Are your plugs not growing properly or do they have Thielaviopsis?

Cooler soil temperatures, over-watering, improper soil nutrients and pH levels and black root rots are common plug issues.

A variety of factors can cause growth problems in plugs: cooler soil temperatures, over-watering, improper soil nutrients and pH levels and black root rots. When these occur, growers go through the check list of potential causes. Has there been a fluctuation in the pH and EC in the root zone? Did the temperature drop? Is this caused by a plant pathogen?

Black root rot is a serious threat to pansies, petunias and vinca. It is caused by the fungus Thielaviopsis, and may also infect cyclamen, poinsettia, primula, impatiens, snapdragon, verbena, phlox, begonia and nicotiana.

The plant’s symptoms are very similar to nutritional issues where seedlings become yellow and chlorotic, growth is stunted and the older leaves become shriveled. Don’t be so fast to rule out a Thielaviopsis just because you believe you have used the best sanitation practices, like disinfecting the benches after each crop, purchasing clean seed stock and plants, and using new plug sheets. There is still a possibility that it is Thielaviopsis. To make sure, submit a sample to MSU Diagnostic Services. View this page to learn how to submit a sample to the lab.

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. With proper identification of a plant pathogen, it is fundamentally important to use products that are effective. Once this fungus is established in a crop or in a greenhouse, an effective fungicide program is needed.

Based on studies conducted in the Hausbeck lab at MSU, 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 or Medallion since these have been shown to be effective in the studies against black root rot.

To learn more about the research conducted by Mary Hausbeck and Blair Harlan (MSU Department of Plant Pathology), read this linked article.

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