Hey mate your fastberry auto is running out of nitrogen and the plant is using nitrogen from those leaves, it's not too much of problem. In the ground I get around this by putting large amounts of manure (not always practical but in wild Wales it's a simple thing) if I'm growing in pots I either use a compost with added slow release fertiliser such as miracle grow or westland with 4 months feed in the compost or use a liquid fertiliser called liquid growmore (at a quarter strength) as well as bloom nutes, I find this way the plant stays healthy for longer into flowering,, happy growing