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This article will give you the information that you need: https://www.gardeningknowhow.com/edible/fruits/raspberry/prune-summer-raspberry-bushes.htm

This does describe powdery mildew! Luckily this can be treated, and doesn't really pose much of a risk to the health of the plant. This article will help: https://www.gardeningknowhow.com/plant-problems/disease/get-the-cure-for-powdery-mildew.htm

Pruning will depend on the variety and when it fruits. This article will help you to determine when to prune your raspberry: https://www.gardeningknowhow.com/edible/fruits/raspberry/pruning-raspberry-bushes.htm

Yes, as long as you follow the pruning guide for whether the specific type is summer bearing or ever-bearing. This article will help: https://www.gardeningknowhow.com/edible/fruits/raspberry/care-of-raspberry-plants.htm

Is this the first year? Autumn Bliss grows vegetatively the first year, and produces fruit thereafter. Once a cane has producted fruit, it can be pruned out. Here is more:

https://www.gardeningknowhow.com/edible/fruits/raspberry/care-of-raspberry-plants.htm
https://www.gardeningknowhow.com/edible/fruits/raspberry/pruning-raspberry-bushes.htm

It sounds like they may have received too much water. These are drought tolerant and not very disease resistant. This can cause a plethora of diseases that will show no other symptoms other that that it will never produce fruit. These diseases are incurable and the only method for control is prevention.

Once it is established in the soil, you will have to plant new plants elsewhere, although it is likely that the infection is in the entire area.

This article will help: https://www.gardeningknowhow.com/edible/fruits/blackberries/blackberries-not-fruiting.htm