Fig. 1. Botrytis produces abundant masses of gray spores on diseased and dead plant parts, appearing as a fuzzy or powdery gray mold.
Photo: Dr. W. Garrett Owen

A new year signals the start of a new growing season. How about propagating and producing high-quality and healthy plugs (seedlings) and rooted liners (vegetative cuttings) as a New Year’s resolution? If young plants are exposed to poor propagation conditions, disease is favored, causing trouble for both propagators and growers. Disease prevention and management during propagation pays dividends throughout production.

Propagation environments and cultural practices

Provide a sanitary and pest-free propagation environment and implement best management and cultural practices to ensure high-quality and healthy young bedding plant transplants (plugs and liners). Monitor and control the propagation environment. Keeping the relative humidity below 85 percent is a challenge, but an environment that is continuously saturated should be avoided. Good air circulation across the crop can keep the leaf surface dry, which discourages foliar pathogens. Daily light integral, and the air and root-zone temperatures are also important components of the propagation environment influencing rooting and quality. Make it a practice to quarantine and inspect all incoming plant material for evidence of insects and disease. Once the new plants are incorporated into the greenhouse, scout the young bedding plant transplants frequently so that problems can be addressed early when the odds of success are highest.

Pathogens that pose the greatest threat

Botrytis is a fungus that loves moisture, causing leaf spotting on foliage and stem blighting on cuttings and young plants during propagation. Two traits make Botrytis a threat. First, any plant can be a target for Botrytis. This pathogen is not picky! Second, Botrytis produces abundant masses of gray spores on diseased and dead plant parts, appearing as a fuzzy or powdery gray mold (Fig. 1). A cloud of gray spores may be seen when diseased plants are moved. These spores may be picked up and carried on air currents and transported to healthy plants, which then become infected. Spores can be produced on plant parts that have been discarded so keep the benches and floors picked up and the trash containers covered and out of the production area.

Black root rot is caused by the Thielaviopsis fungus and can be a problem on plugs and young plants (Fig. 2). This disease is a serious threat to pansies, petunias and vinca. Other crops including cyclamen, calibrachoa, poinsettia, primula, impatiens, snapdragon, verbena, phlox, begonia, and nicotiana may also become infected by the Thielaviopsis fungus. Leaves may turn yellow (chlorosis) and resemble a nutrient deficiency. Black root rot gets its name from the dark-colored spores of this fungus. These spores are long-lived, contaminating growing mats and containers and surviving to infect future susceptible crops. Thorough sanitation measures are necessary to eradicate this fungus from the greenhouse following a disease outbreak. Growing mats and containers associated with the diseased plants belong in the dumpster because standard sanitation measures will not suffice.

Fig. 2. Black root rot caused by the Thielaviopsis fungus can turn young transplant leaves yellow (chlorosis) and roots discolored or necrotic.

Prevention and management

Reducing the amount of moisture in the production area is the best way to keep Botrytis in check. Water at a time of day when the leaves can dry rapidly, avoid oversaturating the growing media, and keep the air circulating. When these environmental control measures are not enough, fungicides can be highly effective with several products to choose from.

Preventing black root rot is best accomplished through strict sanitation throughout the production area, placing incoming plugs in quarantine for observation, scouting for early symptoms, and using a diagnostic lab for suspect samples. This disease is frequently misdiagnosed in its earliest stage when treatment with fungicides can be helpful. Once black root rot has progressed, crop quality suffers and usually cannot be restored by fungicides. In some situations, preventive fungicides are used on the most black root rot-susceptible crops to ensure plant protection.


If young plants become diseased, implement an action plan immediately. If fungicides or biologicals are used, follow the label to maximize their benefit. Using a pest and disease diagnostics lab for diagnosis and contacting your local greenhouse extension educator or university specialist can help too. Cheers to high-quality and healthy young bedding plant transplants in 2019!

Garrett ( is a floriculture and greenhouse outreach specialist in the Department of Horticulture at Michigan State University. Mary ( is a university distinguished professor and extension specialist in the Plant, Soil and Microbial Sciences Department at Michigan State University.