Identifying Marigold Disease
Although marigolds are hardy plants that seldom need special care, they do fall prey to some diseases. These are fairly common garden diseases. At signs of them on marigolds, check all of your garden plants.
Grey mold is spotted in wet or humid weather by examining the plants for brown or spotted areas. These areas are closely followed by the development of grey spores on dead plant tissue. These spores are so numerous that they look like a fuzzy coating on the plant.
Grey mold spores are very easily released so no work on infected plants should be done unless the air and plants are dry. If infected plants can be covered in bags to try to capture the spores, both bag and plant can be burned to remove that local area of infection.
The spores can live over winter in dead plant material so cleaning up the area and destroying infected plants is an important method of controlling grey mold.
Spraying with fungicide in damp or humid conditions can help to prevent the spread of the disease.
When the air is very moist it is possible that the plants take up water much faster than they can lose it through transpiration. The usual situation for this is when plants are grown in greenhouses with insufficient light and ventilation. Oedema causes fluid filled blisters on the bottom of a leaf, usually causing the leaf to turn yellow and die.
Oedema usually responds quickly to proper ventilation and more natural sunlight accompanied by a drop in the amount of water being fed to the plants.
There are two types of microscopic worms (nematodes) that attack marigolds. Hardest to spot are the leaf dwelling worms. These cause damage to the leaves that turns them yellow and kills them. Without microscopic examination it is difficult to know what killed them.
Root dwelling nematodes produce typical root damage symptoms. The leaves fade and may eventually fall off the plant. Worms living in the root will leave patches of the root badly injured.
Because the worms live in the soil it is virtually impossible to cure an infestation. The best treatment is to remove as much soil a possible and replant the area with worm resistant varieties.
Powdery mildew is often mistaken for white dust. It can be brushed off a leaf and leaves no obvious damage. The mildew tends to develop in prolonged periods of high humidity. The leaves turn yellow and fall prematurely and the plant will weaken and die. Generally the mildew that attacks marigolds will not affect other plants. Fungicides are available against mildew but you must choose one that is specifically for mildew on marigolds.
Improving the ventilation around the marigolds and reducing the humidity will eventually clear the mildew.
Because the mildew spores can survive over winter in fallen dead plant material it is important to remove all infected plants and material