Everyone saw this coming! Since 2013 Delta has been adding flights in Seattle (SEA) making it a new Delta west coast hub. Seattle is also a major Alaska hub and that direct competition would inevitably break the partnership.
Effective May 1, 2017, Delta frequent flyers will no longer be able to earn and redeem miles with Alaska and vice-versa. Other perks like free seat selection, upgrades and boarding priority will also go away.
For now, Alaska is keeping the partnership with American Airlines but we will be watching what will happen as Alaska completes the merge with Virgin America.
As a Delta medallion member, I will miss this partnership. I have enjoyed it a few times on flights between Washington DC and the West Coast.