What are the tiny black dots on underside of leaflets of French marigolds (Tagetes patula)?
Your marigold (Tagetes) may have a fungal leaf spot disease caused by Septoria tageticola. The disease starts on lower leaves and progresses upward. Cultural controls are the best way to combat this problem. Start now by picking off and destroying any infected leaves as soon as they appear. Also, regularly clean up and destroy dead plant debris in your garden area. Do not compost infected debris. Rogue out and discard seriously infected plants, together with the soil in which they are growing. A layer of mulch helps prevent fungal spores from splashing from the soil onto plants.
If fungal leaf spot is a particular problem in your area, spray at weekly to ten-day intervals with wettable sulfur, particularly rainy seasons (which is probably when it's happening in the first place). T. patula, French marigold, is fungus-resistant.
Hope this helps.
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- Courtesy of NYBG Plant Information Service