Answer: If you're growing tomatoes now, in late June, the heat is what is causing problems with your plants. When temperatures are over 90F plants stop growing and pollen becomes sterile so any flowers that are opening won't produce fruit.
In the southern deserts of Arizona, we have two growing seasons. Unlike other areas, neither of these are during the hot summer. The first one starts in February (if we're lucky, or March if not), and runs into early June. The second can start in September, and runs until the cold gets here.
These two growing seasons are short. Because our seasons are short, we have the best luck with varieties that were developed for areas like the US/Canadian border. My favorite slicing tomato is the Early Girl. My favorite cherry is the Sweet 100. Another slicing variety that seems to work well is the Champion.
All of these varieties are indeterminate, meaning that they are plants that grow very tall, vine-like, and need to be supported. I've had poor luck growing the bush-like determinate varieties like Big Boy. Your mileage may vary, but I don't waste space on the bushes. I find the climbing varieties to produce far more fruit than the others, though the fruit is smaller than that of a Better Boy, etc.
So, left to my own resources, I plant lots of Early Girl and a few Sweet 100. If you're a compulsive cherry tomato nibbler, you may wish to plant more Sweet 100, but each plant is capable of producing hundreds of cherries.
The only advice I can offer is to plan on growing your tomatoes from February through May and again September through December.
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