| Disease (causal agent) | Symptoms | Survival of pathogen and effect of environment | Comments on control |
|---|---|---|---|
| Powdery mildew * (Podosphaera xanthii) |
White, powdery mildew fungus on surface of leaves and stems. More common on older leaves and older plants. | Disease is favored by moderate temperatures and somewhat shaded conditions; however, powdery mildew may occur on lower leaves of plants in full sun. Spores (conidia) are windborne. | Protect plants with a powdery mildew fungicide. |
| Root rot * (Phytophthora spp.) |
Plants are stunted and somewhat chlorotic. Roots are rotted. | Phytophthora spp. are present in many field soils. Favored by poor drainage, heavy soils, or overwatering. | Plant on raised beds. Drench soil with an oomycete (water mold) specific fungicide. |
| Smut (Entyloma calendulae) |
Circular to irregular (0.25 to 0.5 inch in diameter), greenish yellow to brownish spots, sometimes with a darker brown border. Spots are somewhat thickened and evident on both sides of the leaf. | Spores are windborne or rainborne. Fungus survives on Calendula and Calendula refuse, and probably on other related hosts. Favored by rain and overhead irrigation. | Avoid overhead irrigation. Protect foliage with mancozeb. |
| Pot marigolds are also susceptible to Pythium root rot * (Pythium spp.), gray mold * (Botrytis cinerea), rust * (Puccinia melampodii), spotted wilt (Tomato spotted wilt virus), root knot nematode** (Meloidogyne spp.), aster yellows * (Aster yellows phytoplasma), mosaic (Cucumber mosaic virus), southern blight * (Sclerotium rolfsii), cottony rot * (Sclerotinia sclerotiorum), Alternaria leaf spot (Alternaria sp.), Charcoal root rot (Macrophomina phaseolina), stem rot * (Sclerotinia sclerotiorum), and Verticillium wilt * (Verticillium dahliae). | |||
| * For additional information, see section on Key Diseases. |
| ** For additional information, see section on Nematodes. |