Alternaria leaf spot
July 6, 2015
Fungal leaf spots
Several species are pathogenic on perennials. Together, they have a wide host range, including Aster, Calendula, Coreopsis, Dianthus, Gypsophila, Helianthus, Iris, Ligularia, Pelargonium, Platycodon, Shasta and Stokesia.
Dark-colored leaf spots. Lesions may have concentric rings within them. Spots may coalesce, resulting in blighting of foliage. Petals of some plants may be infected. Dianthus flower buds can be rotted.
Spores are produced on the plant surface and are spread by splashing water or air movement or by moving infected plant material.
Maintain adequate plant spacing to allow for good aeration to reduce humidity levels around plants. Alternaria spp. require leaf wetness for germination and infection; reduce durations of leaf wetness with good timing of overhead irrigation. Regular fungicide applications will likely be necessary to reduce losses on especially susceptible hosts.
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