August is supposed to be the month of non-stop tomatoes. Occasionally things go awry to interrupt those carefully laid spring visions of bountiful harvests, sauce making, and endless tomato sandwiches. Blossom end rot can appear to put an end to the crop production by damaging the ripening and developing fruits. We are seeing and receiving calls in a higher number than more recent years from backyard gardeners complaining about black rotten spots on the bottom of their tomatoes. The spots start as a thickened, leathery spot which sinks in, always on the bottom of the fruit.
Blossom end rot can also occur on peppers.
Blossom end rot is a physiological condition due to lack of calcium. Calcium is needed by plants for proper growth in all functions of cell making, but is most important for cell walls. Without enough calcium either in the soil, or if delivery of uptake of dissolved calcium in soil water is interrupted, cell division stops in the fruit. Tomatoes are especially sensitive to a lack of calcium.
Interruptions in uptake of calcium can happen by repeated cycles of soil drying out, receiving water, then drying out again. Times of drought and hot, humid weather make the problem worse. Plants lose water through their leaves through a process called transpiration, similar to the way we sweat. They then pull up water through their roots. If there is not enough soil moisture, plants wilt. This break is water delivery also limits calcium delivery. Tomato, and to a lesser degree pepper fruits, respond by developing rot on the bottom, the end where the blossom was before the fruit started growing.
High humidity and multiple cloudy days reduces transpiration, thereby reducing water uptake. This leaves plants not able to bring up new calcium rich water to the site making new cells of the fruit. Another interruption of delivery of calcium resulting in blossom end rot. This means that even if you have enough calcium in the soil and you water the soil regularly, the plants still may not be able to move enough calcium to where it is needed to produce a fruit.
Have a soil test done to make sure soil has enough calcium and that pH levels are around 6.5 so nutrients are most readily available. Water regularly so plants receive 1 to 2 inches of water per week for optimum growth. Feel the soil around the root zone to make sure water is soaking in and reaching the roots. Humidity and cloud cover are not obstacles we can help the plant with, so monitor the fruit for rot spots and remove. There are calcium foliar sprays which claim to deliver calcium to be absorbed by the leaves for use by the plant. This won’t help after the rot has already developed, but may help deter future spots on still developing tomatoes.