Alternaria and Septoria are two common forms of fungal leaf spot affecting Shasta daisies. The University of Massachusetts Extension describes the symptoms of Alternaria as spots that start yellow, turn brown and then black, eventually killing the leaf. The leaf spots usually begin at the bottom of the plant. Septoria leaf spot starts the same progression by turning the leaf edges brown, according to the University of Illinois Extension. Water splashing from diseased plants onto healthy plants spreads the infection. So do not water from above, the water should be directed at the base of the plant. Also, if possible, water early in the morning so that leaves can dry before evening. Remove and discard diseased leaves at the end pf the growing season. If the disease is advanced, the University of Massachusetts Extension recommends using a fungicide with such active ingredients as chlorothalonil or mancozeb, applied according to label instructions.

More information can be found here: Shasta Daisy (Leucanthemum x superbum)-Leaf Spot (Leaf Blotch) | Pacific Northwest Pest Management Handbooks (pnwhandbooks.org)

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