 Wilting of calibrachoa caused by Thielaviopsis.1. What plants are most susceptible to black root rot? 2. What are the symptoms of black root rot? Plants with black root rot often show symptoms that mimic nutrient deficiencies such as stunting with the older leaves shriveling. Leaves may turn yellow and the youngest leaves become stunted and tinged with red. In mild infections, older leaves are yellowish-green with the veins retaining their green color. Black root rot may also affect the lower stem on crops such as poinsettia, causing cracks that appear black. 3. How can I avoid black root rot? The best preventive measures against black root rot are to purchase plugs that are free of the disease and use stringent sanitation practices.  Healthy petunia (left), Thielaviopsis infected petunia (right).Quick Control Tips Once Thielaviopsis is established in a crop or in a greenhouse, an effective clean-up and fungicide program is needed. Based on several years of studies conducted in my lab at Michigan State University, I recommend applying fungicides frequently that have thiophanate-methyl (e.g. Cleary’s 3336 F, OHP 6672) as the primary active ingredient at the highest labeled rate. Good rotational products include Terraguard 50W (triflumizole) and Medallion 50WP (fludioxonil) since they have a different mode of action and are shown to be effective in Michigan State studies against the pathogen. Choosing an effective fungicide to control black root rot is critical because a misstep early in the disease epidemic may result in an unsalable crop. If a crop is treated for Pythium root rot when the black root rot is actually caused by Thielaviopsis, not only is time and money wasted, but the disease has a head start in causing plant damage. Ideally, fungicides for black root rot should be applied preventively as no treatment can “cure” the disease once it is established.  Thielaviopsis-infected vinca on left show leaf curling symptom.Potential controls Recent studies at Michigan State show that a couple of new active ingredients show promise in limiting black root rot (See Table). Veranda O (polyoxin D zinc salt) is a biopesticide that is registered on a wide range of crops. It controlled black root rot in our newest trial when applied as a drench. Veranda O also works well against Botrytis (as a foliar spray) and Rhizoctonia (as a drench). Tourney 50WDG (metconazole) fungicide is not registered for use on greenhouse ornamentals but performed well against black root rot. Trinity 2SC (triticonazole) fungicide is not registered for use, but was helpful in the trial. Further studies will be needed to determine if either Trinity 2SC or Tourney 50WDG pose any phytotoxicity concerns to herbaceous bedding plants. Although no negative effects were observed on plant growth from the application of Trinity 2SC or Tourney 50WDG in our trials, additional studies are needed.  This research was funded in part by Floriculture Nursery and Research Initiative of the USDA-Agricultural Research Service under Cooperative Agreement #59-1907-5-553, the American Floral Endowment, and the IR-4 Project. |
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