Symptoms can occur at any stage of development in the tomato. But it seems most common, when the tomato is one-third or half of its full size. The name sorta says it all, it starts only at the blossom end of the tomato. Starting with small water-soaked spots, which get bigger then turns dark quickly as the fruits develop. The spot may get big enough to cover one third or half of the entire tomato's surface! Basically at that point the fight is over, and that tomato is a goner. I usually pick them and toss them away.
If you have seen this tomato sickness, if you have it or had it last year. You know that it usually does not just effect 1 tomato on a plant. It was probably in the majority of your entire crop, and up to 90% of the tomatoes on each plant. The severity can vary, from very bad to maybe just a few bad tomatoes.
I personally have seen them both. I started out several years ago with just a small problem, but over the years it grew to almost taking my entire crop. That is when I decided to learn all about this disease. And I found a simple solution.
Now I could go over the long drawn out reasons of why, and all of that jazz. But it is just a lot of long words and yada yada. Simply said here is "easy" explanation, which probably wont do the disease justice. Basically the tomato plant is having a hard time getting adequate water and nutrition to the plant. Part of that could be due to clay soil. Soil can always be the culprit. Sandy soils will have the same effect. Could be periods of drought, then lots of water. Could be to much water, or inconsistent watering. But for some reason your plant isn't able to grow a good root system, thus making the plants susteptiable to disease.
If you have clay soil can I suggest you add lots of organic material? Clay soil is a common problem in vegy gardens. Clay soil just isn't great for growing, even weeds sometimes. The same can be said of Sandy soil. So add lots and Lots of organic material. Every Year. But that may only just start to solve this problem of Blossom end rot. Although I would hope it would end the problem.
Next make sure that you are consistent in watering. Make a schedule and stick to it. Now, I am not suggesting that you become a Nazi. Just water every other day on a drip system for 15 minutes, simple. In the real heat of the summer, I water every day. But I know lots of people that don't. Just watch your plants, don't let them wilt. (If they are wilting, re-evauate your watering entirely) Other watering suggestions for tomatoes and most all garden plants is, don't water over head. Meaning turn off the rain bird. I will explain more on that in my next post.
If you have taken care of everything else, meaning. Your soil is in good shape, watering is handled and you are taking the opportunity to possibly fertilize with a simple 4-12-4 or 5-20-5 fertilizer, every other week but still have a problem. Add lime. Simple powder lime from the garden department should end this problem entirely. Lime isn't bad for the body (through tomatoes) is organic and just promotes healthy tomatoes. Every year I add 1 tablespoon to the hole that I put my new tomato starts into. I would rather not bother worrying about whether blossom end rot will happen at all.
Several years ago, when I had blossom end rot for the last time. I added lime to the top of the soil next to the tomato, (because I didn't at the beginning of the year) then worked it in around the tomato. Watered as usual. I also plucked all the effected tomatoes off and chucked them. I ended up with a substantially smaller crop because of what I had lost. But everything started to grow 100% rot free after that. I cant promise that will be your experience, but if you remember: Soil, Water, Fertilizer, Lime I think you will have amazing tomatoes every year!