Septoria Leaf Spot of Tomato
Septoria leaf spot, which is caused by Septoria lycopersici, results in spotting and wilting of tomato foliage. This fungal disease causes circular spots with dark brown margins and tan to gray centers dotted with black pycnidia. A narrow yellow halo is often associated with leaf lesions. If the leaf lesions are numerous, some infected leaves turn slightly yellow and then brown, and they wither. Fruit infection is rare.
Septoria leaf spot overwinters on infected tomato debris or debris of solanaceous weed hosts, such as horsenettle. The fungus can also survive on equipment such as plant stakes and cages. Once introduced to a planting area, conidia are spread by splashing water from rain or sprinkler irrigation. Long periods of high relative humidity, high temperatures, and leaf wetness constitute ideal conditions for disease development and spread of the pathogen.
Tomatoes should be treated with registed protective fungicide sprays at a regular intervals during the growing season. There are several fungicides available to home gardeners that are effective in controlling septoria. To be a prudent consumer you need to read the fine print on the product label while you are shopping.
First, the product must be labeled for use on the crop (ex. tomatoes, potatoes, etc.). Second, look at the active ingredient(s) listed on the label. Products that contain chlorothalonil or ethylene bisdithiocarbamate (EBDC) will protect plants from infection. Depending on the particular manufacturer the trade names vary, these may be sold under trade names such as Daconil, Mancozeb, or Fungonil, to name a few. Theses fungicides will need to be reapplied frequently, usually a 7-day interval is recommended. Refer to the product label for specific instructions.
In addition to fungicide sprays, cultural control measures include rotating for at least 1-2 years between tomato crops, control of susceptible weeds (such as horse nettle) in rotation crops, removal of crop debris from the planting area, careful attention to the timing of irrigation to reduce the length of time during which the plants remain wet, and staking plants to improve air circulation and reduce contact between foliage and soil.