain, while many othe ...
, to surround an already infected area can limit the spread of Texas root rot to other areas. By surrounding susceptible crops with immune grasses, a barrier layer in the soil can be created to block root infection of susceptible crops.
Organic amendments are an effective treatment against Texas root rot when applied to the soil before spring planting. The most effective amendment is composed of residues from
wheat
Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus ''Triticum'' ; the most widely grown is common wheat (''T. aestivum''). The archaeologi ...
,
oat
The oat (''Avena sativa''), sometimes called the common oat, is a species of cereal grain grown for its seed, which is known by the same name (usually in the plural, unlike other cereals and pseudocereals). While oats are suitable for human con ...
s, and other
cereal crops
A cereal is any Poaceae, grass cultivated for the edible components of its grain (botanically, a type of fruit called a caryopsis), composed of the endosperm, Cereal germ, germ, and bran. Cereal Grain, grain crops are grown in greater quantit ...
. Though sources disagree, crop rotation is not an effective control method because of Texas root rot’s wide host range of over 2,000 host species.
Overall, it is best to avoid areas known to suffer from the Texas root rot because no management technique is a guarantee to control the pathogen.
Importance
Texas root rot (''Phymatotrichopsis omnivora'') is regarded as one of the most impactful diseases of woody dicotyledon plants in large part because of its wide array of dicotyledon host plants, featuring one of the largest host ranges of any known fungal pathogen with over 2,000 possible host species.
Though Texas root rot has been studied since 1888, there are few effective management tools because of the unique biological characteristics of the pathogen. Namely, its ability to last virtually indefinitely in soil and its capacity to survive on roots of native vegetation without producing symptoms.
The species' broad host range also makes management difficult because disease populations can be maintained across various host species.
Texas root rot presents a serious economic threat to cotton growers with losses of upwards of $29 million in cotton found in Texas as estimated by Texas A&M.
References
External links
Cotton (Texas) Root RotIndex Fungorum - ''Phymatotrichopsis omnivora''USDA ARS Fungal DatabaseEPPO Quarantine pest: ''Phymatotrichopsis omnivora''Persoonia - Molecular Systematics of ''Phymatotrichopsis omnivora''
{{DEFAULTSORT:Texas Root Rot
Phymatotrichopsis
Fungal plant pathogens and diseases
Cotton diseases