Cracks in tomatoes, black rotten spots on the bottom of tomato fruit, and a hard yellow or white area on the inside walls of ripe tomatoes are all physiological problems, not caused by insects or disease. It is a sad sight for gardeners investing so much time and energy to see the actual fruits of their labor turn into less than perfect tomatoes.
Let’s start with why tomatoes crack. Higher moisture levels after a dry period, such as lots of rain after a time of drought, will cause the inside cells to swell and grow faster than the outside skin will grow, resulting in splitting of the skin. To prevent cracking, keep soil evenly moist by watering, and use a mulch to prevent evaporation and keep soil cooler. Cracked tomatoes are still very edible, but not so pretty. Sometimes the cracks are deep, allowing rot to happen inside the meat of the fruit. Plan to use split tomatoes before rotting happen.
Blossom end rot is expressed by a black, sunken area on the bottom, the blossom end, of the tomato. It is caused by a lack of calcium reaching the fruit. The soil could be lacking calcium which can only be determined by having a soil test done for nutrient levels. UConn does a basic soil test for $12.00 at soiltest.uconn.edu. New England is not usually lacking calcium in its soil, it is more likely the cause of blossom end rot is an interruption in the delivery of calcium from the soil to the fruit via water uptake. This is caused by irregular watering, letting the soil dry out, then watering or having a big rain event. Occasionally, high levels of potassium or magnesium fertilizers will compete with calcium uptake by the plants. Only use a balanced fertilizer to avoid an excess of individual nutrients and provide even water levels to the soil to avoid blossom end rot. Portions of the tomato not rotted are also still edible if you cut away the bad part.
Yellow shoulders disorder occurs on the top part of the tomato when areas never turn red, but stay yellow. The flesh underneath can be tough and corky. It can occur only on the top portion or can occur as a grey or white wall just under the skin around the whole fruit.This problem is caused by a number of different circumstances or combinations of them. We do know it is a problem at the cellular level that happens very early as the fruit is forming. Cells in the area are smaller and not aligned normally, and the green chlorophyll areas do not develop red pigment. Causes are thought to be high temperatures over 90 degrees F at time of fruit formation, and possible pH levels over 6.7, and potassium, magnesium and calcium competition among each other. Again, a balanced fertilizer is needed.
The take away message for all of these physiological problems are to have an adequate soil fertility and soil pH without over fertilizing, and have even soil moisture. Hope for summer temperatures to stay at or below 90 degrees F and your harvest baskets will be full of beautiful, delicious tomatoes.