The other thing to check is your pH level 6.5 to 7.0 is a good range, if it is outside of this the calcium intake is affected. To be honest once you have this problem it can be difficult to remedy it. Some people suggest adding crushed eggshells into the compost might help but your best bet is probably stabilizing the water level and making sure the plants are not water logged as originally suggested.

If you have time you could water up to 3 times a day. Just remember not to water over the leaves, put the water in at the bottom of the plant and make sure they are not completely saturated. Tomatoes like to be moist but the roots still need to breathe to take in nutrients. You should get a feel for what is the right amount of water, if the soil is dry its not enough and if you can see water at the surface after a couple of minutes when you have watered then that is too much.

You can treat this with a fungicide sometimes, but I would be inclined to pull the plant out, bag it an bin it if your other plants are OK. If they are all the same then try the fungicide and see if they recover. Don’t forget to get rid of the plants at the end of the season properly though, i.e. don’t be tempted to put them into the compost or anywhere the fungal infection may be able to affect next years crops. Bag and bin as suggested or burn them.