NC State Extension Publications

Introduction

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Target spot is a potentially devastating leaf spot disease in tobacco that is generally first observed around June when the weather is warm and humid. Target spot of tobacco frequently develops in South America, South Africa, the southern United States, and Canada. The disease has been a major threat to American tobacco production since it was first observed within the United States in the 1980s. The disease is caused by Thanatephorus cucumeris, and is more commonly known as its asexual stage synonym Rhizoctonia solani. Spores from the soil-borne fungus are released from the soil surface onto mature leaves closest to the ground. As the disease progresses, it moves up the tobacco plant, while its initial pinhead-sized lesions begin growing outward. Spreading quickly in warm, humid conditions, the disease has the ability to severely reduce yield when it is not adequately managed.

Host Range

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It is important to note that R. solani is not exclusive to tobacco. In fact, the most common symptom observed from R. solani is “damping off,” where infected seeds lose the ability to germinate, or small seedlings rot early in the growing season. R. solani has over 200 known hosts and does not affect each one through the same process or with equal severity.

Pathogen

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R. solani is a soil-borne fungal pathogen within the phylum Basidiomycota. Like many other fungal pathogens, R. solani prefers warm temperatures (> 24°C / 75°F) and high relative humidity (> 98%). The fungus exists primarily as hyphae or sclerotia within the soil and is spread aerially through basidiospores that are formed under optimal conditions (warm, wet weather). The pathogen reproduces on the underside of tobacco leaves, forming translucent hyphae that turn brown with age. Target spot is polycyclic, meaning that if climate is favorable for disease development, then many cycles of infectious spores can occur in one season.

Symptoms

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The first symptoms of target spot are small, 2-3mm white or tan colored primary lesions (Figure 1) that appear on the surface of foliage closest to the soil. As the disease progresses, it spreads outward. A series of necrotic rings around the primary lesion are created, giving the characteristic target symptom (Figure 2). Bordering the outermost necrotic layer is a yellow colored halo of chlorotic tissue. Plants low in nitrogen often display larger necrotic lesions and broader chlorotic rings compared to plants with sufficient levels. When the target spot grows large enough, the necrotic tissue in the center will split or fall out, creating a large hole in the leaf (Figure 3). Fortunately, not all primary lesions grow in size, however, the ones that do have potential to cause a serious amount of tissue loss.

Primary lesions

Figure 1. Primary target spot lesions on tobacco leaf.

Courtesy of Lindsey Thiessen, NC State

Target spot on tobacco

Figure 2. Target spot lesions on a flue-cured tobacco leaf.

Courtesy of Lindsey Thiessen, NC State

Target spot shot-hole symptoms

Figure 3. Shot-hole appearance in tobacco target spot lesion.

Courtesy of Lindsey Thiessen, NC State

Disease Cycle

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R. solani overwinters both as mycelium and sclerotia within the soil and on crop debris. Once environmental conditions become favorable (e.g. warm and humid), the sclerotia extend threadlike hyphae to collect nutrients. The hyphae form a multi-layered mass at the surface of the soil called a hymenium where basidia are produced. The basidia produce basidiospores that are dispersed through the air. When a spore lands on a leaf, it penetrates into a cell so it can begin extracting nutrients and grow within the plant. As the pathogen grows into neighboring cells, the previously exploited cells die and become a visible primary lesion. If the pathogen continues to grow outward, it forms alternating bands of light and dark brown necrotic tissue that gives the disease its signature target-like appearance. When conditions remain favorable for an extended amount of time, R. solani will extend hyphae out from the stomata on the underside of the leaf and release secondary inoculum in the form of basidiospores. If environmental conditions are not conducive for reproduction during the growing season, the dead tissue from the target spot will become brittle, fall to the soil surface, and remain there, ready to begin its cycle again next season.

Diseases With Similar Symptoms

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Brown Spot (Alternaria alternata, Figure 4). This pathogen can cause a brown necrotic leaf spot that also produces concentric rings, but abundant sporulation by this pathogen within lesions can be diagnostic of brown spot. See the Brown Spot of Tobacco factsheet for more information.

Phytophthora Leaf Spot (Phytophthora nicotianae, Figure 5). This leaf spot is typically limited to the lower leaves (spread via splash-dispersal from soil), but can also produce concentric rings within lesions. See the Black Shank of Tobacco factsheet for more information.

Frogeye Leaf Spot (Cercospora nicotianae, Figure 6). This leaf spot can be confused for primary target spot lesions due to the similar white centers; however, this leaf spot also has a dark brown margin and surrounded by a yellow halo. See the Frogeye Leaf Spot of Tobacco factsheet for more information.

Brown spot lesion

Figure 4. Brown spot lesion on tobacco leaf.

Courtesy of Lindsey Thiessen, NC State

Phytophthora nicotianae leaf spot

Figure 5. Phytophthora leaf spot on flue-cured tobacco leaf.

Courtesy of Bryant Spivey

Frogeye leaf spot

Figure 6. Frogeye leaf spot lesions on tobacco leaf.

Courtesy of Lindsey Thiessen, NC State

Management

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The most important environmental factor for the occurrence of target spot is high moisture levels. Harvesting mature leaves allows vital airflow to move around the bottom canopy and aid in the release of trapped moisture that encourages disease development. Nitrogen-deficient plants are more susceptible to target spot; therefore, it is critical to maintain the proper nutrient level to help suppress the pathogen’s devastating effects. In general, practices that increase air circulation and reduce leaf wetness are the most important to avoid disease formation. Chemical control may be necessary in years with conducive conditions or in fields with a history of disease. Azoxystrobin (e.g. Quadris) is labeled for use in the field and greenhouse. Mancozeb (e.g. Manzate Pro Stik) has a 24c label for use in tobacco in North Carolina, and may be moderately effective for reducing target spot. Several biologicals and organic fungicides are also available, but their efficacy may be limited based on environmental conditions, disease pressure, and number of applications made in the growing season. More information on fungicides available for tobacco can be found in the North Carolina Agricultural Chemicals Manual.

Other Useful Resources

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Authors

Undergraduate Research Assistant
Entomology & Plant Pathology
Assistant Professor & Extension Specialist
Entomology & Plant Pathology

Find more information at the following NC State Extension websites:

Publication date: Aug. 17, 2020

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The use of brand names in this publication does not imply endorsement by NC State University or N.C. A&T State University of the products or services named nor discrimination against similar products or services not mentioned.

Recommendations for the use of agricultural chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by NC State University or N.C. A&T State University nor discrimination against similar products or services not mentioned. Individuals who use agricultural chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact your local N.C. Cooperative Extension county center.

N.C. Cooperative Extension prohibits discrimination and harassment regardless of age, color, disability, family and marital status, gender identity, national origin, political beliefs, race, religion, sex (including pregnancy), sexual orientation and veteran status.