10nl 6max: Obvious straight completes on river, easy fold?

[a nonymous]

$0.10 BB (6 handed)

Hero ($10)
BB ($9.85)
UTG ($9.75)
MP ($8.55)
CO ($10.35)
Button ($2.35)

Preflop: Hero is SB with 8[img]/images/graemlins/heart.gif[/img], 6[img]/images/graemlins/heart.gif[/img].
<font color="#666666">1 fold</font>, MP calls $0.10, <font color="#666666">2 folds</font>, Hero completes, BB checks.

Flop: ($0.30) 6[img]/images/graemlins/diamond.gif[/img], 8[img]/images/graemlins/spade.gif[/img], 9[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(3 players)</font>
<font color="#CC3333">Hero bets $0.2</font>, BB folds, MP calls $0.20.

Turn: ($0.70) J[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets $0.5</font>, MP calls $0.50.

River: ($1.70) 5[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">MP bets $1.7</font>, Hero ?

I couldn't figure out what I could beat really. At the same time it didn't really add up for him to bet pot there with anything but a 7 therefore I thought air was the other possibility but I had no read suggesting he would float flop and turn with air (Maybe float flop with a hand with a ten in it or some hand with two diamonds and then the turn isn't a float? Doesn't seem particularly likely, but his river bet could def be a missed draw IF he floats flop or if I'm ignoring some hands that would make sense to call with on the flop that would pick up draws on the turn.) and if he had a non-7-hand he thought was best I don't think he'd bet this river I think he'd just check it.

Is this a really easy fold?