Marigolds are considered the workhorses of the garden, thriving in poor soil, growing quickly from seed and even repelling garden pests like nematodes. But marigolds are vulnerable to a few diseases such as botrytis, oedema and powdery mildew, according to the Cornell University Plant Clinic.
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Powdery mildew appears on marigolds as a grey or white powder. It may start in small circles but eventually spreads to cover leaves, stems and even flowers. Left untreated, the fungi may cause distorted growth or even kill the plant.
Powdery mildew thrives in humid, warm conditions and spreads through spores that travel on the wind. During the winter, the fungus may survive in plant debris left on the ground to reinfect marigolds the following year, says Cornell University.
Gardeners should remove any infected plant parts at the first sign of disease or, if necessary, discard the entire plant. Drip irrigation keeps leaves dry, minimising the chance of powdery mildew. Promptly remove and discard dead vegetation in the fall to prevent the fungi from overwintering.
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