Septoria leaf spot of tomatoes

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

We are currently seeing quite a few tomato samples with Septoria leaf spot. This is a fairly common disease on tomato, both on plants in home gardens and in commercial fields.

Septoria leaf spot, which is caused by Septoria lycopersici, results in spotting and wilting of tomato foliage. Symptoms develop first on the lowest leaves of the plants.

This fungal disease causes circular spots with dark brown margins and tan to gray centers dotted with black specks (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 wither. Fruit infection is rare.

S. lycopersici
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 are ideal conditions for disease development and spread of the pathogen.

Tomatoes should be treated with registered protective fungicide sprays at regular intervals during the growing season. There are several fungicides available to home gardeners that are effective in preventing fungal diseases of tomato. 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 tomatoes. 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 seven-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 one to two years between tomato crops, control of susceptible weeds (such as horsenettle) 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.

Septoria leafspot
Septoria leafspot.
Photo Credit: William M. Brown Jr., www.bugwood.org

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