Texas root rot

Phymatotrichopsis omnivora is a necrotic fungal pathogen that has a very broad host range, attacking almost 2000 dicotyledonus species. It is known to inhabit in the alkaline, calcareous soils in Southwest United States.[4] It particularly targets dicots as most monocots are immune.[5] Economically important plant host affected by the species include: peanuts, cotton, alfalfa, apple, pecans, and ornamental trees.

First symptoms of disease is often chlorosis on the leaves. This is then followed by browning and wilting of the leaves. Eventually after two weeks of the first symptoms the plant dies.[4] 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 will show extensive discoloration.[4] Underneath the soil, more observable signs are present. Distinctive cuniform branched hyphae are found on infected root tissue which are observable with compound microscope (Figure 1). In addition, taproots of the infect plant will be covered in myclieal strands.[6]

Figure 1: Cruciform Hyphae is a unique sign observed using a compound microscope

Another macro sign is during favorable high moisture environmental conditions where spore mats appear.[4] Despite the name, the purpose of these spore mats are not known to aid in dispersal. Although presence of the condial phase on the spore mats is known the function of the produced condia remains unknown since condia germination is rarely observed[2] The spore mats are tan and white, found on the soil surface near the infected plant.[4]

The highest concentrations of Texas root rot (Phymatotricum omnvorum) have been found in the southwestern United States and Northern Mexico; with cases also reported in India and Pakistan.[7] Texas root rot is a pathogen that has shown a preference for alkaline and calcareous soils (pH between 7-8.5), leaving the potential for control via soil amendment.[8] Scientific research has discovered that both high precipitation and high temperatures (below 93 degrees Fahrenheit) are further environmental attractors for the Texas root rot, increasing its virulence. The pathogen earned its common name because of the geographical area where it is most prevalent, the cotton-growing region of Texas.[8]

Texas, an area located in the southern half of the United States, receives an annual rainfall between 127cms and less than 25 cms.[7] Scientists collected data on Texas root rot virulence over a 13 years at the Blackland Research center using cotton plants infected with Texas root rot and found that virulence of Texas root rot correlates somewhat with precipitation in the range of 36–100 cm of rainfall.[9] Higher virulence of Texas root rot was observed after large precipitation events.[9] These experimental findings support the conclusion that Texas root rot preferentially develops in wet over dry soil conditions.[9] The same study also found that virulence of Texas root rot develops inversely to air temperatures over 93 degrees Fahrenheit.[9]

The symptoms of Texas root rot are most severe during hot summers, summers are considered hot if the air temperature can increase the average soil temperature over 80 degrees Fahrenheit. The elevation in temperatures produces Texas root rot’s most severe symptoms: wilting and bronze colored leaves.[10] 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.[10]

Figure 2: P. Omnivera disease cycle

The disease overwinters as sclerotia or as mycelium on dead plant tissue. Once spring to early summer arrives, germination phase with hyphae growth continues. Following this, root colonization occurs. In Mid and late summer you begin to see the disease at its infectious stage, which is when you start to observe associated symptoms.[2] The pathogen will penetrate the host, and colonize plant root tissue causing root rot. This initially causes the first symptoms of chlorotic leaves and eventually wilting.[2] Root rot disease rings enlarge and the field can be categorized into three zones based upon plant status: asymptomatic, disease front and survivor.[11] The pathogen will then eventually disseminate infecting neighboring plants, with infected plant tissue serving as a secondary inoculum fueling the disease. In situation of high moisture, conidia are produced on spore mats but its role in dispersal is currently unknown since very rarely conidia were found to germinate and facilitate the spread of disease.[2]

P. omnivore will form several differentiated hyphae. Initially, germ tubes emerge from soil residing sclerotia that overwinter.[2] The sclerotia structure acts as the primary inoculum in affected fields.[12] The pathogen hyphae will either infect the host root or form mycelium with a differentiated rind. Upon contact with host roots, P. omnivore forms a mycelial mantle on the root's surface.[12] 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.[12] The roots at later stages of infection show extensive vascular discoloration due to root necroses. The mycelial strand and symptom development in field-infected roots are even more conspicuous on cotton.[12] 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.

As a soil borne pathogen, Phymatotrichopsis omnivore enters the plant host via the roots.[2] 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.[2] 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.[12] This suggests that in order to avoid plant defense Phymatotrichopsis omnivore suppresses the production of these phytochemical defenses to ensure success.[12]

The most common management strategy to limit the spread and damage produced by Texas root rot (Phymatotricum omnvorum) is soil manipulation. The goal of the soil manipulation would be to create a soil environment that is not favorable to Texas root rot and can limit its activity. The most used approach is to change the soil acidity/soil pH because of Texas root rot prefers alkaline soils. Recommended methods of decreasing soil pH are through the use of ammonium sulfate or ammonium phosphate fertilizer at around 4.5 kg per 9.3 m^2. Applications of fertilizer in this manner will cause the soil pH to decrease making to less favorable to Texas root rot which has a preference for a higher more basic soil environment.[13] This management method will prove impracticable for any moderate or large scale agricultural operation because of the cost required to amend large areas of soil. However, it would be well suited to 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.[14] 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.

The planting of resistant grass crops, such as sorghum, to surround an already infected area can limit the spread of Texas root rot to other areas. The pathogen commonly invades its hosts through the root system incurring wilt when water transport is disrupted. By surrounding susceptible crops with immune grasses, a barrier layer in the soil can be created to block root infection of susceptible crops.[4]

Organic amendments are an effective treatment against Texas root rot when applied to the soil before spring planting. The most effective amendment has been composed of residues from wheat, oats, and other cereal crops. Though sources disagree, crop rotation has been shown as an ineffective control method because of Texas root rot’s wide host range of over 2,000 host species.[8] Overall, the most effective method to avoid Texas root rot is to avoid areas known to suffer from the Texas root rot because no management technique is a guarantee to control the pathogen.

Texas root rot (Phymatotricum omnvorum) has been 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.[5] 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.[10] The species'-wide 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.[13]

• Cotton (Texas) Root Rot
• Texas Root Rot
• Index Fungorum - Phymatotrichopsis omnivora
• USDA ARS Fungal Database
• EPPO Quarantine pest: Phymatotrichopsis omnivora
• Persoonia - Molecular Systematics of Phymatotrichopsis omnivora
• NC State Pathogen Profile