Cannas are tropical plants, you live in the desert. I think that's the big issue. They like rich dark compost to grow in, lots of humidity, moisture & lots of water - about an inch per week. 108F is extremely hot & Arizona is very arid. Not many plants can cope.

thanks for putting me out of my misery Bathgate. Couldn't work out what Az meant. Geography is not my strong point in the UK never mind the USA.

As said its is the lack of water especially humidity around the leaves. how you solve it depends if you can group your cannas together and sit them on a tray of damp gravel to encourage high humidity.

I live in Phoenix, AZ, and I've found that Cannas do much better here with sun in the morning, and shade in the afternoon. They are also originally marsh plants, and do much better if they get plenty of water: 2-3 times a day in containers, or 2-3 times a week in the ground. Because of the frequent watering they need, they also need regular feeding to keep new flowering stems coming. A 3-1-2 formula has worked best for me.
For growing plants in general in the heat, here are some tips:
Water deeply and not too often. The more deeply you can train the roots to grow, the less they will suffer from the heat.
On most non-native plants, a deep, coarse, organic mulch is very helpful. It helps keep the soil temperature and moisture more even, conserves water, and gradually improves the soil--a big bonus here in the desert.

your answer Tug is much better than mine. in most parts of the uk canna's tend to be grown as container plants.