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Introduction
Thielaviopsis basicola is a plant pathogen that causes what is commonly known as black root rot. The disease has been known since the early 1900s. At present, the disease has been recorded in more than 100 plant species in 33 families. The name Thielaviopsis basicola has several synonyms, including Chalara elegans, due to an attempt by Nag Raj in 1975 to group together all fungi which have the Chalara state. However, this state is found in some discomycetes and pyrenomycetes and as such is a product of convergent evolution. Because of this fact, the name was changed back to T. basicola.

Black root rot is a destructive fungal disease found in a wide range of host plants in all regions of the world, most commonly in areas with cool climates. Host plants for the pathogen include cotton, peanut, alfalfa, bean, beet, carrot, celery, pea, tomato, sweet potato, tobacco, sesame, soybean, lettuce, onion, citrus, pansies, vincas, and a variety of ornamental house plants, such as holly and poinsettia.

Thielaviopsis basicola is known to be an opportunistic pathogen and is considered to be a weak or facultative parasite, meaning it has the ability to survive long periods without a host by way of biotrophic use of soilborne organic matter and has been classified as a hemibiotroph. Two types of spores are produced by the fungus, endospores (Endoconidia) and aleuriospores (chlamydospores) produced on the root surface. Endoconidia most often germinate immediately, while chlamydospores (resting spores) are able to remain in the soil and persist for extended periods of time.

Symptoms
Black root rot is often confused with Pythium rot. Above-ground symptoms of both diseases are similar, resulting in death of the plant. The best time to observe roots for symptoms is about the time fruits begin to color.

The pathogen most commonly attacks the root system of a plant, causing a cortical rot and can occur at any time during the growing season. The disease will deteriorate the cortical and epidermal tissue of the seedlings, resulting in damping off. It will also damage the roots, as well as hypocotyl. This usually result in loss of vigor of the plant, as the damaged roots are no longer able to properly absorb nutrients. Affected roots appear dark brown to black, usually starting with the root tips and advancing to larger roots. The off-color of the roots is due to the decay of root cells and from the presence of the fungus on the roots. The roots might also show signs of elongated narrow lesions on infected areas, first appearing red, then black. In above-above ground parts, the plants may show stunting of terminal growth with a shortening of internodes, as well as interveinal chlorosis. Plants infected with the fungus will usually decline over months and may die if stressed by dry conditions.

A common symptom of black root rot has been seen on carrots. Physical injury makes the plant more susceptible to infection. The soil is the source of the pathogen, and the infestation usually occurs post-harvest and develops later during storage. The disease can be minimized by washing soil from the carrot, cooling them as quickly as possible, and rinsing them in chlorinated water before storage.

Transmission
If the pathogen has become established in agrowing area, it is often difficult to control. Spores can begin to infect roots of the host plant within 24 hours. Soil temperatures between 55 and 65 F favor growth of the fungus, especially in wet soil. The fungus is spread when healthy roots are put in close proximity to infected roots or into infected soil. Sanitation is the most efficient means to prevent the disease from spreading to other plants and/or growing areas. It also has the potential to disperse via infested wind blown dust or in growing media.

In 1999, there was a research project conducted that showed supporting evidence from a commercial production facility that adult shore flies may also be involved in the aerial transmission of T. basicola. It was shown that a large percentage of flies and larvae collected in close proximity to naturally infected plants were observed with chlamydospores in frass excreted by the larvae and flies and in the intestinal tract of larvae. This showed the flies were acquiring the fungus by direct feeding, which was later shown to be possible through feeding experiments involving the flies. Frass deposits containing propagules were observed within minutes on the hypocotyl of seedlings visited by internally infested adults. A larger percentage, 50 to 60, of plants in two separate experiments developed symptoms of the disease within 14 to 21 days. It is also believed that passage through the intestinal tract of the flies aides in germination of the spores.

Control/Management
At present, there are few management options for control of T. basicola. Prevention is currently the most effective measure for controlling infestation. Treatment after the plants are infected is rarely effective. Infected plants should be discarded and kept clear of any work areas. It is recommended that only new potting mix and containers be used for any container plants, as spores of the fungus can be spread from pot to pot through water. If pots or equipment is to be reused, it should be thoroughly cleaned and sterilized using bleach, quaternary ammonium compounds, or other available disinfectants. As spores can survive in greenhouses for several months on plant debris, it is recommended that before introducing any new plugs into the growing area, the roots should be spot-checked for healthy, white color. In larger field crops, crop rotation is recommended.

Commercial peat moss has been shown to contain T. basicola. It is recommended to only use the best potting media that is available. Steam pasteurization or fumigation with methyl bromide may be used to stop the fungus from propagating in growing media in nurseries.

The disease is also enhanced by stress to the plant, such as suboptimal temperatures, excessive moisture in the root zone, and large amounts of soluble salts in the soil. Although certain commercial fungicides may be useful, excessive use of fungicides or other production chemicals can have an adverse effect and intensify the severity of the disease. Effective fungicides, such as 3336® (thiophanate methyl) and Terraguard® 50WP (triflumizole) should be used as a preventative or at the first sign of infection, and should be used routinely for best results and efficiency. Control of soil pH is also helpful for controlling severity of the disease, which has been shown to be enhanced at soil pH > 5.6 and suppressed in soils with pH < 5.2.

Other means of prevention have been tested for specific species of plants. In the case of hollies, some researchers have noted that efficient cultural practices may allow some plants to grow even with the disease. If the plant is in early stages of infection, sufficient water and fertilization will help the plant to survive. In certain large commercial crops, species such as Nicotiana glauca and cotton are being bred with resistance to the disease. The process of growing strawberry plants in compost-filled mesh tubes has also been used, which significantly reduces the risk of getting the disease. Studies involving the use of garlic extract as an effective control of T. basicola spores have also been conducted. The garlic extract was found to be effective on several other pathogens, but research is continuing on the effect for T. basicola.