MSU Extension

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

While root rots caused by Pythium and Rhizoctonia are the most common among greenhouse crops, black root rot is a serious threat to pansies, viola, petunias and vinca. Black root rot is caused by the fungus Thielaviopsis and may also infect cyclamen, poinsettia, primula, impatiens, snapdragon, verbena, phlox, begonia and nicotiana. Plants with black root rot often show symptoms that mimic nutrient deficiencies such as stunting with older leaves shriveling. Leaves may turn yellow and the youngest leaves become stunted and tinged with red. In mild infections, older leaves are yellow-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.

Sanitation is the best preventive measure against black root rot. Once this fungus is established in a crop or in a greenhouse, an effective fungicide program is needed. Based on studies conducted in my lab, we recommend fungicides that have thiophanate-methyl as the primary active ingredient (Cleary’s 3336 F is an example) be used frequently at the high labeled rate. A good rotational product is Terraguard 50W since it has a different mode of action and has shown to be effective in my studies against black root rot. Also Medallion is another product to control this disease.

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 the crop is treated for Pythium root rot when black root rot caused by Thielaviopsis is really the problem, not only will time and money have been wasted but the disease will have a head start in causing damage to the crop before it can be halted with the correct fungicide.

Dr. Hausbeck's lab conducted two trials to look at how Cleary’s 3336 F and Terraguard 50W compare with new fungicides and biocontrol agents in controlling black root rot. We used pansy and vinca crops because of their frequent problems with black root rot. We evaluated plant height and death as a measure of whether the fungicide provided protection. In our pansy study, only drenches of Terraguard 50W or Cleary’s 3336 F limited plant death to 25 percent or less, compared to plant death of over 70 percent when no fungicide was used. Also, drenches of Terraguard 50W or Cleary’s 3336 F prevented the severe stunting that occurred when fungicides were not used. In our black root rot trial with vinca, few plants died overall even without fungicide treatment. We did, however, see differences in the ability of the fungicides to prevent plant stunting caused by black root rot. Drenches of Cleary’s 3336 F or Terraguard 50W or Medallion provided plant protection better than all other fungicide drenches tested in this trial.

Sometimes new isn’t always better and for black root rot the standard program of Cleary’s 3336 F rotated with Terraguard 50W is the way to go. Since this was our first look at some of the products included in these studies, it is possible that higher rates and shorter intervals may be helpful in improving black root rot control. Until further testing is done, stick with these proven fungicides to ensure the best control.

Here are some additional cultural practices that growers should keep in mind if they have had a positive identification of Thielaviopsis black root rot in their greenhouse crops:
First, do not reuse old plug trays. We suggest you can disinfest them with a 10 percent sodium hypochlorite solution or some other sanitizer. This disease has a persistent spore stage due to a specialized cell wall that the spore produces, which allows it to persist for several years on the floor, bench or on the plug tray. We recommend buying new trays every year.

Second, if a certain location in your range is where black root rot was found in 2008, rotate away from any black root rot susceptible crops in succeeding years. This is similar to crop rotation used by farmers in outdoor growing systems. We have noticed black root rot problems right to the "foot print" where the infested flats had the problem a previous year when the pansies, as an example, were put down there the next season. Remember, the spore can survive for several years.

And finally, we have recommended the removal and replacement of ground weed mat from greenhouses where the flats were set on if black root rot was diagnosed. One cannot adequately sanitize the ground weed mat to insure a clean start, so start with new fabric on the floor. If you combine these suggestions with a preventative fungicide program as described earlier, you can reduce the risk of reintroducing black root rot into your greenhouses.

Dr. Hausbeck's work is funded in part by MSU's AgBioResearch.