Cercospora leaf spot is a common fungal disease that affects a wide assortment of weeds, shrubs and plants, including flower and vegetable plants.  In addition to leaves, the fungus can also affect other parts of the plant like stems, fruit, and seeds.  Some of the crops that are more susceptible to cercospora are beets, spinach, Swiss chard, okra, and carrots.

How Do Plants Get Cercospora?

The spores overwinter on plant matter, soil, compost, or fruit from the prior season.  When the conditions are right, usually beginning in late spring when temperatures are warm and wet, the dormant spores will be activated.  Spores are often transmitted by water droplets.  Thus, when water splashes on the soil it can cause the spores to go airborne and land on the plant’s leaves.  From there, the spores invade leaf tissue.  Spores can also be transported by wind, and an existing infection on an older leaf can become a source for a new infection.

How to Identify

Cercospora leaf spot is characterized by small, round or angular-shaped lesions that generally measure between 1/8 and 1/4 inch (.3 to .6 cm) in diameter.  The spots have maroon or purple borders with gray to brown centers. The lesions may also have a yellow “hallow” around the border.  Coloration may vary somewhat depending on the species and host plant.

Damage

As the disease progresses, the lesions will enlarge and a gray or purple fuzz will appear as spores develop.  This is especially problematic for crops that are grown for their edible leaves like spinach, Swiss chard, beets, turnip greens, etc.  If left untreated, leaves will often shrivel and fall off.  Lesions can also spread, connect with other lesions, and eventually cover entire leaves making photosynthesis difficult if not impossible.  The plant will eventually die.

How to Treat Organically

Once you find evidence of cercospora, the sooner it is dealt with, the more successful treatment will be.  Here are several options that should be used in combination:

Remove infected leaves because they will become a source of spores and new lesions.

Do not compost any portion of an infected plant, including fruit, seeds, etc.  Most compost piles do not maintain high enough temperatures to kill the spores.

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Antifungal agents can be helpful in cercospora management.  This is the gallon pump sprayer I use for larger applications like the ones described below.

• Apple cider vinegar. Add one tablespoon of apple cider vinegar (5% acidity) to one gallon of water.   Do a test spray on one leaf and wait 24 to 48 hours.  If there are no adverse effects, spray the top and underside of leaves and  the soil.  Repeat every three days as needed.  Do not spray in sunlight as the acidity of the vinegar and sunlight can burn plant tissue.

• Hydrogen peroxide. Mix 6 ounces (about 12 tablespoons) of hydrogen peroxide (3%) with one gallon of water.  Do a test spray on one leaf and wait 24 to 48 hours.  If there are no adverse effects, spray the top and underside of leaves and the soil.  Do not use the mixture on seedlings as it will burn them. Repeat once a week as needed.

• Milk spray. Like powdery mildew, which is also a fungal infection, cercospora can be treated with a milk-based spray.  Proteins found in animal milk is what makes the spray effective.  Therefore, coconut, almond, and other kinds of nut milk will not work.  Some recommend adding 3 ounces of animal milk to one gallon of water.  However, I use 1 part milk to 8 parts water.  Be sure to do a test spray as described above.  Apply the spray during daylight hours as the proteins require sunlight to be activated.  Repeat once a week as needed.

• Neem oil. Many gardeners know that neem oil is an effective organic insecticide, but it can also be an effective fungicide.  Be sure to use a cold-pressed neem oil with the naturally occurring chemical called Azadirachtin.  This is the brand I use.  Be sure to follow directions and do a test spray. 

• Liquid Copper. Some recommend the use of copper, a known fungicide, especially for severe cases of cercospora leaf spot. However, I personally do not advocate its use.  While copper is a naturally occurring mineral, it is combined with copper octanoate, or copper soap, to combat fungal infections in plants.  However, other chemicals, like copper sulfate, are also added.  Although copper sulfate is classified as being organic and said to be safe to use in the garden, several studies have found that it can be toxic to humans, animals, and bees.[1]  The Genetic Literacy Project reports:

 It has been assumed for years that pesticides that occur naturally (in certain plants, for example) are somehow better for us and the environment than those that have been created by man. As more research is done into their toxicity, however, this simply isn’t true, either. Many natural pesticides have been found to be potential – or serious – health risks.[2]

The use of liquid copper calls for wearing protective gear including clothing to protect the skin, eyewear protection, filtered mask, gloves, etc.  In my opinion, I would not use liquid copper in my garden, especially, when there are other alternatives.

How to Prevent

Proper plant spacing creates better airflow and thus dryer plants, a deterrent for the fungus which requires warm, damp conditions to propagate.

Crop rotation is a helpful measure to take against the buildup of cercospora spores in the soil.

Careful watering.  Use an irrigation system, soaker hose, or water by hand with a low-pressure at ground level to minimize splashing.  Also, water in the morning so that plants can dry before evening.

Mulch creates a protective barrier and thus helps minimize the spread of spores from the soil.  Mulch will also help the soil retain moisture so that you can water less frequently, thus keeping the “splash” factor to a minimum. 

Remove plant debris regularly because it can harbor cercospora spores. 

Practice good garden hygiene.  Make sure garden tools are cleaned with rubbing alcohol after use so that spores are not transferred from one plant to another.  (You can learn about other effective options for disinfecting tools in this helpful article.)  Also, disinfect containers and seed pots before repotting with new seeds or plants.  A bleach solution of 1 part bleach to 9 parts water works well.

Neem oil, as described above, can also help as a preventative measure.  Be sure to do a test follow directions and do a test spray. 

Cercospora leaf spot is a common fungal disease that, if caught and treated early, should not interfere with a bountiful harvest.

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[1] Andrew Porterfield, “Organic Fungicide Copper Sulfate Poses Dangers to Humans, Animals, Insects—How Does It Compare to Conventional Pesticides?”  Genetic Literacy Project, 11-16-18, updated 7-23-20, accessed 5-18-21, https://geneticliteracyproject.org/2018/11/16/organic-fungicide-copper-sulfate-poses-dangers-to-humans-animals-insects-how-does-it-compare-to-conventional-pesticides/

[2] Ibid.