Brown spot is one of the most economically damaging foliar diseases of tobacco in North Carolina. The disease typically affects lower or mature tobacco leaves. Because brown spot populations increase exponentially during the growing season, plants that are in the field longer than normal are most damaged by this disease.
The causal agent of brown spot is Alternaria alternata, which is an opportunistic pathogen of many crops. The pathogen survives in crop debris and disperses as conidia (asexual spores). Conidia germination requires moisture for germination and colonization of host tissue. Once tissues are colonized, abundant conidia are formed within lesions that are later dispersed to new tissues.
Alternaria alternata has a wide host range and can infect tobacco, pears, apples, wheat, tomatoes, beans, cotton, brassicaceae, peppers, cucurbits, potatoes, strawberries, beets, and many other common host plants. Although it is capable of colonizing a wide number of hosts, wounding is often required for disease to develop.
Small circular to irregularly-shaped spots, beginning small (0.25 in) and expanding as they mature (1.25 in). Lesions are dark brown with yellow halos surrounding the spots (Figure 1). Within the lesions, dark rings may be observed that contain thousands of spores that may be splash- or wind-dispersed to newer leaves. Lesions may coalesce, causing the necrotic tissue to tear away, leaving the leaves ragged. Brown spot can continue to develop under curing conditions, causing moldy growth on cured leaves.
Diseases with Similar Symptoms
Target Spot (Thanatephorus cucumeris, syn. Rhizoctonia solani) (Figure 2)
Target spot lesions are not limited to mature leaves. Spots are brown with concentric rings and lack spores.
Frogeye Leaf Spot (Cercospora nicotianae) (Figure 3)
Frogeye leaf spot lesions affect leaf tissues and may appear similar to early brown spot lesions. Frogeye leaf spot is characterized by small white lesions with yellow halos and black spots in the center (spores).
Leaf spots caused by Phytophthora nicotianae are limited to the lower leaves and will not spread upward. The lesions are dark brown with concentric rings, but do not contain dark, brown spores like brown spot.
Disease Cycle and Favorable Conditions for Disease
Alternaria alternata survives on crop debris or other weedy hosts. Conidia production begins when warm, moist conditions are present in the spring. Alternaria alternata conidia disperse via air currents, and their release from the lesions can be triggered by rainfall. When the conidium lands on a leaf, it will wait until the nighttime dew, and then germinate. It can either enter through the stomata, or penetrate directly through the top of the leaf, infecting the leaf within 12 hours. New production of conidia can occur in as few as ten days after the first symptoms appear and can continue for a long period of time thereafter.
Brown spot management is geared to reduce fungal inoculum build-up in the production system and delay initial onset of disease. Crop rotation away from tobacco will allow for crop debris to completely decay limiting fungal survival. Quickly destroying crop debris after harvest will also limit inoculum survival on tobacco tissues.
Infections by other pathogens that impact general plant health may also increase brown spot incidence. High nematode pressures, black shank, and Granville wilt may also increase brown spot. Limit nematode and other disease pressures where possible.
Adequate nutrition (not deficient nor excessive) for encouraging vigorous growth of leaves and limiting excess growth may reduce the impact of brown spot. Additionally, proper plant spacing and sucker control will limit humidity in plant canopies, reducing brown spot incidence.
The NCSU Plant Disease and Insect Clinic provides diagnostics and control recommendations.
The NC State 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. The manual recommendations do not replace those described on the pesticide label, and the label must be followed.
The Tobacco Diseases Information page, created by the Shew lab at NC State University, provides in depth information on several tobacco diseases found in North Carolina.
For assistance with a specific problem, contact your local N.C. Cooperative Extension agent.
This factsheet was prepared by the NCSU Field Crops and Tobacco Pathology Lab in 2019.
Publication date: Sept. 27, 2019
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