Controlling Black Root Rot of Bedding Plants Keeping Plants Stress-Free is the Key By Dr. Stephen Nameth Professor and Associate Chair Dept. of Plant Pathology The Ohio State University Columbus, OH Black root rot is a common and destructive fungal disease on a variety greenhouse bedding and pot crops. The disease is widespread, having been reported on many different hosts including poinsettia, fuchsia, pansy, vincas, petunia, and calibrachoa. The fungus that causes Black Root Rot is capable of living in soils as a saprophyte (without causing disease) and surviving in soil, soiless media and dust for years via tiny, thick-walled spores called chlamydospores. Plants that are stressed are more susceptible to infection making stress prevention key to controlling the disease. Symptoms Black root rot (BRR) can be difficult to diagnose in the early stages of disease development and can be confused with other common diseases and disorders. Above the media symptoms include yellowing, symptoms of nutritional deficiencies, stunting and under server conditions wilting and the eventual death of the plant (Figure 1 and Figure 2). Sometimes, plants affected with BRR may have black stem lesions at or near the soil line, however, stem lesions are rare in greenhouse crops. Black stem lesions are more commonly associated with plants infected with Impatiens necrotic Spot Virus or advanced cases of Pythium Root Rot. Symptoms of Black Root Rot 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 Pythium can be seen in the infected roots with a microscope. Black Root Rot, however, begins by attacking the middle of the root and forms small, black cankers. The BRR cankers can be seen relatively easily by washing roots free of growing media and then viewing them carefully with a 10X or a higher power hand lens. A university or private plant disease diagnostic lab should be consulted if you suspect black root rot. A diagnostic clinic can confirm the presence of the pathogen and give recommendations as to what control methods should be initiated. Be sure and tell them you think BRR may be involved so that the lab will examine the roots through a microscope. If the pathogen is present, black, barrel-shaped chlamydospores will be easily seen (Figure 3). The clinic or lab may also culture the fungus from the tissue. Since the pathogen grows slowly onto the culture plate and timely diagnosis is important, culturing is rarely done in a diagnostic lab. The presence of the thick-walled resting spores in the roots is enough positive evidence for a sound diagnosis. Causal Fungus and Disease Development Black root rot is caused by the fungus, Thielaviopsis basicola (TB). As mentioned, the soilborne fungus is very common and widespread. It has a very wide host range and affects many other hosts besides greenhouse floral crops. These include many woody ornamentals and vegetable crops such as holly and cauliflower. Thielaviopsis can spread between greenhouses or between crops within a greenhouse in many ways. Long distance spread between greenhouses occurs via the movement of infested soil, media, pots or plant material. Spread within the greenhouse takes place via splashing water or airborne spores in dust. Bringing in infected plant material, media or pots are not the only way your crop can get black root rot. Thielaviopsis also has the potential to enter a greenhouse via wind blown dust from outside. Greenhouses located in areas where building construction or agronomic activities such plowing or field cultivation should be very careful not to allowing the dust these activities generate into the greenhouse. Once within a greenhouse, there are many situations that can result in the pathogen becoming a long term resident. As mentioned, Thielaviopsis can grow and reproduce as a saprophyte and its saprophytic nature (can survive in soil and debris as a non-pathogen) may allow it to continue to grow, spread and survive on many plants and plant debris in the greenhouse. Thielaviopsis produces resistant "resting" spores in tremendous numbers. These spores can be splashed about or blown about in dust in the air. They will be present on flats, pots, or trays that may be brought into a work area for reuse. In addition, the pathogen produces a second type of spore that is spread by splashing water. It may be the spread of these small spores that allows the disease to develop so quickly once it gets started. All in all, the BRR pathogen is well suited to becoming a permanent, though unwelcome, resident in your greenhouse. Disease Management and Control Avoid Plant Stress: The #1 Method of Control TB is considered an opportunistic pathogen. This means it can survive as a saprophyte in the soil and on debris as a nonpathogen. However, once it comes in contact with a host plant under stress the saprophyte can take this opportunity and aggressively attack and infect the stressed host. Almost any factor that stresses the host plant can lead to further parasitic development of this disease. On-the-other-hand, if the disease is identified early on, correcting the factors causing the stress will allow infected plants to "outgrow" the disease and eventually recover. Again early detection is key. What types of stresses would help predispose a host to infection by TB? Plants grown under conditions that are too cool or too warm for a particular crop are often subject to this disease. For instance, growing violas too warm or petunias too cool may predispose them to infection. Nutritional imbalances are frequently associated with disease development. Excess of ammonia nitrogen appears to be particularly troublesome for pansy growers dealing with BRR. Growing media with a pH above 5.5-6.0 is also conducive to BRR development. Growing calibrachoa at too high of a pH will predispose this susceptible host to an on slot of TB. A well-drained medium provides an environment favorable for the seedling and somewhat less favorable for the pathogen. Carefully executed watering practices are very important. When the medium is irrigated thoroughly and less often, conditions at or near the surface of the medium tend to remain slightly drier and are less favorable for growth of the pathogen. When producing plugs, plug trays must be kept evenly moist and sufficiently warm to enable seed to germinate rapidly and seedlings to emerge promptly. Careful attention to the details of water relations, media pH, soluble salts and temperature are the keys to growing a good crop and essential in preventing BRR. Sanitation Thielaviopsis is well adapted to survival in the greenhouse. Some relatively simple sanitary practices are crucial for controlling this disease. Do not reuse containers or trays that housed plant material infected with BRR the previous year. Do not create dust, especially when the dust can settle on piles of growing media nearby. Disinfesting potting media will eliminate the organism and should be carried out if there is any topsoil or sand to be used in the growing media. Topsoil and sand is guaranteed to contain BRR resting spores. Sanitize benches when possible with a good disinfestant 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 when dealing with Black Root Rot. Fungicide drenshes should be considered only when all else has failed and you are trying to save your crop. There are some very good chemicals with efficacy against BRR. Applications at labeled rates with products such as Cleary's 3336, Banner MAX, and Terragard will be very effective in controlling BRR and if the disease has not progressed too far many infected plants can recover and be saved. In Summary As with most plant diseases the best control is prevention and Black Root Rot is no exception. Paying very close attention to the horticultural conditions that are best for the plant is imperative. Do this and it will assure the plant is stress-free and a stress-free plant is less likely to be a host for this destructive and persistent disease.