French marigold is a compact annual grown for steady flowering from early summer to frost. It is widely used as a border, edging, and container plant, and is often planted in vegetable gardens because its root secretions suppress some soil nematodes.
This plant is a warm-season annual with no frost tolerance. It germinates quickly and blooms within about 8 weeks of sowing. Deadheading keeps plants flowering, and it tolerates heat and poor soil better than most bedding annuals.
Powdery mildew
White or grey powdery coating on leaves — usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.
Botrytis (grey mould)
Grey fuzzy mould on petals and stems, worst in cool wet conditions.
Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.
Spider Mites
Fine stippling or bronzing on leaves, faint webbing on undersides, foliage drying out during hot, dry weather.
Spray foliage with a strong jet of water to dislodge mites. Apply insecticidal soap or horticultural oil to leaf undersides; repeat every 5–7 days as needed.
Aphids
Clusters of small soft insects on new growth and flower buds.
Knock off with a strong jet of water. Ladybirds and lacewings are natural predators. Insecticidal soap as last resort.
Marigold Leaf Spot
Dark brown to black spots on leaves, sometimes with yellow halos. Spots enlarge and merge, causing leaves to yellow and drop in wet conditions.
Remove infected leaves promptly. Water at the base to keep foliage dry, improve air circulation by proper spacing, and apply a fungicide if spread continues.