Black currant discolouration

Both of my black currant bushes seem to do the interveinal chlorosis thing. This is a pretty recent development - they were planted 2 months ago, did great. Then, a couple of weeks ago, I noticed the colour. They're planted at the edge of a regrowth forest. Pretty shady, with lots of other plants around. Everyone else is green and happy, except for these two... Soil is mainly clay. Any ideas?

Anyone? Help?

I'll try to add some potassium and a bit of mycelium-filled wood chips, but would love to have some solid ideas for what the heck is wrong with these guys...