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Thielaviopsis basicola is responsible for black root rot of a wide range of host plants.
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.
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.
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.
Cosmopolitan, but not yet documented in Vietnam.
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.
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).
None as yet.
T. paradoxa is the causal fungus of a range of diseases including bud rot, leaf spot, heart rot, bleeding trunk canker and root decay of coconut and pineapple butt rot, black rot and white leaf spot. Butt rot of pineapple is a common disease of crowns, slips and suckers used for establishing new plantings. Black rot is a post-harvest disease occurring only on injured pineapple fruit. White leaf spot is only of minor importance. The pathogen also infects sugarcane and palms.
In pineapple, only freshly cut or injured tissue is infected, and a soft black rot with dark coloured mycelium and arthrospores develops. In coconut initial symptoms in the trunk include a soft yellow decay. Affected trunk tissue blackens with age. A reddish-brown liquid may bleed from the original point of infection. The sap flow may extend several feet down the trunk, discolouring the trunk black as it dries. Heart rot of the trunk may develop as a result of several point infections coalescing. In other palms, T.paradoxa causes stunting, necrosis of lower leaf pinnae, defoliation then death. The pith tissues of diseased palms decay to leave a hollow trunk.
Tropical and subtropical regions, but not yet documented in Vietnam.
Pineapple, sugarcane, coconut and other palms.
Infection in either the bud or trunk of the coconut palm can lead to plant death.
If infected pineapple tissue is allowed to dry prior to planting, the rot can cause only limited damage. There is a high correlation between rainfall prior to harvest and disease following harvest. For the control of this disease, freshly cut pineapple should not be planted unless treated with a fungicide or dried out. To prevent the spread of the pathogen, avoid wounds to tissue and remove infected pineapple plants and palms.