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Cercospora Leaf Spot: Another Annoying Fungus

Have you noticed large dark spots forming on the center of leaves of your plants that get larger as time goes on, and eventually cause the death of entire leaves? Did this occur during the wet and rainy summer season? You could be dealing with one of the many species of fungus diseases in the cercospora genus, called cercospora leaf spot. 

There are dozens of species in this particular genus. They are all specialized per the species of plant they tend to affect. Overall, there are over 1200 species of cercospora. The most common among these is Cercospora beticola. All species are characterized by small spots that group and develop into lesions, and finally turn leaves brown causing what is known as “full leaf collapse” or defoliation. Certain species are more susceptible to the spread of this fungal disease, and some naturally have resistance to infections. 

It would be naive to say the first signs of cercospora on leaves isn’t cause for alarm, because at this point gardeners need a good plan of attack. Allowing an affected beet plant or crop to get to the point of severe yield loss is not a great option. Catch these lesions early, and employ successful management to ensure your neighbors don’t have the same problems you are experiencing. With the right resources and right action, you’ll be able to stop the infection from growing beyond the point of resistance. And you’ll be able to prevent cercospora fungus species from developing on other plants you love to grow.

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What Is Cercospora Leaf Spot?

Cercospora leaf spot on beets
Cercospora leaf spot on beet greens. Source: photofarmer

Warm and consistently damp environmental conditions with high humidity (especially warmth and humidity at night) are favorable for all species of the fungus cercospora. Temperatures of 77 to 95 degrees Fahrenheit are optimal for spore growth, so early summer or late spring season prevention is essential. Proper management of irrigation is important in this regard. A crop infected with cercospora leaf spot shows symptoms when small dark circular lesions less than one inch in diameter appear on leaves. These lesions have a purple to maroon border and a gray or brown interior. Sometimes spots that appear are covered with a light gray or purple fuzz. You’ll notice these lesions first on mature leaves of the plant, and if left to reproduce, they’ll move to younger leaves.

Early on, lesions group and begin at the base of the plant or the outer diameter of the plant and move upward and inward. As these lesions occur and the disease progresses, nutrients divert from fruit, vegetable, and flower production into leaf production, making further development of non-leaf elements impossible. This disease can reduce your yield by a large percentage; anywhere from half to all of your efforts could be thwarted. 

This varies across species of plants; some are more susceptible to leaf collapse than others. Scientists in agricultural fields have been studying varieties that are naturally resistant to CLS. Gardeners in historically warm and humid climates might take note of those species, rather than risk spreading the pathogen that causes CLS, or losing 50 percent of their crops in severely affected plants. 

Common crops susceptible to the fungus cercospora leaf spot are those in the beta species: swiss chard, sugar beets, and spinach. Often the pathogen spreads from weeds that are host to the disease. Managing the disease is simple, but requires painstaking attention to detail as even remaining seeds and plant debris can be compromised. 

Types Of Cercospora Fungi

Gardeners and farmers commonly come into contact with Cercospora beticola, which attacks the beta group like the ones listed in the previous section of this article (sugar beets and leafy greens). But there are many other cercospora leaf spot species. Roses are susceptible to Cercospora rosicola. Hydrangeas are inclined to contract Cercospora hydrangea. Eggplants are vulnerable to Cercospora melongenae. The number of species is a testament to how adaptable this fungus is, and how easily spores can spread from one plant to another. Proper management and control of the spread is essential for the health of your garden and those around you.

Not all cercospora species present the same symptoms across affected species, and not all are called cercospora leaf spot (CLS). For instance, CLS on soybeans is commonly called cercospora blight. That same term is applied to cercospora damage on juniper trees, celery, and carrots. CLS is the end result of infections of particular cercospora pathogens. Cercospora on corn is called gray leaf spot. 

Life Cycle Of The Cercospora Fungus

Cercospora species begin their reproductive cycle by spreading from infected leaves, seeds, or other plant detritus, usually by wind and rain. They then attach themselves and make their way into the cellular structure of healthy leaves. Then symptoms of infection appear. The fungus creates lesions on the plant that allow further asexual reproduction of spores which can perpetuate the cycle further. Sometimes the origin of this pathogen comes from infected garden beds, tools, or nearby weeds. It is worsened when an infected seed is planted or when detritus is left in garden soil. When the night is warm, humid, and temperatures remain in the 77 to 95 degree range, conditions are optimal for CLS. 

Symptoms Of Leaf Spot

Symptoms of CLS or cercospora blight present differently on different species. Sugar beet infections show disease through light tan spots with a maroon border. Eggplant CLS contagion symptoms look very different from sugar beet infections with light brown spots that have no border. Fruits can also suffer infection that looks almost like a caterpillar has taken to the eggplant for a snack. 

Cercospora rosicola appears dramatically as very distinct purple necrotic lesions with a light tan to grey center. Cercospora hydrangea looks similar, but the center of the spots tends more toward light grey to white. Kale CLS symptoms are varied across varieties but typically show light brown discoloration. Sometimes kale will have the characteristic purple border at each lesion. Spinach cercospora leaf spot is like that of kale and doesn’t always have a dark maroon border. Severely affected plants all have the same general symptoms, though: leaves are brown and dead at the defoliation point. 

There are other pathogens that look a lot like cercospora leaf spot but are caused by bacteria. Black leaf spot on roses could potentially be confused for CLS if careful consideration isn’t taken. Black leaf spots will be clustered without necrosis. They also won’t be as round. This is one way to distinguish between a fungal and a bacterial infection. Use a small magnifying glass to see if the fuzz has developed on spots. Light gray or light purple hairs are symptoms of a fungal infection rather than a bacterial infection. Knowing the difference between each disease gives you a good indication of the tools needed to control the problem and prevent its spread. There are many resources out there for disease identification, so feel free to do some research in the determination process. One great resource is your local agricultural extension office. 

Controlling Cercospora Fungi

Cercospora on Ipomoea leaf
Ornamentals like this Ipomoea pes-caprae are susceptible to cercospora too. Source: Starr

Although fungicide can be applied for the control of cercospora species, the most effective way to prevent cercospora leaf spot spread is good garden hygiene. It’s important to look out for cercospora leaf spot in times when the summers are particularly wet with high humidity that lasts through the night. These conditions are favorable for cercospora spores, and can further disease development. Management is essential when temperatures are high. Environmental conditions are your first point of understanding how to control CLS. 

Cercospora Treatment

Organic methods of treatment are slightly less effective than chemical methods, but they exist and are in fact viable. They’re also much more widely available to the home gardener than the most common chemical methods, which is helpful!

In comparative studies of organic vs chemical treatment on Cercospora beticola in beet crops, it was determined that the most effective treatment used two things: a good-quality liquid copper fungicide, and Bacillus amyloliquefaciens. The latter is a bacteria that acts as a natural fungicide, and the former is copper that has been dissolved and turned into a liquid. Both are effective fungicidal agents, but when paired they had an extremely good impact against cercospora species. 

Neem oil is also used, but more as a preventative than as a treatment. Apply protectant fungicide prior to conditions where CLS disease is expected to appear. If you know your summer crops are about to endure high temperatures and humidity, use protectant fungicides as a prevention method.

Typically, there are two types of chemical fungicide used to control CLS and cercospora leaf blight: protectant fungicides or systemic fungicides. One of the popular protectant fungicides applied to leaves is triphenyl tin hydroxide (Super Tin). This is sprayed early on varieties of beticola and others susceptible to disease development. Other chemical systemic fungicides that are applied to leaves are Benlate and Topsin M. 

Do note that there are strains of cercospora leaf spot that are completely resistant to fungicide. Check with your local agricultural extension to determine if spraying systemic fungicide will further exacerbate the reproduction of resistant strains of cercospora disease. Your local ag extension is a great resource for common diseases that occur in your area, and the best time to apply fungicides.

Preventing Leaf Spots

The only sure-fire way of preventing cercospora leaf spot is to properly manage the site where you grow with proper irrigation, clean beds free of debris, and limited contact with infected areas and plants. Improper irrigation will increase the moisture and humidity that cercospora prefers. Do not allow foliar contact with an infected crop, and do not allow plants with CLS to go to seed. Along the same lines, avoid planting infected seed, as this will only prolong the life cycle of cercospora leaf spot and give it more time to develop resistance to treatments. 

Check each leaf of your sugar beet plant, roses, or those most susceptible to cercospora leaf spot for symptoms, especially in humid hot summers when the disease is more likely. 

The best ways to manage CLS are environmental cultural preventative methods. Clean tools after every use. Keep infected plants out of your garden beds and compost piles. Rotate crops, and don’t plant next to a previously diseased site, since spores typically hang out in areas where an infection has occurred. Choose seeds for plants that are resistant to defoliation caused by CLS. Some resistant strains of plants have been identified by agricultural and horticultural scientists.

Frequently Asked Questions

Cercospora on okra
Severe cercospora can cause okra leaves to skeletonize. Source: JIRCAS

Q: Is leaf spot contagious?

A: Yes. Cercospora leaf spot is a highly contagious disease that can build resistance over time and must be aggressively controlled. The best methods for managing cercospora leaf spot are cultural methods that involve removing all damaged material, rotating crops, and avoiding cultivation near prior outbreak sites. Planting a resistant species of roses, sugar beets, and beticola crops is the best way to prevent this disease.

Q: Can you eat Swiss chard with cercospora leaf spot?

A: You can, but at the point when spores have developed on a leaf or crop, it’s best to remove the damaged foliage and bag it up for disposal. Leaves with diseases like cercospora leaf spot will have a significantly different structure compared to healthy ones. The texture of the plant could be woody and bitter. Severe infections cause the total death of leaves. These cannot be consumed for nutritional value.


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