A thin dark line connecting the stem and blossom ends develops on fruit. It is caused by poor pollination due to cold temperatures. To avoid it, grow resistant varieties, and cover young plants with floating row covers. Just the opposite of blotchy ripening, prevent it by keeping plants leafy and fruits shaded. Both are caused by fungi.
Fusarium wilt causes lower leaves to yellow and die, ultimately killing the plant.
Verticillium wilt is less lethal. Plants often show no signs until they are bearing heavily, then the margins of older leaves begin to yellow and die. Fungi are spread by infected soil, debris and seeds. To avoid wilt, plant varieties that are resistant to these diseases. There should be a V, and F or both on the plant label.