The Heartbreak of Botrytis

Decay at the terminal portion of the flowering stem of
poinsettia, due to infection by Botrytis cinerea

Dark blotches are bract
infections by B. cinerea

On Monday, December 16th, we received a sample of poinsettias that for all commercial purposes had been ruined by Botrytis blight. Botrytis cinerea is a pernicious and ubiquitous fungus that particularly infects wounded or senescent tissue such as old flowers. From this foothold it can spread to other plant parts. When the fungus sporulates, the colors of the black conidiophores (spore-bearing threads) and white conidia (spores) combine to give the appearance of a gray mold. The spores are easily carried around on air currents.

Botrytis cinerea can wreak havoc with many different host species, even causing canker on rose canes and fruit rot on plants such as strawberry. More specialized species of Botyrtis also exist. One is Botrytis elliptica, which affects primarily lilies. Botrytis tulipae causes a disease called "fire" on - you guessed it - tulip.

Closeup of conidiophores and conidia of Botrytis cinerea,
growing on cyathia (true flowers) of poinsettia. Black bar = 1mm.

The Achilles heel of Botrytis is its need for abundant moisture. In greenhouses, care must be taken to ventilate, even for a while after sunset, in order to keep the relative humidity down. Watching watering practices (timing, drainage) is also important. Fungicides are sometimes needed. Another essential element in Botrytis management is prompt removal of dead plant material from the house. For more information on Botrytis in greenhouses, see our blog from May 18, 2012.

Pansy bed at NCSU in early 2012. Botrytis blight.

Be on the look out for Botrytis blight on pansies in the landscape, especially in the late winter and early spring. To prevent problems, avoid overhead watering if possible, and make sure the plant spacing and surrounding shrubbery allow for good airflow. For more information on Botrytis in the landscape, see the May 31, 2013 blog.

Light colored dead blotches on flowers, spreading to leaves,
are a hint that you might have Botrytis blight in your pansies.

Just a reminder to check our holiday closing schedule. We look forward to seeing you in 2014!

Mike Munster and Kelly Ivors

Controlling Botrytis in the Greenhouse

Due to weather conditions, Botrytis epidemics are currently occurring in greenhouses across the state. A wide range of floriculture crops can be affected. Symptoms of Botrytis infection range from flecking of blossoms, blossom blight, and even leaf and stem rot. On rose canes it can cause a tan-colored canker. The characteristic gray mold may be visible under high humidity conditions. 

Botrytis sporulating on petunia stem (Photo: M.J. Munster)

The two keys to Botrytis management are keeping the relative humidity below 85% and maintaining the greenhouse free of dead or injured plant material (spent flowers, fallen leaves, pruned branches, culls, etc.) on which the fungus can produce new spores. Irrigate at times of day when foliage will dry quickly, and if possible ventilate greenhouses in the evening to bring down the humidity. Avoid wounding plants, which allows Botrytis to invade healthy tissue.  Keep fertilization at optimal levels to avoid premature leaf senescence. Plants with bloom infection or crown rot should be discarded along with the potting mix. NEVER REUSE POTTING MIX WITH THIS PATHOGEN. Clean up all plant debris from the block and discard it. Do not compost any of this material, as the sclerotia of the fungus are capable of surviving adverse conditions. Spores of this fungus can be windborne, so be sure there are no cull piles nearby on which the fungus could produce them.

Botrytis stem rot on lavender (Photo: M.J. Munster)

If re-using pots, first clean thoroughly and then sanitize with either steam (150-160F for at least an hour at the center of the pile) or one of many chemical disinfectants available.

Petal spotting by Botrytis (Photo: M.J. Munster)

Fungicides may help prevent new infections, but won't cure plants that already have symptoms like those submitted. Effective products for Botrytis control include Chipco 26019/26 GT (Iprodione), Decree, Medallion and Pageant. Chlorothalonil (e.g. Daconil) can also be used; however chlorothalonil can cause phytotoxicity on blooms so this product should not be used on flowering plants. Decree is probably the most effective product for Botrytis control; however, it is only labeled for controlling Botrytis and should be used in a rotational program. In addition, there is a new Syngenta product named Palladium that just got registered for use in greenhouses and it is very effective against Botrytis. Be sure to rotate fungicides of different modes of action (FRAC groups) so as not to pressure the fungus into becoming insensitive (resistant) to any particular chemical. Note that one of the active ingredients in Palladium (fludioxonil) is the same as that found in Medallion. Get good coverage of the stems/crowns. Test any new treatments on a small number of plants first, to ensure that there are no adverse effects.


Written by: Mike Munster and Kelly Ivors


For more information on Botrytis in the home garden, click here