Brown spot (Alternaria alternata)

Brown spot: biology

Brown spot is a fungus that survives over winter on decomposing or dead plant tissue, including weeds. Spores are widely dispersed by wind. The disease causes premature leaf shedding which, in turn, affects yield potential by up to 10 percent. Brown spot infects and progresses more quickly on stressed plants. For example, after a prolonged dry spell, plants are under moisture stress. A change in weather, either a storm front or a significant drop in temperature, can trigger a spore release.

Brown spot is typically more prevalent in warm seasons, and though it can occur at any time in the season, it generally develops in later summer after the rows close. The fungus can also lead to tubers becoming infected at harvest leading to additional losses.

Initial symptoms of brown spot include small, round dark spots that can grow up to 10 mm in diameter; they appear first on older leaves in the middle of plants. Spots have concentric rings and eventually coalesce with other spots, forming black patches of dead tissue. These necrotic areas usually form near the edges of leaflets; leaf drop typically follows. Brown, elongated small lesions may also form on plant stems when disease pressure is high.

Brown spot symptoms are often confused with early blight symptoms, and it is not unusual for a plant to be infected by both pathogens.

Infected tubers exhibit small black holes (black pit) that are similar to pits caused by common scab but are deeper, narrower and blacker.