Black Root Rot in the Landscape

Written by: Emma Lookabaugh and Dr. Barbara Shew


Root rot is one of the most commonly diagnosed disease problems of woody landscape ornamentals in North Carolina.   Every year we receive boxwoods, Japanese hollies, rhododendrons, azaleas, camellias and other woody shrubs and trees that have one of the “big three” root rot diseases: Phytophthora root rot, Thielaviopsis black root rot, or Armillaria root rot.  Earlier, we discussed Armillaria root rot. With fall landscaping season in full swing, now is a great time to focus on black root rot.

Black root rot affects many plants, including woody ornamentals, annuals, and even field crops. It is the number one disease we see on Japanese hollies.  Black root rot also is an important disease in pansy, annual vinca (Madagascar Periwinkle), and other bedding plants. 

In the landscape, symptoms of black root rot on Japanese hollies include stunting and lack of plant vigor, yellowing of the foliage, and eventual leaf drop.  The most obvious symptoms of this disease are black lesions that occur on the tips of feeder roots, hence the name black root rot.  These lesions expand as the pathogen colonizes the root tissue, and plants decline as the root rot advances. Young holly plants in the nursery can be killed within a few weeks from severe infections, whereas mature plants decline more slowly. 

Stunting symptoms associated with BRR in the landscape
(Photo: Charles Hodges)

BRR Symptoms: Notice the black root tips
(Photo: Plant Path Departmental Slide Collection)

On pansy, symptoms include yellowing of new growth, stunting, and root rot.  If you rinse away the soil and examine the roots, black root rot will appear as small black lesions that are found along the length of the root.  The lesions are microscopic at first, but later will become visible as the lesions expand.  In more advanced cases, the entire root system is blackened and eventually rots away. 

Stunting Symptoms of BRR on Pansy
(Photo: Mike Munster)

Root rot and stunting symptoms of BRR:
Look closely at the roots on the left and you can see black lesions on the stem
(Photo: Mike Munster)

Black root rot is a caused by the fungus Thielaviopsis basicola. This fungus is a common inhabitant of many soils in the US. It can easily be distinguished by its characteristic chlamydospores, which are black and barrel-shaped. These are the primary survival structures of the pathogen. In addition, Thielaviopsis basicola produces abundant conidia in infected root tissues.  Both spore types are spread in soil water or in infested soil or root fragments. 

Dark chlamydospores and clear conidia of Thielaviopsis basicola
(Photo: H.D. Shew)

Chlamydospores inside pansy root tissue
(Photo: Mike Munster)

Infected plant material from the nursery and infested soil in the landscape are the most common sources of infection in Japanese hollies.  In the greenhouse, Thielaviopsis basicola may enter on infected cuttings or other diseased plant material.  It may also be present in infested potting medium or on contaminated pots or trays. Previously infected transplants, growing media, or garden soil are common sources of infection for pansies and other bedding plants in landscapes. 

Example of potential source of inoculum:
Notice the dirty pots and cull pile right next to "healthy" plants
(Photo: Emma Lookabaugh)

Cultural practices are the main means of black root rot control.  In landscapes with a history of black root rot, plant resistant holly varieties or other plants that are not susceptible to black root rot.  Infected Japanese hollies should be removed and these susceptible species should not be replanted in the same location. American and Yaupon holly are moderately resistant, while English and Chinese hollies are highly resistant to black root rot.  Many woody ornamentals used in NC landscapes, including boxwoods, azaleas, and rhododendrons are not susceptible to the fungus, providing many options when replacing diseased plants.

Check pansies and other bedding plants before you buy them. Choose plants with white, healthy root systems that have colonized the entire plug or pot. Avoid transplants with black or rotted roots.  Resist the temptation to plant beds of pansies while the weather is hot. Wait a few weeks for lower temperatures, which will reduce stress on new transplants and help the plant resist infection. 

Sanitation is the most important defense against black root rot in greenhouse/nursery production.  Infected plants should be removed and destroyed.  Use only disease-free cuttings and propagation material.   Proper sanitation of trays and benches is crucial in managing this disease.  For more information on cultural practices see Cultural Control Practices Guide.  For more information on chemical sanitation see Sanitation Table.  

Poor sanitation practices:
Notice the standing water and stack of diseased/discarded plants and pots next to healthy plants
(Photo: Emma Lookabaugh)

Fungicide drenches with thiophanate-methyl can be helpful if applied preventatively. See the NC Ag Chemicals Manual (http://ipm.ncsu.edu/agchem/agchem.html) for more information. Fungicides will not “cure” infections that are well-established, so it best to use them preventatively. 

For more information:

http://www.ces.ncsu.edu/depts/pp/notes/oldnotes/od7.htm


Special Thanks to Dr. Kelly Ivors for helping with this post!