Thielaviopsis basicola is the plant-pathogen fungus responsible for black root rot disease. This particular disease has a large host range, affecting woody ornamentals, herbaceous ornamentals, agronomic crops, and even vegetable crops. Examples of susceptible hosts include petunia, pansy, poinsettia, tobacco, cotton, carrot, lettuce, tomato, and others. Symptoms of this disease resemble nutrient deficiency but are truly a result of the decaying root systems of plants. Common symptoms include chlorotic lower foliage, yellowing of plant, stunting or wilting, and black lesions along the roots. The lesions along the roots may appear red at first, getting darker and turning black as the disease progresses. Black root lesions that begin in the middle of a root can also spread further along the roots in either direction. Due to the nature of the pathogen, the disease can easily be identified by the black lesions along the roots, especially when compared to healthy roots. The black lesions that appear along the roots are a result of the formation of chlamydospores, resting spores of the fungus that contribute to its pathogenicity. The chlamydospores are a dark brown-black color and cause the "discoloration" of the roots when they are produced in large amounts.
Environment As a poor saprophyte and obligate parasite, T. basicola is often dependent upon favorable environmental conditions. Although the pathogen is able to grow in a variety of soil moistures, wet soil is optimal for greater infection since spores are able to move easily in water. Water plays a role in dispersal of spores and can lead to an increased infection rate. Soil temperatures also play an important role, as temperatures between 55 and 65 °F are favorable for the pathogen. However, temperatures that are higher than 86 °F are unfavorable for the fungus and only traces of the pathogen can be found. At lower temperatures, the severity of the disease increases since the temperatures become unfavorable for and induces stress on the hosts. Alkaline clay soils have proven to be conducive to pathogenicity and also favor the pathogen. This can be attributed to the fact that the pathogen is suppressed at soils with pH less than 5.2, so increasing pH is favorable for severity of disease. There are also cultural conditions which may induce stress on the host plants that favor the pathogen including high soluble salts, excessive nitrogen fertilizer, low organic matter, etc. When the plant undergoes stress due to cultural conditions, there is an increase in susceptibility to opportunistic pathogens such as T. basicola. For this reason, it is important to practice proper cultural conditions such as maintaining proper temperatures, amount of nitrogen fertilizer, and pH of the soil to reduce stress of host plants and decrease susceptibility to disease.
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