|My plum tomatoes are brown-black on the bottom end, on both green and ripe fruit. What is causing this problem? My other types of tomatoes seem okay.|
|Your tomatoes have "blossom end rot" (BER) a physiological condition caused by a lack of calcium at the growing tip of the fruit. While your soil may have adequate calcium, fluctuations in soil moisture content from dry to wet really increase the incidence of BER. It is especially bad on the early fruit each summer and in sandy soils.
The damage occurs as cells die at the tip of the fruit. In time (and as the fruit grows) the spots enlarge and turn black. So, by the time you see it, the damage actually has already occurred some time back.
Remedies for BER include: having a soil test to make sure calcium levels are adequate, adding organic matter to a sandy soil to increase its moisture holding capacity, keeping plants evenly moist, especially during the development of the first fruits (mulch helps maintain soil moisture), and spraying plants with a Blossom End Rot spray (contains calcium) which can usually be purchased from your local garden center.
Don't wait until you see it to spray with Blossom End Rot spray. If you have an annual problem with BER treat next time once fruit reach marble size. However, usually the other cultural practices will control the problem without the need for spraying.