Thanks for your question.

The fact these changes occurred in your raspberries “a few weeks ago” leads us to consider that they are experiencing sun scorch, also known as sun scald.  This can occur during extremely high temperatures and drying winds.  These conditions increase the rate at which water is lost from the leaves – a process termed transpiration.  This excessive water loss from leaves leads to curling on the leaf’s edges.  It also contributes to a loss of chlorophyll which results in the yellowing and browning that you are observing.  The leaves most susceptible are the lowermost leaves, which is the case for your plants.  The uppermost leaves are younger and generally not as affected by sun scorch.  Thankfully this is largely a cosmetic effect and has no long lasting effect on the plant. These bleached leaves will not recover.  Just remove and discard them.  You need to keep watering the raspberries at their base.  Do this as early in the morning as possible.  See:

https://extension.unl.edu/statewide/dodge/leaf-scorch/

https://extension.umn.edu/yard-and-garden-news/sunburn-raspberries-apples-and-grapes

http://www.thedrummer.com/drummer/ground/sunburn-raspberries-apples-and-grapes

https://blogs.cornell.edu/berrytool/raspberries/raspberries-leaves-are-yellowing-or-bleaching/

Also take a look at the following site describing various diseases that can infect raspberries.  Perhaps you will see there some symptoms that are not easily seen in your pictures.  Note that several of these diseases affect the canes.

https://www.alberta.ca/diseases-of-raspberries.aspx

If your raspberries develop this yellowing over the entire plant, please get back to us and we will revisit things.

Good Luck!!