Symptoms observed on low, oldest leaves of transplants in a greenhouse.
Small dark brown spots with tan centers containing very tiny black speck-sized structures visible upon close scrutiny are characteristic of the disease Septoria leaf spot. The black structures are where the fungal pathogen, Septoria lycopersici, produces spores. Images of these spores in their tendrils are below and closer views are on the lettuce Septoria leaf spot page.
The pathogen causing Septoria leaf spot can be seed-borne. When the seed coat sticks to the cotyledon, any pathogen spores on the seed can easily be moved to leaf tissue by splashing water, which is how this pathogen is spread.
Symptoms observed on plants in the field.
These symptoms usually first appear on low (oldest) leaves after fruit start to ripen.
The characteristic tendrils of spores of Septoria can be seen in the leaf spots upon close examination. They are more likely to be present on leaves that have been in high humidity, such as often occurs overnight or when a leaf has been on a wet paper towel in a plastic bag. Images of spore tendrils taken under a dissecting microscope are on the lettuce Septoria leaf spot page.
Management: Greenhouse environment can be ideal for development of this and other diseases due to plants being packed close together, daily watering that involves splashing water, and high humidity. Plants should be inspected routinely for disease symptoms, especially when grown from seed not tested or treated for Septoria or other pathogens. Keep varieties as separate as possible in case a disease develops in one. Minimize the time that leaves are wet by using fans, watering sparingly early in the day, spreading trays out, and by moving seedlings out of the greenhouse as soon as they are ready. Dithane and some copper fungicides (including Cuprofix, Champ and Champion) can be used for Septoria leaf spot in the greenhouse. Organocide is a botanical oil approved for organic production and labeled for greenhouse use that provided good control in a field experiment at LIHREC in 2008.