Controlling Black Root Rot of Bedding Plants By Steve Nameth

Black root rot (BRR) is a common and destructive fungaldisease that attacks a variety of greenhouse bedding and pot crops. The diseaseis widespread, having been reported on many different hosts, includingpoinsettia, fuchsia, pansy, vinca, petunia and calibrachoa. The fungus thatcauses black root rot is capable of living in soils as a saprophyte (withoutcausing disease) and surviving in soil, soiless media and dust for years viatiny, thick-walled spores called chlamydospores. Stressed plants are moresusceptible to infection, making stress prevention key to controlling the disease.


BRR can be difficult to diagnose in the early stages ofdisease development and can be confused with other common diseases anddisorders. Above media symptoms include yellowing, stunting, other signs ofapparent nutritional deficiencies, and under server conditions, wilting and theeventual death of the plant. Sometimes, plants affected with BRR may have blackstem lesions at or near the soil line; however, stem lesions are rare ingreenhouse crops. Black stem lesions are more commonly associated with plantsinfected with impatiens necrotic spot virus or advanced cases of Pythium rootrot. Symptoms of BRR may sometimes be confused with those of Pythium root rot;however, Pythium usually attacks roots from the ends or tips, causing a soft,brown rotting as it progresses up the root into the stem. Oospores of Pythiumcan be seen in the infected roots with a microscope. BRR, however, begins byattacking the middle of the root and forms small, black cankers that can beseen relatively easily by washing roots free of growing media and then viewingthem carefully with a 10x or higher-powered hand lens.

A university or private plant disease diagnostic lab shouldbe consulted if you suspect BRR. A diagnostic clinic can confirm the presenceof the pathogen and give recommendations as to what control methods should beinitiated. Be sure to tell them you think BRR may be involved so the lab willexamine the roots through a microscope. If the pathogen is present, black,barrel-shaped chlamydospores will be easily seen. The clinic or lab may alsoculture the fungus from the tissue. Since the pathogen grows slowly onto theculture plate and timely diagnosis is important, culturing is rarely done in adiagnostic lab. The presence of the thick-walled resting spores in the roots isenough positive evidence for a sound diagnosis.

Causal Fungus and Disease Development

BRR is caused by the fungus, Thielaviopsis basicola. As mentioned, the soilbornefungus is very common and widespread. It has a wide host range and affects manyother hosts besides greenhouse floral crops. These include many woodyornamentals and vegetable crops such as holly and cauliflower. Thielaviopsiscan spread between greenhouses or between crops within a greenhouse in manyways. Long-distance spread between greenhouses occurs via the movement ofinfested soil, media, pots or plant material. Spread within the greenhousetakes place via splashing water or airborne spores in dust.

Bringing in infected plant material, media or pots is notthe only way your crop can get BRR. Thielaviopsis also has the potential toenter a greenhouse via wind-blown dust from outside. Greenhouses located inareas where building construction or agronomic activities such as plowing orfield cultivation occur should be very careful not to allow dust from theseactivities into the greenhouse.

Once inside a greenhouse, there are many situations that canresult in the pathogen becoming a long-term resident. As mentioned,Thielaviopsis can grow and reproduce as a saprophyte, and its saprophyticnature (survives in soil and debris as a non-pathogen) may allow it to continueto grow, spread and survive on plants and plant debris in the greenhouse.

Thielaviopsis produces resistant “resting” sporesin tremendous numbers. These spores can be splashed about or blown about withdust in the air. They will be present on flats, pots or trays that are broughtinto a work area for reuse. In addition, the pathogen produces a second type ofspore that is spread by splashing water. It may be the spread of these smallspores that allows the disease to develop so quickly once it gets started. Allin all, the BRR pathogen is well suited to becoming a permanent, thoughunwelcome, resident in your greenhouse.

Disease Management and Control

Avoid plant stress: the number-one method of control. Thielaviopsis basicola is considered an opportunisticpathogen. This means it can survive as a saprophyte in soil and debris as anonpathogen. However, once it comes in contact with a host plant under stress, thesaprophyte will seize the opportunity and aggressively attack and infect thestressed host.

Almost any factor that stresses the host plant can lead tofurther parasitic development of this disease. On-the-other-hand, if thedisease is identified early on, correcting the factors causing the stress willallow infected plants to “outgrow” the disease and eventuallyrecover. Again, early detection is key.

What types of stresses would predispose a host to infectionby T. basicola?Plants grown under conditions that are too cool or too warm are often subjectto this disease. For instance, growing violas too warm or petunias too cool maypredispose them to infection. Nutritional imbalances are frequently associatedwith disease development. An excess of ammonia nitrogen appears to beparticularly troublesome for pansy growers dealing with BRR. Growing media witha pH above 5.5-6.0 is also conducive to BRR development. Growing calibrachoa attoo high of a pH will predispose this susceptible host to an onslaught of T.basicola.

A well-drained medium provides an environment favorable forthe seedling and somewhat less favorable for the pathogen. Carefully executedwatering practices are very important. When the medium is irrigated thoroughlyand less often, conditions at or near the surface of the medium tend to remainslightly drier and are less favorable for growth of the pathogen. Whenproducing plugs, plug trays must be kept evenly moist and sufficiently warm toenable seeds to germinate rapidly and seedlings to emerge promptly.

Careful attention to the details of water relations, mediapH, soluble salts and temperature are important in growing a good crop andessential in preventing BRR.

Sanitation.Thielaviopsis is well adapted to survival in the greenhouse. Some relativelysimple sanitary practices are crucial for controlling this disease. Do notreuse containers or trays that housed plant material infected with BRR theprevious year. Do not create dust, especially when the dust can settle on pilesof growing media nearby. Disinfesting potting media will eliminate the organismand should be carried out if there is any topsoil or sand in the growing media.Topsoil and sand are guaranteed to contain BRR resting spores. Sanitize bencheswhen possible with a good disinfectant such as Green-Shield, Physan or ZeroTol.These products are formulated to inactivate the resting spores of BRR.

Chemical Controls.The use of chemical fungicides should be considered as a last resort whendealing with BRR. There are some very good chemicals with efficacy against BRR.Applications at labeled rates with products such as Cleary’s 3336, Banner MAXXand Terragard will be very effective in controlling BRR, and if the disease hasnot progressed too far, many infected plants can recover and be saved.

In Summary

As with most plant diseases, the best control is prevention,and BRR is no exception. Paying very close attention to the horticulturalconditions that are best for the plant is imperative. Do this, and it willassure the plant is stress-free, and a stress-free plant is less likely to be ahost for this destructive and persistent disease.

Steve Nameth

Steve Nameth is professor in the Department of Plant Pathology at The Ohio State University, Columbus, Ohio. He may be reached by phone at (614) 292-8038 or by E-mail at [email protected]

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