Botrytis blight conditions expected in greenhouses

Greenhouse growers will be challenged by the seven-day weather forecasts for cloudy and rainy conditions favoring Botrytis blight. Remembering cultural practices and correct fungicides will help until we get more sunshine.

Photo 1. Stem canker caused by Botrytis. Photo credit: Mary Hausbeck, MSU

Photo 1. Stem canker caused by Botrytis. Photo credit: Mary Hausbeck, MSU

The forecast for the next week, as of April 28, 2014, indicates cloudy, cool and rainy weather conditions. These low light, humid conditions combined with many of our greenhouse floral crops being in full flower in area greenhouses can lead to Botrytis blight outbreaks. Crops like geraniums, Gerbera daisy, petunias, fuschia and calibrachoa, to name just a few, can be especially vulnerable to this disease now since they have a full flower canopy at this time and most greenhouses are filled to the maximum allowable space.

Remember that Botrytis is a fungal disease that can cause leaf spots, petiole blighting and stem cankers (Photo 1) on many different annuals and perennials. It will produce large masses of “fuzzy looking” spores (Photos 2- 3) that are most often called “gray mold.” These spores or conidia will be spread on wind currents and can readily travel from infected to uninfected plants in that manner. The spores can survive for upwards of 21 to 24 days before they germinate on a plant.

Botrytis blight Botrytis blight

Photos 2-3. (Left) Sporulation and blighting caused by Botrytis. (Right) Botrytis sporulation close-up. Photo credits: Photo credit: Mary Hausbeck, MSU (Left) and Tom Creswell, Purdue University (Right)

Michigan State University Extension suggests these cultural control practices that will reduce the conditions that favor Botrytis infections, including: reducing the relative humidity in the greenhouse below 85 percent; making sure plants do not remain wet for six or more hours in a 24-hour period; and if possible, heat and vent on mornings and evenings for at least a half-hour or more to reduce humidity thus removing the humid, warm air allowing for plant surfaces to dry.

If plants are seriously infected and need to be removed from the growing area, do not just remove the plants and throw them on the compost pile out back as the spores can blow back into the facilities on wind currents. Bag up plants where they were growing, seal the bags and remove them from the facilities, thus reducing the risk of spores dislodging and infecting other plants in the greenhouse. This process should also be used when cleaning plants to remove dead foliage. Bag it and remove the spent blooms or leaves as quick as possible so the spores are not released in a clean greenhouse.

As for preventative fungicides, MSU plant pathologist Mary Hausbeck recommends the following fungicides that contain the following active ingredients: chlorothalonil, fenhexamid, iprodione, pyraclostrobin + boscalid, cyprodinil +fludioxonil or polyoxin D zinc salt.

Apply any of the above mentioned products as foliar sprays and be sure to have complete coverage and do not go longer than seven days between applications for best results. Some of the above products may “spot” open flowers or leave a residue.