Septoria Leaf Spot caused by the pathogen Septoria lycopersici can become a serious problem in tomato fields when the first two weeks of July are excessively hot and wet. Symptoms (see picture) usually show up by the third week of the month and if severe enough defoliation can occur rapidly. The pathogen can overwinter on infected plant debris, infected weeds, and in or on seed. Relative humidity must be 100% for at least 48 hours (can be non-continuous) for infection (via stomata) to occur. Symptoms can appear 6 days after infection and pycnidia which produce spores for secondary infections develop about 2 weeks after the initial infection. Control of Septoria Leaf Spot is best with crop rotation and chemical control.