Black root rot (BRR) is a common and destructive fungal
disease that attacks a variety of greenhouse bedding and pot crops. The disease
is widespread, having been reported on many different hosts, including
poinsettia, fuchsia, pansy, vinca, 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. Stressed plants are more
susceptible to infection, making stress prevention key to controlling the disease.
Symptoms
BRR can be difficult to diagnose in the early stages of
disease development and can be confused with other common diseases and
disorders. Above media symptoms include yellowing, stunting, other signs of
apparent nutritional deficiencies, and under server conditions, wilting and the
eventual death of the plant. 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 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 Pythium
can be seen in the infected roots with a microscope. BRR, however, begins by
attacking the middle of the root and forms small, black cankers that can be
seen relatively easily by washing roots free of growing media and then viewing
them carefully with a 10x or higher-powered hand lens.
A university or private plant disease diagnostic lab should
be consulted if you suspect BRR. A diagnostic clinic can confirm the presence
of the pathogen and give recommendations as to what control methods should be
initiated. Be sure to tell them you think BRR may be involved so the lab will
examine the roots through a microscope. If the pathogen is present, black,
barrel-shaped chlamydospores will be easily seen. 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
BRR is caused by the fungus, Thielaviopsis basicola. As mentioned, the soilborne
fungus is very common and widespread. It has a 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 is not
the only way your crop can get BRR. 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 as plowing or
field cultivation occur should be very careful not to allow dust from these
activities into the greenhouse.
Once inside 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 (survives in soil and debris as a non-pathogen) may allow it to continue
to grow, spread and survive on plants and plant debris in the greenhouse.
Thielaviopsis produces resistant "resting" spores
in tremendous numbers. These spores can be splashed about or blown about with
dust in the air. They will be present on flats, pots or trays that are 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 number-one method of control. Thielaviopsis basicola is considered an opportunistic
pathogen. This means it can survive as a saprophyte in soil and debris as a
nonpathogen. However, once it comes in contact with a host plant under stress, the
saprophyte will seize the 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 predispose a host to infection
by T. basicola?
Plants grown under conditions that are too cool or too warm 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. An 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 onslaught of T.
basicola.
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 seeds to germinate rapidly and seedlings to emerge promptly.
Careful attention to the details of water relations, media
pH, soluble salts and temperature are important in 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 in the growing media.
Topsoil and sand are guaranteed to contain BRR resting spores. Sanitize benches
when 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 when
dealing with BRR. There are some very good chemicals with efficacy against BRR.
Applications at labeled rates with products such as Cleary's 3336, Banner MAXX
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 BRR 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.
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