Disease	Symptoms	Pathogen/Cause	Management
Alternaria Leaf Spot	Purplish spots form on leaves and stems.	Alternaria	Avoid overhead irrigation. Apply iprodione, fludioxonil, neem oil, myclobutanil, azoxystrobin, copper hydroxide, chlorothalonil, mancozeb, or thiophanate methyl + mancozeb.
Bacterial Leaf Spot	Small (2-5 mm) circular dead spots form on leaves and petioles. Spots have purple margins.	Pseudomonas tagetis	Destroy infected plants. Avoid overhead irrigation.
Botrytis Flower Blight	Flower parts brown and die. Gray masses of spores form on the infected tissue when wet.	Botrytis cinerea	Avoid overhead irrigation. Apply azoxystrobin, fludioxonil, chlorothalonil, mancozeb or a mixture of the two.
Fusarium Wilt	Seedlings are killed. In older plants, black streaks darken the vascular tissue up one side of the plant. Plants wilt. Roots on the greatly reduced root system are rotted. During wet weather, salmon-colored spore masses form on infected stems.	Fusarium oxysporum	Plant in potting mix free of pathogens. Destroy infected plants.
Leaf Burn	The tips and margins of leaves yellow and die.	Excess boron, manganese, or molybdenum	Measure and apply micro nutrient solutions carefully. Manganese should not be above 55 ppm, molybdenum above 24 ppm, or boron above 3 ppm.*
Septoria Leaf Spot	Oval to irregular gray to black spots with tiny dots peppering their surface (fungal fruiting structures) form first on lower leaves and then spread upward.	Septoria tageticola	Avoid overhead irrigation. Apply chlorothalonil.

* Chun Ho Pak and Chi Won Lee. 1991. "How to detect micro nutrient toxicity in marigolds." Greenhouse Grower (Jan. 1991) 9:80, 82.

COMMON NAME	TRADE NAME
azoxystrobin	Heritage
chlorothalonil	Exotherm Termil
copper hydroxide	Kocide
fludioxonil	Medallion
fosetyl-Al	Aliette
iprodione	Chipco 26019
mancozeb	Dithane, Mancozeb
myclobutanil	Systhane
neem oil	Trilogy
thiophanate methyl	Clearys 3336, Domain FL
thiophanate methyl + mancozeb	Duosan, Zyban