Answer: Blossom end rot can occur when soil moisture is inconsistent which binds up calcium in the soil and makes it unavailable to the plant. While the sprays can help, a better approach is to plant your tomatoes in well amended soil and water them deeply on a regular basis. The dark rings you're finding on the leaves indicate a fungal disease called late blight. It may be too late to save your tomato plants this year so next year I would recommend planting disease resistant varieties in a different spot in the garden, one that you've amended with organic matter to help it drain quickly yet hold just the right amount of moisture. You can begin preparing the new bed now by spreading 4-5" of compost or aged manure over the top and digging it in to a depth of 8-10". Level the bed, then spread another 2-3" of organic matter over the top, to help suppress weeds. Next spring, when it's time to plant your disease resistant tomatoes, you can dig in the organic mulch, plant your seeds or transplants, then mulch over the bare soil with an additional 2-3" of organic matter. Next year your tomatoes should produce a bountiful harvest.
Best wishes with your tomato plants!
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