Recent weather has created an increased threat to local tomato plants from Septoria lycopersici, a disease commonly known as Septoria Leaf Spot.|
According to Mahfuz Rahman, WVU Extension Service plant pathology specialist, "Pairing the large quantities of rain we've had this summer with high humidity levels, it is prime breeding ground for this plant disease."
Rahman adds it can sometimes be difficult for the untrained eye to tell the difference between Early Blight and Septoria.
Septoria is most easily identified by the leaf spots, which can grow up to 1/8 inch in diameter. The spots may become gray or tan in the center, with dark brown borders. As the spots mature, they may grow a dark brown, pimple-like fungus inside of the spots. Eventually the leaf turns yellow and then dies. Unlike Early Blight, Septoria doesn't usually affect stems or fruits, although if the plant loses too many leaves, the fruit can be susceptible to sunscald.
Early Blight usually manifests as five to ten brown, circular spots that first appear on the leaf. These spots can reach up to one half inch in diameter and have concentric rings or ridges that are surrounded by a yellow halo. Dark, sunken spots in the fruit are a clear indication that Early Blight has spread.
To treat Septoria, Rahman suggests removing the lower, infected leaves from the garden and burying or burning them as soon as possible. Dry leaves help to prevent the disease from spreading, and a plastic canopy can help to protect the leaves from moisture. If you have an outbreak, the same products are effective against both Early Blight and Septoria.
For organically grown tomatoes, use copper hydroxide (such as Kocide 101) or Serenade on a 10-day schedule, starting pre-bloom or at first sight of blight spots. For conventional tomatoes, use chlorothalonil (such as Daconil 2787) or azoxystrobin (such as Quadris, Amistar, etc.) if the disease is detected and the weather remains damp and rainy. Do not use azoxystrobin more than two consecutive sprays or a total of five sprays in a growing season to keep the fungus from developing resistance.
If you need help identifying what's affecting your plants, or want more information, contact Daisy Fryman, WVU Extension Agent- Agriculture in the WVU Extension Service Calhoun County office, 304-354-6332.
You can also find help about Early Blight, Septoria, and Late Blight on the WVU Extension Service website at ext.wvu.edu