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Thielaviopsis Root Rot (Black Root Rot) Thielaviopsis root rot is less common now than it was 30 to 40 years ago. This is probably due to the elimination of soil from the substrate (medium) and to warmer growing temperatures. This disease has become more important over the past 10 years on pansy and vinca plugs. This raises concern about re-introduction of this fungus into greenhouses.
The above ground symptoms of Thielaviopsis root rot and Pythium root rot are very similar. Affected plants are stunted with badly rotted roots. Initial root symptoms are speckled brown lesions. As the disease progresses, the root system may turn brown and later black. Longitudinal black cracks develop on lower portions of stems. These black cracks on the lower portion of rotting poinsettia stems infected with Thielaviopsis distinguish it from Pythium, Phytophthora and Rhizoctonia root rots. Thielaviopsis root rot is favored by cool soil temperatures of 13 to 16°C (55 to 60°F). This root rot is most likely to develop later in the production cycle when plants are grown cool to retard flower development and intensify bract color. Disease development is favored by soil moisture holding capacity of 70% or higher. Thielaviopsis root rot is also favored by a substrate pH of 6.0 or above. Management of Root Rots Successful poinsettia production requires a good sanitation program from transplanting through bloom. Sources of possible introduction of fungal root pathogens after potting include infested crop debris from previous crops, transplanting of infected cuttings, recycled potting mix or containers. Bench tops and walkways with debris from previous crops. In addition, for Pythium and Phytophthora root rots the possibility of contaminated irrigation water as a source for introduction of these fungi is possible, if recycled water is used. A sanitation program that addresses disinfestation of surfaces or elimination of the above sources of fungal introduction is critical to management of root rots in poinsettia. In general, little is known about the relative resistance of any of the popular poinsettia cultivars to the root rot pathogens. Given the common occurrence of root rots in poinsettia, plant resistance is not likely to be useful management tool for root rot control, however. Although root rot pathogens may take several days to a few weeks to rot roots, above-ground symptoms may show up overnight once roots are rotted. At this point, that is when foliar symptoms are evident, it is too late to apply any effective control measure. Once foliar symptoms are apparent, poinsettias will not be suitable for sale. Therefore, growers are well advised to continue the preventative fungicide treatment program begun in propagation. Several fungicides are available for root rot control (see: Disease Management Strategies Table). Unfortunately, most fungicides are selective in the fungal pathogen that is controlled. For instance Daconil 2787 TM is very effective against Rhizoctonia solani but it does not control Pythium or Phytophthora. On the other hand, Subdue TM controls Pythium and Phytophthora but it does not control Rhizoctonia. If a grower has had specimens diagnosed at a plant disease clinic, then an appropriate fungicide for the pathogens present can be selected. Conversely, when a growers does not know which pathogen or pathogens is attacking the poinsettia crop, a broad spectrum fungicide like Banrot TM or another combination is needed that controls Rhizoctonia, Pythium and Phytophthora.
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