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Past Articles Library | Plant Diseases of Marigolds

Marigolds are an annual favorite that can be used as a bedding plant, container garden plant and even a vegetable garden companion.  As flexible as they are, they still suffer from plant diseases but one thing they do not play prey to is pests.  Believe it or not, the marigold can keep insects at bay above and below ground.  The smell of the marigold is strong and unpleasant enough to keep nematodes away for up to three years.

Many of the plant diseases are caused by overhead watering and/or overwatering.  To prevent this, only water the soil and allow the soil to dry up a little bit between watering.  How do you know when to water?  Do a moisture test.  This is easily done with your finger and only requires you to stick your finger into the soil.  Do not just go surface deep but instead go down to where your roots are.  Once you reach that zone, pull your finger up, and take a look.  If it is clean, it is time to water.  If it is covered in soil, do not water.   Having said that, below are a few of the plant diseases that marigold’s can suffer from.

Gray mold is a common marigold disease and is brought on by overcrowding of plants.  In other words, the plants get wet and there is not enough air circulation around them to dry them off quickly.  The name really describes what this plant disease looks like.  It can be found on the leaves, stems, and even the flowers.  The mold will start out as gray spots and then change to brown.  The mold will continue until the entire area is covered, which includes leaves, stems or flowers.

To prevent this from happening, make sure that you have enough space between the marigolds.  Also, make sure that you do not injure your plant.  Gray mold will set up in wounded areas.  Finally, watch the watering and avoid getting the plant itself wet.

While you can apply a chemical to treat the gray mold, it is better to just pull the plant up and start over.  When using the latter approach, make sure there is no plant material left from the diseased marigold.  One leaf with gray mold left behind is enough to start the disease over again.

Collar rot is another common disease that comes from overwatering.  It appears as black lesions that can be found on the stem and on the collar of the flower.  If you see this in your marigolds, remove the plant immediately.  Once that is done, remove the soil or sterilize it and replant.  After that is done, adjust your watering schedule so that you are not watering as often and only watering the soil.

Powdery mildew is a fungus disease of marigolds that looks like the plant has been dusted with flour.  The easiest approach is to pull up plants that have this fungus disease.  Why you may ask?   Well, this disease appears later in the growing season and in some situations it appears so late that you are already to plant something new. 

To prevent powdery mildew, do not water from above.  Second, marigolds that are labeled powdery mildew resistant should only be planted.  This will not only help the marigolds but any other plant that suffers from this plant disease.

Southern bacterial wilt is a disease that hits marigolds grown in warm areas.  The disease starts out as the foliage wilts and the growth is stunted.  This in turn, causes the foliage to change color from vibrant green to light green and finally to grey-green.  Once that happens, the plant will be dead in one to two weeks.

To treat this disease, one must simply pull up the plant and throw it away.

Botrytis blight is a plant disease that appears when an area has received an abnormal amount of rain.  Marigolds with this disease will have large brown spots on the foliage, which can turn into dead tissue.  If you take a closer look at the spots, you may find silver dots.  These are spores that are germinating. 

If you find this in your marigold planting, the only choice you have is to pull up the plant and start over. 

Fusarium wilt is not a plant disease that is caused by a wet environment.  Instead, it is caused by disease carrying soil.  This disease is fatal to seedlings but older plants can survive for awhile.  Wondering if you have this plant disease in your marigolds?  If so, take a look at the plant.  Is it wilting?  If the answer is yes then take a look at the vascular tissue.  When looking at the tissue, you will find that one side has a black streak.  Roots on the side of the black streak will be reduced in size and can be rotted.  During times of wet weather, you will find orange colored spores along the side with the black streak.

What do you do if you find your marigolds have this plant disease?  Well, the first thing is to pull up the plant material, dispose of the soil, and start over.  If you do not want to dispose of your diseased soil in the trash, consider sterilizing it yourself.  This can be done through solar sterilization or baking in the oven.

While leaf burn is not normally considered a plant disease, it is a problem that marigolds suffer from.  It appears as a yellowing of the tips and margins of the leaf.  This will continue to spread until the foliage has died.  You may think it is caused by some type of disease, it is not.  The true cause is improper use of micronutrients.  Marigolds need boron, molybdenum, and manganese.  Check your levels of these micronutrients, which should not be over 3 ppm for boron, 24 ppm for molybdenum, and 55 ppm for manganese.

Alternaria and bacterial leaf spot have similar symptoms.  Both have spots that appear purple or are purple on the margins.  Alternaria leaf spot attacks the leaves and stems while the bacterial leaf spot can be found on the leaves and petioles.  Overhead watering creates the environment by which this plant disease likes.  There are fungicides that can be applied but the best approach is to just remove the diseased plant material from the garden space.

Finally, septoria leaf spot is a marigold disease that appears as black spots that can be oval or irregular in nature.  These spots work their way up the plant from the bottom to the top.  This disease is brought on by overhead watering.  If you find this in your marigolds, pull them up, dispose of them in the trash, and start over.

As you can see, many of the problems that marigolds have can be reduced or even prevented with proper watering and spacing.  While many of the approaches listed for dealing with their plant diseases entails removing the plant, never compost them.  Throwing them away is a great way of ending that plant disease’s life cycle and gives you a “clean” garden slate by which you can plant.


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