How to Identify, Treat, and Prevent Cercospora Leaf Spot

Have you noticed large dark spots forming on the center of the leaves of your plants? Did they get larger and eventually cause the death of entire leaves? Did this occur during the wet and rainy summer season?

If so, you could be dealing with cercospora leaf spot (CLS). 

There are dozens of species in this particular genus. All are characterized by small spots that group and develop into lesions. Finally, leaves turn brown, causing what is known as ‘full leaf collapse’ or defoliation.

Certain species of plants are more susceptible to the spread of this fungal disease, and some naturally have resistance to infections. Some are even bred for resistance to leaf spot. Plant these to prevent proliferation of CLS in your garden. 

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 action, you’ll be able to stop the infection from growing beyond the point of resistance.

What is Cercospora Leaf Spot?

Overall, there are over 1200 species of cercospora. They are all specialized per the species of plant they tend to affect. The most common among these is Cercospora beticola, which is a frequent one that tends to show up in many gardens.  

Warm and consistently damp environmental conditions with high humidity (especially at night) are favorable for all species of the fungus.

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. 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 the disease progresses, nutrients divert from fruit, vegetable, and flower production into leaf production, making further development of non-leaf elements impossible.

This varies across species of plants, and some are more susceptible to leaf collapse than others. Common crops susceptible to the fungus include 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 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 (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.

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

Cercospora species begin their reproductive cycle by spreading from infected leaves, seeds, or other plant debris, usually by wind and rain. They then attach themselves and make their way into the cellular structure of healthy leaves.

The fungus creates lesions on the plant that allow further asexual spore reproduction, which can further perpetuate the cycle. It often originates from infected garden beds, tools, or nearby weeds. It is worsened when an infected seed is planted, or debris is left in garden soil.

Warm, humid evenings with temperatures between 77F and 95F are optimal for CLS. 

Symptoms

Symptoms of CLS present differently in different species. Sugar beet infections show disease through light tan spots with a maroon border. Eggplant CLS symptoms look different, 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. 

Here are a few other examples of presentations in different plants:

• Cercospora rosicola appears dramatically with 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 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.

However, severely affected plants all have the same general symptoms: leaves are brown and dead at the defoliation point. 

Other pathogens 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.

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.

Controlling Cercospora Fungi

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. 

Treatment

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

In comparative studies of organic vs chemical treatment on Cercospora beticola in beet crops, 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 dissolved and turned into a liquid.

Both are effective fungicidal agents, but when used in succession to one another, they have an extremely good impact against cercospora species. 

Neem oil is also used, but more as a preventative than as a treatment. Apply protectant fungicide before 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 is triphenyl tin hydroxide (Super Tin). This is sprayed early on varieties of beticola and others susceptible to disease development. Other chemical systemic fungicides are Benlate and Topsin M. 

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.

Prevention

The only surefire way to prevent cercospora leaf spot is to properly manage the site 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 and 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. Agricultural and horticultural scientists have identified some resistant strains of plants.

Frequently Asked Questions