Growing tomatoes, peppers, squash or melon is sometimes frustrating when all the new fruit you are anxiously anticipating rots. Blossom end rot is not a disease, rather a physiological disorder in the plant. It is most often seen when conditions in temperature and soil moisture change rapidly, drought stress, root injury and excessive Nitrogen fertilization. The plant gets confused and cannot move calcium in the plant to the right places. This sometimes occurs even when growing conditions are favorable. However, the condition corrects itself in a week or two and further fruit development is unaffected. For more information about Blossom End Rot, here’s a link from K-State Research and Extension.
https://hnr.k-state.edu/extension/info-center/common-pest-problems/common-pest-problem-new/Blossom-end%20Rot.pdf
https://hnr.k-state.edu/extension/info-center/common-pest-problems/common-pest-problem-new/Blossom-end%20Rot.pdf