Black root rot - Thielaviopsis basicola
Introduction
Thielaviopsis basicola is responsible for black root rot of a wide range of host plants.
Symptoms
This pathogen commonly causes a cortical rot of seedling roots. The fungus infects the cortical and epidermal tissue of the root system of seedlings and the portion of the hypocotyl below the soil surface. Elongated narrow lesions appear on the infected tissue, initially red, then black and the tap root growth is stunted. Lesions may coalesce and blacken the entire root. However it can also cause internal collar rot of mature plants. Shallow adventitious roots may develop following infection and disease of the main root system. These support the plant when moisture is available, but cannot sustain it under drought conditions when the surface soil is dry. Within a few weeks, the diseased tissue may even be replaced with healthy tissue.
Internal collar rot can follow black root rot, where in the mid-late growth stages the leaves and stem tissue wilt and collapse.
Other symptoms include red-brown lenticels, swelling of the crown, purplish black discolouration of the stele in the crown area, then general stunting, and premature defoliation.
In citrus, black localised necrotic lesions up to 1.5cm in diameter are produced on fibrous roots. The damage is associated with stunting during the seedling stage and is most severe during cool wet springs. Cotton is a common host in Australia and other countries.
On peanuts, T. basicola causes small scattered black lesions on the pod surface. The black lesions contain abundant black arthrospores. The seed can also be discoloured and the pegs may rot, resulting in a large number of pods remaining in the ground after harvest. On sesame, the fungus causes rotting of the roots and stem with a red discolouration.
The Pathogen
Thielaviopsis basicola is characterised by two distinctive types of spores (conidia), endophialoconidia and arthrospores. Endophialoconidia are produced from elongate phialides composed of rounded bases with a long, narrow cylindrical neck or barrel. The endophialoconidia are typically rectangular with rounded corners, subhyaline and non-septate. In culture, the arthrospores are formed within hyphae in chains from appressed mycelium at or near the surface of the medium. The brick shaped arthrospores gradually disarticulate after maturity. The arthrospores are also formed abundantly in diseased roots, a diagnostic feature of the pathogen. The pathogen can be isolated from soil and infected tissue using fresh carrot discs or selective media.
The carrot disc technique involves the placement of a small amount of soil or infected root tissue directly onto fresh, aseptically cut carrot discs which are incubated on filter paper soaked with 0.1g/L streptomycin solution in a petri dish. The dark coloured hyphae of Thielaviopsis basicola colonise the disc from which it can be subcultured onto other media. The fungus will readily grow on potato dextrose agar and carrot agar.
Distribution
Cosmopolitan, but not yet documented in Vietnam.
Host Range
Thielaviopsis basicola has a wide host range including cotton, peanut, alfalfa, bean, beet, carrot, celery, pea, tomato, sweet potato, tobacco, sesame, soybean, lettuce, onion and citrus.
Epidemiology
Endophialoconidia only survive a few months in the soil whereas the arthrospores of T. basicola survive for several years. The arthrospores germinate and the fungus colonises the outer surface of the tissue and gain entry into plant cells via an infection peg formed under a clump of 2-5 hyphae. Arthrospores form in the dark coloured hyphae in the root tissue. Infected root and stem residues left in the field after harvest are the primary source of inoculum. The arthrospores will germinate under neutral to high pH and low temperature, excess soil moisture, shading, and generally cool damp soil during the growth of the plant. The fungus grows most abundantly at high temperatures (25-28°C) but the disease is generally more severe at lower temperatures (15-20°C).
Control
- Rotation is a key control measure. Resistant monocotyledonous crops should be rotated with susceptible crops.
- The disease can be minimised by planting into warm soil, above 16°C, for example, cotton. As temperature rises, the severity of black root rot declines.
- The herbicide chloramben can increase the severity of T. basicola root rot.
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Diseased and healthy cotton seedlings - Photo: David Nehl
Black root rot of cotton - Photo: David Nehl
Black discolouration of the xylem tissue - Photo: David Nehl
Pure culture of T. basicola and a soil dilution plate
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