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Black Root Rot
by Edward Luersman, Elizabeth Roche, Nancy J. Taylor & Francesca Peduto Hand
Black Root Rot is caused by the fungus Thielaviopsis basicola. The pathogen is found worldwide on more than 100 plant species and it is widespread in both the greenhouse and the landscape environment. Plants can be affected at any growth stage. Flower crops particularly susceptible to the disease include begonia, african daisy, pansy, and madagascar periwinkle among others.
Plants affected by black root rot may show above-ground symptoms that are not diagnostic in of themselves and can be confused with other root rot diseases or general nutrient deficiencies. These may include yellowing of leaves, plant stunting, wilting or even plant death. As the name would suggest, a diagnostic feature of black root rot is the presence of black lesions on the roots, which are visibly very different from healthy white roots. These lesions occur in the middle of the root and expand to form cankers. Root discoloration initially is brown and becomes dark black as abundant black-colored spores (chlamydospores) form on and in the root. Sometimes black lesions can extend into the crown and on the plant stem near the soil line causing stems to be necrotic and soft and leaves to wilt.
A disease that is commonly confused with black root rot is Pythium root rot. However, Pythium initially attacks the root tips, causing a soft brown rot as it progresses down the root. Wash infected roots and study the lesions with a hand lens. If black root rot is suspected, a plant diagnostic lab can confirm the presence of Thielaviopsis basicola if black, barrel-shaped spores are seen on the lesions under a microscope.
For assistance in identification, contact the C. Wayne Ellett Plant and Pest Diagnostic Clinic.
Thielaviopsis basicola is a soil-borne plant pathogenic fungus commonly found growing in agricultural and non-agricultural soils. When presented with the opportunity, it will infect susceptible plants, particularly those that are stressed. Introduction of Thielaviopsis to a greenhouse usually occurs through long distance movement of infested soil and growth containers, as well as infected plant material, particularly plugs. Movement of the fungus within the greenhouse can occur via spores that spread through water splash during irrigation, via wind-blown dust, and through insects such as fungus gnats and shore flies. There are two types of infective spores: endoconidia and chlamydospores. Endoconidia are typically dispersed short distances inside the greenhouse and chlamydospores are resilient “resting” spores which can persist in the greenhouse in soil and on pots, trays, and benches for long periods of time. The fungus can also persist by growing in soil until it can infect a host plant. Once infective spores are introduced to the greenhouse, they are stimulated to germinate by root exudates. The fungus infects the roots, causing the described symptoms and produces a new generation of infective chlamydospores.
Favorable Environmental Conditions
- Alkaline soil pH (5 to 8.5)
- Soil temperature 55-70°F (12-21°C)
- Saturated soils or growing media
- Scout for uneven plant growth
- Scout for leaf chlorosis. If found, pull plants out of the pot and inspect the roots for the presence of black lesions
Once the disease has become established it is often difficult to manage or control, so preventing infections is the best option for T. basicola. This can be achieved with a combination of the following:
- Sanitation: Good sanitation is key to disease prevention. Inspect newly purchased plant material for symptoms, such as wilting and darkened roots before introducing them to the greenhouse. Remove any and all infected plant material and infested potting medium from the building. Avoid reusing pots and plug trays. Sanitize all equipment and keep the bench areas clean of soil and dust.
- Reduce Plant Stress: Stress can cause plants to become susceptible to black root rot. Conversely, infected plants can experience “recovery” if stress is removed. Take preventative and reactive measures to reduce plant stress. Allow saturated soil to dry between watering. Monitor nutrient levels to prevent nutrient imbalances, particularly over-fertilization. Do not allow plants to be exposed to extreme high or low temperatures.
- Soil pH: Maintain the soil pH below 5.5. Acidic growth medium conditions are less ideal for the growth and development of Thielaviopsis.
- Appropriate Watering: Regulate watering practices. Ensure that water splash is minimized to decrease spore dispersal. Also, water plants thoroughly and less frequently to prevent saturated soil.
- Chemical Control: There are many fungicides that have been shown to be effective against black root rot, but must be applied preventatively or at first detection of the disease for maximum effect. Chemical control alone will not stop the disease, so if necessary, use with the cultural practices listed above. For an updated list of available fungicides that may be right for your particular situation, you should contact Extension specialists in your State.