NC State Extension Publications

Introduction

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Frogeye leaf spot disease on flue-cured tobacco is caused by the fungal pathogen, Cercospora nicotianae. This disease has historically been an issue for ripe tobacco, but has become more common over the last couple of years. The disease is found most commonly on lower, more mature leaves of the plant, but can also affect green tissues. Infections are not often severe enough to impact yield, but, under conducive environmental conditions, can cause severe damage to leaves.

Pathogen

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Frogeye leaf spot is caused by the ascomycete fungus, Cercospora nicotianae. This fungus is economically important in tobacco; however, it can also reproduce on several common weeds. Like many other leaf spot pathogens of tobacco, this fungus is favored by warm, humid environments.

Upon landing on host tissues, asexual spores (conidia) germinate and form new lesions that contain the hyphae of this fungus. From within these hyphal mats, conidiophores, the structures that bear conidia, erupt from plant tissues via asexual reproductive structures called stroma. From each conidiophore, conidia are produced. Conidia are long, multi-septate, and brown pigmented. After dispersal to susceptible tissues, conidia germinate to form new lesions.

Symptoms and Signs

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While usually found on low growing leaves, frogeye leaf spot can spread to all leaves of the plant. Lesions begin as small, sunken white/tan spots (Figure 1). Mature leaf spots are round or ovoid and approximately a cm in diameter (Figure 2). Spots are characterized by a dark brown margin with a pale tan/white middle. On green leaves, a yellow halo can be seen extending out from the dark margin. When the dark colored fruiting bodies (stroma) are created, a black area can be seen in the center of the spot that resembles pepper flakes. The conidia emerge from these stroma, and under magnification appear as long, thin, and hair-like with a gradual, uniform curve along its length.

Frogeye leaf spot lesions

Figure 1. Frogeye leaf spot lesions of varying ages, including early lesions with white/tan spots and lesions with yellow halos.

Photo Courtesy of Maryanna Waters

Mature Frogeye Leaf Spot Lesion

Figure 2. Mature frogeye leaf spot lesions with sporulation inside of lesion tissues

Photo courtesy of Lindsey Thiessen, NCSU

Diseases with Similar Symptoms

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Brown spot (Alternaria spp., Figure 3).

This disease produces dark brown lesions that become irregular shaped as they coalesce to large lesions. Lesions begin as small yellow spots that may be confused with early frogeye leaf spot lesions.

Target spot (Rhizoctonia solani, Figure 4).

This disease produces brown lesions, and commonly concentric rings of growth within the brown lesions can be observed. Because lesions begin as small, yellow spots, it may also be confused with early infections caused by frogeye leaf spot.

Brown spot lesions

Figure 3. Brown spot causing damage to tobacco leaf tissues

Photo Courtesy of Bryant Spivey

Target spot on tobacco

Figure 4. Target spot lesions on tobacco leaf characterized by concentric rings and yellow halo surrounding lesions.

Photo courtesy of Lindsey Thiessen, NCSU

Disease Cycle

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This disease overwinters as conidia in plant debris and soil. The conidia are dispersed by water and air to reach their new host. Spores will then germinate and infect the tissue of the leaf. As hyphae spread the leaf tissue dies, forming a necrotic lesion. New fruiting bodies (stroma) are formed within the leaf lesion, and erupt from plant tissues to release spores.

Favorable Conditions for Disease

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Warm conditions (25-30°C), combined with high humidity are favorable for growth and reproduction of Cercospora nicotianae

Disease Management

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Cultural practices are important to limiting damages by this pathogen. Crop residue destruction at the end of the growing season can help reduce disease incidence and severity of frogeye leaf spot from one season to the next by limiting inoculum build-up. A crop rotation schedule of non-host species can help break the life cycle of the disease. Avoid transplanting new plants near more mature plants that can harbor and spread the disease. While frogeye leaf spot is not currently common in greenhouses, infected transplants may spread disease into a previously unaffected field. Adequate row spacing can also reduce incidence and severity.

The use of fungicides may provide some reduction of disease pressure; however, strobilurin fungicide resistance has been found in North Carolina. There are currently no other conventional fungicides available in flue-cured tobacco that have efficacy on frogeye leaf spot. Until new chemistries become labeled, use of cultural practices is important to limit losses by this disease.

Useful Resources

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The NCSU Plant Disease and Insect Clinic provides diagnostic and control recommendations.

The NCSU Extension Plant Pathology Portal provides information on crop disease management.

The North Carolina Agricultural Chemicals Manual provides pesticide information for common diseases of North Carolina Crops. Recommendations do not replace those described on the pesticide label, the label must be followed.

Your N.C. Cooperative Extension agent.

Acknowledgements

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This factsheet was prepared by the NCSU Field Crops and Tobacco Pathology Lab in 2020.

Authors

Research Assistant
Entomology and Plant Pathology
Assistant Professor
Entomology and Plant Pathology

Publication date: Jan. 6, 2020

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North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons without regard to sexual orientation.