High humidity, free water and moderate temperatures favor rust growth so make sure your mums have adequate air circulation all around and try to keep water off the foliage. You can apply Banner or Daconil to your mums. Either of these fungicides should control the problem. You may want to prune off the affected plant parts and remove them from the garden to keep the fungal spores from splashing onto your other plants. Good luck with your mums.
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