Cercospora Leaf Spot

Family: Mycosphaerellaceae

About Cercospora Leaf Spot

Life Cycle

Cercospora leaf spot can be caused by many different Cercospora fungal pathogen species depending on the plant type infected. For example, Cercospora beticola infects sugar beets whereas Cercospora rosicola infects rose plants. This is considered a foliar disease and can be especially devastating to sugar beet crops in North Dakota and Minnesota. This disease is sometimes misdiagnosed as black spot. Infection will begin at the bottom of the plant and will work up toward leaves with new growth. This happens when the fungal spores germinate and enter through natural openings of leaves when conditions are optimal. If leaves do not have high moisture levels on their outside tissue for at least 11 hours, new spores are unlikely to enter the plant. While infection is most commonly found on leaves, sometimes stems, bracts, or fruit can also be affected. Once spores infect a plant, it will take 5 to 21 days for symptoms to appear. This fungus has the ability to overwinter in plant debris that remains in a field after harvest.

Cercospora Leaf Spot Identification and Habitat

Identification

As the name suggests, this disease causes spots on foliage. The circular blemishes typically have an average diameter of 3 millimeters that is between brown and reddish-purple in color, bordering a gray center. However, if spots have just developed, the gray center will not yet be visible. When this fungus experiences favorable conditions, it may progress through 4 or 5 cycles in one season. Each progressive cycle is often more severe than the previous one, with spots sometimes growing to have a diameter of 10 millimeters. As these spots kill the plant cells, leaves will begin to fall from the plant, causing defoliation. A plant’s yield will be reduced from the limited photosynthetic capacity and can also result in more juice impurities and less sucrose extraction for sugar beets.

Habitat

Cercospora leaf spot favors weather that is wet, warm, and humid; often most prevalent following canopy closure. During the day this fungus thrives when temperatures are between 80 and 90 degrees Fahrenheit and at night when temperatures exceed 60 degrees Fahrenheit. The disease will not develop well if temperatures reach 93 degrees or higher. Humidity levels are preferred between 90 and 100 percent. More susceptible conditions for Cercospora involve areas near waterways, plants close to other fields that were infected in the past, and those near shelter-belts.

Cercospora Leaf Spot Management and Control Methods

Cultural Control

Since infection grows worse with time, early control is essential for effective management of this disease. In order to do this, scout fields frequently to catch any infection early, especially in highly susceptible areas. Also, whenever possible keep vulnerable plants away from areas that are most commonly affected by this disease. For example, plant sugar beets no closer than 100 yards from other previously infected areas. Practice fall tillage to bury infected plant debris that could be a possible overwintering site for Cercospora spores. Rotate crops, giving sugar beets a 2 year break in-between and replacing them with a crop type that is less susceptible. It is important to note that having satisfactory control is often dependent on using an integrated management system that includes the use of both cultural and chemical methods.

Chemical Control

Fungicides should be applied once first symptoms appear. Most often, additional applications will be needed to continue effective control, especially when conditions are favorable for this fungus; however, if using Topsin with a protectant (e.g., Supertin), this application should only be used once per season. Other fungicides with the following active ingredients have also shown to be effective for the control of Cercospora: copper oxychloride, sulfur, maneb, mancozeb, chlorothalonil, propiconazole, and neem oil. Always be sure to carefully read the fungicides label for cautions and proper application before use. In order to avoid this fungi building a resistance to a certain chemical, ensure the same fungicide is not being used back-to-back. Additionally, if only one application type is being used annually, do not use the same type of application. High spray pressure is best, often 100 psi, and also high water volume. Using both of these methods can create more effective control. On average, fungicides can provide protection for 2 weeks. Also, treatment methods in place for powdery mildew and black spot have also helped in the management of Cercospora.

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