Thank you for the question. Many gardeners this year are noticing problems with their tomatoes and one of the diseases that can cause quick death of the plant and tomato fruit is late blight, a fungal disease of particular importance during wet, cooler than normal weather. It is very important to bury or throw away diseased plant parts in the garbage. Next year, plant tomatoes in a different location. Fungicides can be used prior to infection when weather conditions that favor the disease are present. You can't use these now because your plants are already infected.
These publications provide more information:
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