Black Root Rot

Blackened roots, failure to thrive, yellowing leaves, and irregular stunting may all be signs of root rot, but not all root rots are black root rot. 
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​Root rot might refer to the cabbage family’s black rot, asparagus’ Fusarium crown and foot rot, or phytophthora root and crown rot, which attacks several varieties of plants and trees. And then there is black root rot. To tell the difference, you would need a microscope. But knowing what to watch for can reduce your losses.

Black root rot of carrots (Public Domain)

Black root rot is a fungal disease. This fungus has a strange name, Thielaviopsis basicola. [THEE-lay-vee-OP-sis.] But being able to pronounce the Latin isn’t as important as recognizing this plant disease before it spreads. Before looking for symptoms, however, you need to know which plants are susceptible to this fungal disease.

Black root rot host plants

Black root rot is a frequent problem for commercial growers of ground covers, cotton, rice, herbaceous perennials, snapdragons, tobacco, and our holiday poinsettias. Those lovely spring vinca, pansy, and viola plants can all carry this disease to your garden, even though they might look healthy in the store, which is why quarantining new plants is so important.

In addition to those nursery crops, black root rot can appear on several garden plants, including beans and peas, carrots, citrus, cucurbits, horseradish, lentils, melons, peanuts, soybeans, berries, potatoes and tomatoes. Many times, you won’t see damage to roots until after harvest.

Identifying black root rot

Black root rot first appears as irregular growth patterns followed by uneven stunting. At this point, dig up a sample plant, wash its roots, and look for signs of infection. If the roots or stele exhibit elongated, reddish-purple lesions, it is probably black root rot. [The stele is the center, darker portion of a root.] Over time, those lesions will turn black. Infected roots may also be enlarged, with rough, black cracks. Infected roots are usually severely stunted.

Conditions that favor black root rot

Black root rot occurs most often in moist soil at 55° and 61°F. Fungus gnats, shore flies, splashing rain or irrigation water, infected flats, containers, and garden tools can spread black root rot. Soggy soil, poor drainage, and too much fertilizer all increase the likelihood of these soil-borne fungi taking hold of your plants. This condition occurs more often in alkaline soil.

Preventing black root rot

Commercial nurseries use chemical fungicides as preventive measures, but once the infection has occurred, the plant is a goner. [Always throw diseased plants in the trash bin.] In severe cases, soil solarization may be necessary. 

The best way to avoid black root rot is to provide plants with good drainage, avoid overwatering and excessive fertilizer, and control fungus gnat populations with yellow sticky paper. Acidifying the soil can help somewhat, but soil pH is difficult to change without ongoing treatments. Crop rotation can also interrupt this disease cycle.

Remember, mulching with arborist wood chips is one of the best ways to improve soil structure and drainage, reducing the chance of black root rot finding its way to your garden.