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.

Black root rot of pansy on the right (MSU Extension)

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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 a wide variety 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 is caused by a fungus (Thielaviopsis basicola). Yes, I know. It’s a strange word. [It is pronounced THEE-lay-vee-OP-sis.] But being able to pronounce the Latin isn’t as important as being able to recognize 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 of carrots (Public Domain)

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Black root rot host plants

Black root rot is a serious problem for commercial growers of ground covers, cotton, rice, many 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. This is why quarantining new plants is so important.


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

Black root rot is first seen as irregular growth patterns, followed by uneven stunting. At this point, a sample plant should be dug up, its roots washed off and inspected. If elongated, reddish-purple lesions are seen on the roots or in the stele, it is probably black root rot. [The stele is the center, darker portion of a root.] Over time, those lesions will turn black. You may also see enlarged roots with rough, black cracks. Infected roots are usually severely stunted.

Severe stunting of soybeans by black root rot (U of Ark)

Elongated red-purple lesions due black root rot on melon seedling (PSU Extension)

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Black root rot of soybeans (U of Ark)

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Conditions that favor black root rot

Black root rot occurs most often in cool, moist conditions. It is most likely when temperatures are between 55° and 61°F. Black root rot can be spread by fungus gnats and shore flies, and it is more commonly found in alkaline soil, such as we have here in the Bay Area. Fungal spores can also be spread via splashing rain or irrigation water, or on infected flats, containers, and garden tools. Soggy soil, poor drainage, and too much fertilizer all contribute to the likelihood of these soil-borne fungi taking hold of your plants.


Preventing black root rot

Once infection has become well established, the plant is a goner, so prevention is your only course of action. [Always throw diseased plants in the trash bin.] In severe cases, soil solarization may be needed to prevent infecting the next plants installed in that location. In commercial nurseries, chemical fungicides are used as preventive measures only.


The best way to avoid black root rot is to provide plants with good drainage, avoid overwatering and excessive use of fertilizer, and control fungus gnat populations with yellow sticky paper. Acidifying the soil can help somewhat, but soil pH is very 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.