Sounds to me like Blight. Have you removed the bottom stems from the stalk? I always remove them so the plant
can get some air circulation. If they need calcium, crush up some egg shells fine and mix with used coffee grounds and sprinkle around the plants
then water. Not too close to the stems, you really want this in the soil. I always use this mixture in my area where I am going to plant
tomatoes. The "blight" if that is what is is will go away if the bottom leaves are removed.
As far as the leaves curling, do you notice that before they are watered, because I have found if you watch your plants
, they will tell you what they need. Curling to me means it needs water.
Have you inspected the leaves, under the leaves, to make sure there are no insects? Lot's of questions. But I could mean that they need water twice a day if it's that hot and if they are in containers. Container plants
require more water because they dry out faster.
Hope some of this helps. But do try the mixture of used coffee grounds and egg shells, always works for tomato plants