Tomato Septoria Leaf Spot is a fungal disease of Solanaceous crops caused by Septoria lycopersici. Symptoms appear on the leaves as circular, tan-to-gray spots with darker brown margins and dotted with dark, raised pycnidia inside the tan portion of the lesion. These lesions are often surrounded by a yellow halo, and typically develop on lower leaves first. Stem lesions appear similar to leaf lesions, but are often darker, and fruit lesions appear similar but are uncommon.
The pathogen causing this disease overwinters on diseased Solanaceous crop or weed debris in the soil, and spreads to older leaves via splashing water, workers, equipment, and several insects. The pathogen then spreads to newer leaves from resulting lesions in a similar way. Favorable conditions include poor airflow, high moisture, and moderate temperatures.
Cultural management involves planting resistant varieties, rotating away from susceptible crops, staking or trellising plants, destroying host weeds and infested plant debris, avoiding over-irrigating, and properly mulching. Preventative application of conventional or organic fungicides can also prevent disease development.