Two diseases that attack tomatoes early in the growing season around blossom set are Early Blight and Septoria. Both start on lower leaves of the plant and result in leaf spots that may eventually kill the leaf. Defoliation of the plant is harmful and also leads to sunscalded fruit. Disease development is favored by warm temperatures, abundant rainfall and high humidity. Mulching, caging, no overhead watering and good air flow are helpful to reduce disease pressures. To learn more about these early tomato diseases, here are articles from K-State Research and Extension and University of Minnesota.
https://hnr.k-state.edu/extension/info-center/common-pest-problems/common-pest-problem-new/Early%20Blight%20of%20Tomatoes.pdf
https://extension.umn.edu/diseases/early-blight-tomato
http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/fungal-spots/septoria-leaf-spot-of-tomato.aspx
https://hnr.k-state.edu/extension/info-center/common-pest-problems/common-pest-problem-new/Early%20Blight%20of%20Tomatoes.pdf
https://extension.umn.edu/diseases/early-blight-tomato
http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/fungal-spots/septoria-leaf-spot-of-tomato.aspx
Early blight
Septoria leaf spot
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