Greenhouse grown pansies may have problems with Cercospora leaf spot. Leaf spots tend to be on the older leaves or leaves that are close to the substrate surface. Initially leaf spots are 1- 3 mm in size with a sunken center. The leaf spots become dark gray or purplish in color, and have a feathery appearance. If left untreated the leaf spots will continue to expand, coalesce, and may cover the whole leaf. These leaves may turn yellow and drop off the affected plant. Sanitation is important in controlling Cercospora leaf spot – plants with severe symptoms should be discarded. Keep the leaves free of moisture as much as possible. Irrigating plants early in the morning will allow the leaves to dry off quickly and remain dry going into the coolness of the evening. There are a variety of fungicides that are labeled for this disease on pansy including mancozeb (Dithane T/O, Protect DF and others), thiophanate methyl (Cleary’s 3336), fludioxonil (Medallion), and chlorothalonil + thiophanate methyl (Spectro 90 WDG).
Note small spots on lower leaves of this pansy plant caused by the Cercospora fungus.
Information and photo from the October 16, 2009 edition of the Greenhouse TPM/IPM Bi-Weekly Report from the University of Maryland Cooperative Extension Central Maryland Research and Education Center http://www.ipmnet.umd.edu/09Oct16G.pdf