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 that is fairly common in Mexico and the southwestern United States resulting in sudden wilt and death of affected plants, usually during the warmer months. It is caused by a soil-borne fungus named Phymatotrichopsis omnivora that attacks the roots of susceptible plants. It was first discovered in 1888 by Pammel and later named by Duggar in 1916.
Host and symptoms
Phymatotrichopsis omnivora is a necrotrophic 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 symptom of disease is often chlorosis of the leaves then followed by browning and wilting. Two weeks after the first symptoms appear, the plant dies.
thumb|Figure 1: Cruciform hyphae is a unique sign observable using a [[compound microscope]]
Another macroscopic sign is tan and white spore mats developed on the soil surface near the infected plants during favorable high moisture environmental conditions. Texas root rot is caused by a pathogen that prefers alkaline and calcareous 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 ) are further environmental enhancers for Texas root rot by 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. Higher virulence of Texas root rot was observed after large precipitation events. The pathogen is only active in the summer months when the high average temperatures can heat the soil microclimate one foot deep over .
Disease cycle
thumb|Figure 2: disease cycle
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. 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. This suggests that P. omnivora is able to suppress the production of these phytochemical defenses to ensure disease success. 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 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. By surrounding susceptible crops with immune grasses, a barrier layer in the soil can be created to block root infection of susceptible crops.