TB03 - Tobacco Disease Note 3
T. A. Melton, Philip Morris Professor and Department Extension Leader
H. D. Shew, Professor of Plant Pathology
Brown spot, one of the most destructive leafspot diseases on tobacco, was only a minor problem in North Carolina until the mid-1950's. From that time until the mid- 1970's, disease losses averaged about 0.5% per year in North Carolina. Losses increased during the 1980's but have decreased back to 0.5% or less. Because brown spot populations build up higher and higher throughout the growing season, and because aging tissue is more susceptible to the brown spot fungus, plants that remain in the field longer than normal are most damaged by the brown spot disease. Other important conditions that lead to brown spot are continuous tobacco culture, frequent rains, and very sensitive varieties.
The leafspots caused by this fungus appear as circular spots, ranging from 1/4 inch to 1 1/4 inches in diameter, and are found primarily on the lower leaves of the plant. Initially, these target-like spots have a yellow or yellowish-green halo around them. The fungus causes the leaf tissue in the area of the spot to age prematurely, which results in a yellow halo around the lesion. Each of the dark rings in the target are made of thousands of tiny spores (seed-like structures) that may be splashed or blown up into the upper leaves where additional spots will occur. As spots enlarge and coalesce, dead tissue often tears and falls out of the leaf making the entire leaf ragged and worthless. Late in the season, on very sensitive varieties, spots may occur on suckers, petioles and even on the stalk. When stalks and suckers are infected, girdling of the plant can occur and the plant dies. The pathogen can also act systemically and affect the overall plant metabolism resulting in premature death. Brown spot may continue to develop in the curing barn at low temperatures.
Brown spot is caused by an air-borne fungus, Alternaria alternata. This fungus overwinters primarily in tobacco stalks. When spring-time brings warm weather, spore production begins on old stalks that are still exposed on top of the soil. These spores are then blown and/or splashed up onto the lower leaves of the new plants where they germinate and penetrate the leaves directly. Spots produced on the lower leaves form new generations of spores that are splashed and blown onto other leaves. Each of the spores requires moisture to germinate on a tobacco leaf. Therefore, the leaves must be wet. If dry conditions exist, spores cannot germinate and new spots cannot develop. Alternaria alternata is a common fungus associated with at least 56 different species of plants. However, under natural conditions it rarely, if ever, causes disease in plants other than tobacco. It has been found to be a wound pathogen of closely related plants such as peppers and tomatoes. In other words, it can only attack those plants after the plants have been wounded and one of the spores of the fungus lands in the wound.
Brown spot management should fit into the overall disease management program. The goal is to delay disease build-up. Rotation with another crop will allow time for the tobacco debris to decay therefore, leaving little or no food for the brown spot fungus. Stalk and root destruction helps prevent early infection by reducing availability of inoculum. Early infection means faster build-up and greater disease severity by the end of the season. High nematode levels also lead to increased brown spot damage. Therefore, nematodes should be controlled. Proper levels of potassium also should be available to the plant to insure vigorous growth. Infected leaves should be primed as soon as possible to reduce the number of spores available for pathogen spread. Proper plant spacing will decrease humidities around the plant, thus decreasing the incidence of brown spot. Efficient sucker control also aids in reducing losses. Excessive fertilization, especially with nitrogen, may lead to greater losses to brown spot and should be avoided. In areas with a history of severe brown spot, brown spot sensitive varieties should be avoided.
For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service
of specific chemicals are based upon information on the manufacturer's
label and performance in a limited number of trials. Because environmental
conditions and methods of application by growers may vary widely, performance
of the chemical will not always conform to the safety and pest control
standards indicated by experimental data. All recommendations for pesticide
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regulatory agencies. Last printed 10/90/1000