Friday, May 18, 2012

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