Septoria lycopersici

Septoria lycopersici infects the tomato leaves via the stomata and also by direct penetration of epidermal cells.^[3] Symptoms generally include circular or angular lesions most commonly found on the older, lower leaves of the plant.^[1] The lesions are generally 2–5 mm in diameter and have a greyish center with brown margins.^[4][5] The lesions are distinct characteristics of S. lycopersici and contain pycnidia in the center which aid when trying to identify the pathogen. Pycnidia can be found in the center of the said lesions. Pycnidia are fruiting bodies of the fungus. When the lesions become numerous often the leaves turn yellow, then brown, shriveling up and eventually dropping off the plant altogether.^[1]

Septoria lycopersici prefers warm, wet, and humid conditions. Disease development occurs within a wide range of temperatures; however, the optimal temperatures lie between 20 and 25 degrees Celsius.^[4] High humidity and leaf wetness are also ideal for disease development.^[2] The initial source of inoculum for S. lycopersici results from overwintered resting structures such as mycelium and conidia within pycnidia which can be found on and in infected seed and within infected tomato debris left in the field.^[6] Spores spread to healthy tomato leaves by windblown water, splashing rain, irrigation, mechanical transmission, and through the activities of insects such as beetles, tomato worms, and aphids.^[1] Provided the environment is conducive for disease development, lesions usually develop within 5 days of infection.^[1]