Most tomato gardeners have seen or experienced blossom-end rot on tomatoes. It appears at the base of the tomato…the portion that is fastest growing and forms an unattractive black decayed appearance. Also a fruit so affected generally ceases to grow or grows slowly.The following information applies to all tomato gardeners, whether growing in ground, containers, or straw bales.
Generally, it is the oblong Roma or paste tomatoes that are most often affected. I have never seen blossom end rot on a cherry tomato plant and I think it is because the fruits growth cycle is faster.
First it is a physiological problem not a disease. A lack of soil calcium or a defect in the plant’s utilization and uptake of calcium is considered the cause. A few controllable conditions lead to this.
A simple lack of calcium is a factor which can be remedied by adding lime, gypsum, bone meal or in small gardens even ground up eggshells to the soil before planting.
An excess of ammonium nitrate can be a problem…straight substantial amounts of chemical nitrogen which some old articles on straw bale gardening recommend. Don’t use it. An excess or imbalance of potassium or magnesium. All these minerals and nutrients are needed by tomatoes, just no one in excess so use a balanced fertilizer along with compost. Consider also applying foliar sprays of liquid seaweed , Maxicrop is a good product.
The third and perhaps most significant cause is fluctuations in watering. Plants which dry out are not absorbing nutrients or water. This is followed by a heavy watering. Cycle through this a few times and it sets the stage for blossom end rot. If your climate is somewhat humid this moisture problem may be compounded. Drip or trickle irrigation either with emitters or leaky pipe will maintain even and optimal moisture. With straw bales and containers I like to give plants a thorough soaking from time to time.
The more oblong Roam or San Marzano tomatoes are most susceptible to blossom end rot. If your plants are producing defective fruits consider picking everyone that looks as though it will have this condition. The fruits will likely have a very poor taste and by picking plants nearly clean new fruits will set.