Rot in Tobacco Greenhouses
TB06 - Tobacco Disease Note No. 6
W. A. Gutierrez, H. D. Shew, and T. A. Melton
routinely cause seedling damage in float-type greenhouses. One of
the most potentially devastating of these diseases is collar rot, caused
by Sclerotinia sclerotiorum. Incidence of this disease has
increased as the percentage of transplants produced under greenhouse conditions
of Collar Rot
of the disease are usually observed in the second half of the transplant
production period after the canopy has formed among seedlings, or
approximately 7 to 10 days after first the clipping. First symptoms
of collar rot are observed as small foci (small group of plants with
symptoms) 4'' to 16'' in diameter in different areas in the greenhouse.
Early stages infection are characterized by the presence of a soft
dark green (water-soaking) lesion at the base of the stem and the presence
of a cottony white mycelium in the surface of the infected tissues with
In very advanced
infections the cottony mycelium will form a sclerotia, and infected plants
melt down in the foci, leaving a blank of dead plants on the tray.
The sclerotia, which is the resting structure, is black, 0.1'' –0.5''
long, and resembles mouse droppings or a black bean. These sclerotia will
germinate as a thread-like mycelium (not observed in tobacco greenhouses)
or as an apothecium after overwintering in the soil.
In the spring, apothecia release ascospores which will infect seedlings.
Ascospores are wind disseminated from a few feet to 1.5 miles, but most
spores travel less than 100 yards from their source of origin. This
steep dispersal gradient increases the possibility of finding and eliminating
sources of primary inoculum of the pathogen. The presence of ascospores
can be easily monitored by trapping them with the use of a semi-selective
The infection of collar rot can either start from the upper leaves, growing
down through the petiole, infecting stems, and other plants; or start
from the lower senescent leaves, colonizing them first and then infecting
the stems, and killing the plants.
that Affect the Development of Collar Rot
Seedling age. Seedlings 45 and 52-days are the most susceptible
to collar rot.
and inoculum concentration. High numbers of spores and the presence
of a food supply on the leaf surface of seedlings favor the development
of collar rot. However, the presence of leaf sap is so conducive
to disease development that it mitigates the effect of inoculum levels.
Clipping effects. Tobacco seedlings are typically
clipped several times during the production period to increase plant
uniformity at transplanting. Clipping alone has little effect on
the incidence of collar rot, but when clipped debris (macerated tissue
and sap) is left on seedlings, incidence of collar rot is significantly
increased. Apparently this is favorable for germination of S.
sclerotiorum and infection by the fungus of tobacco seedlings. Large
clumps of debris are most favorable.
of leaf injuries. During the transplant production period, injuries
other than clipping may occur. Injuries may predispose tobacco seedlings
to infection by S. sclerotiorum. The most commonly observed
injuries in commercial greenhouses are those due to poorly adjusted heating
systems, which may blow dry, hot air over seedlings and desiccate leaf
margins. Propane gas leaks in the presence of high humidity may result
in the condensation of propane onto leaves in water drops and cause small
necrotic lesions. Incompletely burned gases, damage due to cold temperatures,
or injuries caused by oil drippings from mower machines may also occur.
Inadequate pest control also may cause foliar damage. High incidence of
collar rot is observed with injuries that results in necrotic areas on
leaves that serve as infection courts for ascospores of S. sclerotiorum.
trays no more than 55 to 65 days in advance of transplanting.
- Reduce the moisture inside the greenhouse by increasing air circulation
- Reduce early stage injuries of seedlings by correct setting of
heat systems and correct application of chemicals used in greenhouses.
- Use good clipping practices that reduce the production of leaf
debris. Increasing the frequency of clippings and reducing the amount
of leaves clipped at a time. Reduce the deposition of debris and
pieces of leaves on seedlings (use mower machines with high vacuum power).
- Discard leaf debris at least 100 yds from the greenhouse or bury
- Dispose of infected tobacco seedlings apart from leaf debris,
burying them in the soil.
- Avoid having family gardens or weeds close to the greenhouse
because it is possible that plants other than tobacco also are involved
in sclerotia and ascospore production. The fungus has a host range of
more than 300 plant species.
that affect infection of collar rot
that favored the presence of the primary inoculum
cycle of Sclerotinia sclerotiorum in tobacco greenhouse system
Symptoms in greenhouses
medium (semi-selective medium)
A method to
induce apothecia formation
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local governments cooperating.
July 2001 by Plant Disease and