Marigolds (Tagetes spp.) are perennial or annual members of the sunflower family prized for brightly coloured, plentiful blooms and a strong scent that attracts beneficial insects yet repels pests. Various pests and pathogens, as well as certain site or soil characteristics, could cause leaf spotting on marigolds. Pot marigold (Calendula officinalis) is affected by many of the same pests and diseases as true marigolds, as well as by the leaf spot disease smut.
Fungal Leaf Spots -Alternaria leaf spot, caused by the fungus of the same name, appears as purplish spots on marigold leaves and stems. Botrytis cinerea, the fungus responsible for Botrytis blight or gray mold, is most problematic when humidity is high and causes flower spotting or discoloration and leaf discoloration, wilt and decay; grayish spore masses eventually develop. Pot marigolds are also susceptible to a leaf spot disease known as smut that is caused by the fungus Entyloma calendula. This disease appears as thickened greenish-yellow to brown spots up to 1/2 inch across on both leaf surfaces. Fungal diseases are best prevented or treated by using good sanitation practices that include removing infected portions of marigolds and spent blooms with clean, disinfected tools, avoiding overhead watering or working around the plants during wet weather and, if necessary, treating the plants with a fungicide.
Tomato Spotted Wilt Virus -Tomato spotted wilt virus is transmitted to a range of ornamental plants and food crops by thrips. Symptoms of this disease typically appear on marigolds in summer and could include spots with yellow or brown rings, dead leaf spots, black streaking and tip dieback. This virus is prevented or addressed by controlling the thrips population, such as composite thrips and western flower thrips. Thrips are tiny, slender winged insects whose feeding causes tissue discoloration and distortion, in addition to the spread of tomato spotted wilt virus. A thrips infestation is best addressed by removing spent flowers, removing weeds near plants that could harbor thrips, avoiding excessive use of nitrogen fertilizer and removing and destroying infected marigolds if there is a chance they could spread tomato spotted wilt virus to other desirable plants nearby.
Mites and Stippling -Twospotted spider mites appear as tiny moving dots on leaves, and their feeding damage shows as a stippling of light dots. A severe infestation causes leaf yellowing and premature leaf drop, and webbing is often present. Mites are most problematic when conditions are hot and dry. Regularly spraying marigold foliage with water to address dust, removing heavily infested portions of marigolds and avoiding the use of broad-spectrum insecticides that harm natural mite predators generally result in adequate mite control.
pH and Nutrient Considerations -The soil pH for marigolds should ideally remain between about 5.8 and 7. Too low a soil pH, or too acidic a growing media, may result in harmful nutrient deficiencies or toxicities. A low pH or excessive micronutrient fertilizer applications can cause a disorder referred to as bronze speckle, which is related to overly high iron and manganese availability. High soil acidity could also lead to dead spots on older leaves, necrotic leaf margins and tip dieback.
Verticillium Wilt -Verticillium wilt, caused by fungi in the genus of the same name, forces leaves on infected plants to first turn yellow along the margins and between veins before the foliage turns brown from the bottom of the plant upward. Verticillium problems are avoided by planting only healthy, disease-free specimens in pathogen-free soil and providing plants with good cultural care.