Black root rot (BRR), caused by the fungus Thielaviopsis basicola

January 2016 Plant Disease Advisory

Fig. 1. Darkly pigmented survival spores of the black root rot fungus in root tissue

Black Root Rot Survival Spores in Root Tissue
Fig. 1. Darkly pigmented survival spores of the black root rot fungus in root tissue

Black root rot (BRR), caused by the fungus Thielaviopsis basicola, is diagnosed on many greenhouse bedding plants. Pansy and viola are very commonly afflicted with BBR, but annual vinca, fuschia, impatiens, kalanchoe, Nicotiana, petunia, begonia, Salvia, calibrachoa, and phlox, in addition to certain other herbaceous and woody plant species, are also susceptible to BRR. Typically, BRR is associated with poor sanitation practices. One common means of BRR spread is on diseased plant material, such as cuttings or plugs. The fungus produces long-lived soilborne spores (Fig. 1), so contaminated soil, potting media, flats and pots can all be a source of fungal inoculum, in addition to dirty benches and other greenhouse surfaces.

Aboveground symptoms of BBR include stunting, off-color foliage, wilt and dieback (Fig. 2). Symptoms of yellowing or off-color foliage and stunting caused by BBR may be mistaken for a nutrient deficiency, so examine roots of affected plants for blackening typical of root disease. In the early stages of BRR, blackened roots may not be present, so it is advisable to submit a plant sample to a diagnostic lab. Once symptoms are observed, root loss is severe and fungicides will not be effective.

Pansy diagnosed with black root rot and showing symptoms of stunting, off-color foliage and wilt.

Pansy with Black Root Rot
Fig. 2. Pansy diagnosed with black root rot and showing symptoms of stunting, off-color foliage and wilt.

 

Good sanitation practices are key to avoiding BBR problems. Avoid re-using flats and pots and, if you do re-use, power wash to remove all potting media and plant debris before surface disinfestation with a commercial disinfectant or an overnight soak in 10% bleach solution. Keep watering hoses clean and off the floor. Surface-disinfest greenhouse surfaces regularly. Promptly remove plant debris or diseased plants from the greenhouse. Maintaining potting media pH at 5.5 can deter the BRR fungus, which is favored by higher pH. Also use potting media that drains well. 

The dark survival spores produced by the black root rot pathogen are apparent in the roots of this diseased coral bell; however, the dark spores are not always apparent in roots of diseased plants.

Black Root Rot on Coral Bell
Fig. 3. The dark survival spores produced by the black root rot pathogen are apparent in the roots of this diseased coral bell; however, the dark spores are not always apparent in roots of diseased plants.

Propagate only from healthy mother plants and avoid propagation from field-grown plants, which are more likely to harbor spores of the BBR fungus. Control insects, which can spread the BBR fungus. Fungicides can be used preventatively. Fungicides containing the active ingredient thiophanate-methyl are very effective against Thielaviopsis, but since this is a pesticide that is at high risk for development of pesticide resistance, rotation to fungicides with other modes of action is necessary (refer to product label directions). Fungicides containing the active ingredient fludioxonil are a good rotation option.