It’s been an increasingly difficult problem for us the past few years,especially during cool,wet weather. Our blooms become spotted,then brown and finally completely rotten. The cause,of course,is Botrytis blight or gray mold fungus disease. Exactly which species is the causative agent in our garden is unknown but is likely the most common one,Botrytis cinerea. Indeed once our rotten blooms become dry,spores can be seen coming off infected blooms if they are touched. Additionally,on dead leaves or stems I have also seen small black spots that I presumed to be the sclerotia described in the literaturBotrytis. These structures provide a site where the fungus can overwinter.
In querying other growers I learned that sanitation plays a major role in preventing outbreaks of the disease. Indeed,just a few old infected blooms here and there in the garden,perhaps hidden in the center of thickly growing varieties,can provide sufficient spores to cause a major outbreak. In some gardens the disease can be managed solely by inspection and sanitation. Unfortunately we are not so lucky! Our garden is nestled in the center of a stand of tall pine trees. Rain or heavy dew stays on the plants until mid morning,and,the surrounding trees interfere with good air circulation.
Last summer I attended a presentation on fungus diseases given by a plant pathologist from the University of Georgia. She actually mentioned Botrytis and stated that a chemical sold as Medallion was very helpful in preventing and even eradicating the disease on roses. I could not thank her enough for providing such a revelation!
Indeed Medallion seemed to work well on our roses. I told everyone about it! Oddly enough,almost every grower had a similar tale to tell…but with a different chemical cure. If reports were to be believed,Daconil,Benlate,Dithane,Chipco 26019 and Banner Maxx all provided some measure of control for a period of time. Could it be that this fungus was clever enough to become resistant to the materials commonly used to combat it? Indeed,I had used most of these sprays at one time or another with limited lasting success. And then,just when I felt very confident in Medallion it also seemed to become less effective. I decided to alternate applications of the two chemicals I had on hand,Medallion and Chipco 26019 to test my theory. This year I misted these two products over the tops of the plants,from two sides,usually mixed with Orthene,at least once a week. I was astounded by the very positive results. No,I did not gain perfect control of Botrytis,but it was no longer a significant problem in our garden. I discussed my findings with Robin Ross of Eden Bioscience during the District Show this fall. She explained that a rotation of chemicals with different modes of action was the key to good control of fungus diseases in general,including Botrytis. Will this clever fungus disease find a way to break through my new spray program? Since the materials I am using have two entirely different modes of action it is unlikely.
If,however,resistance is observed I will attempt to find another product with a different mode of action and immediately add it to the spray rotation! If you are plagued with this troublesome fungus,continuous inspection,careful sanitation and a rotation of fungicides may work for you,too!
Life cycle of Botrytis from Plant Pathology,4th edition,by George Agrios
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