Cercospora Leaf Spot on Hydrangea Macrophylla
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Cercospora leaf spot (pictured right) is the most common disease we see in nursery production of Hydrangea macrophylla from July through leaf drop. I have had a few questions from growers lately about managing this disease so here is some important information.
Tissue is infected in May but leaf spots do not show up until later in the growing season (July or after) so management practices have to start well before you see the leaf spots.
Sun and shade effects:
I know from past experience and reading that this leaf spot is typically worse in full sun production settings versus 40 to 60% shade. Consider growing this in production areas with shade from scattered pines or in shade cloth covered structures.
Frequent overhead watering increases the disease intensity as well so consider drip or micro-irrigation and/or growing in poly top structure with open sides so you can better manage water and air circulation.
There is information on a few cultivars that I have found related to resistance. Researchers from Tennessee State University and University of Tennessee have found these cultivars to have good resistance: Blue Deckle, Fasan, Lilacina, Ami Pasquier, Ayesha, Forever Pink, Fuji Waterfall, Fujinotaki, Seafoam, Taube, Tricolor, Veitchii.
I did some online searches as we all do and found the same information from many State’s Extension web pages with three older active ingredients (chlorothalonil, myclobutanil, and thiophanate-methyl). The NC Agricultural Chemicals Manual lists many products for leaf spots but there are no specifics for this disease on this plant. Knowing there had to be better information, I contacted our Woody Ornamental Pathologist, Sara Villani, who put me in touch with Fulya Basel-Gurel from Tennessee State University, who has done extensive efficacy work on this disease with newer active ingredients and combination products. Based on her research, here are the best options to use in rotation:
Murial (group 11, group 7) -azoxystrobin,benzovindiflupyr
Orkestra (group 7, group 11) -fluxapyroxad, pyraclostrobin
Postiva (group 7, group 3) -pydifumetofen, difenoconazole
Concert II (group M05, group 11) -chlorothalonil, propiconazole
Regalia (group P5) -Reynoutria sachalinensis (biofungicide)
-pick at least two as from different fungicide groups to rotate with and you may need to spray every two weeks starting May 1 until leaf drop if plants are in more sun and have frequent overhead irrigation or experience periods of excessive rainfall. Always follow label instructions for mixing, handling, and application.
Remove old and diseased leaves that drop from plants during the summer and in the fall after leaf drop to reduce spread within current or next year’s crop.
–If you have questions about Commercial Ornamental Nursery & Greenhouse Production in North Carolina contact one of our Area Specialized Agents:
Eastern Region – Danny Lauderdale
Central Region – Stacey Jones
Western Region – Sam Marshall