Sorry, I was just a little busy laughing because I once hear the lead story on an Atlanta television news station say, during a drought, "The National Weather Service has predicted a 50% chance of above average rainfall in the next 30 days." I just laughed and laughed. (Okay, it's not the Median, but it's close enough for rainfall and for harvests.)
Can you build a tree?
Disease & Bad ==> .3*.8 = 0.24
Disease & Good ==> .3*.2 = 0.06
No Disease & Bad ==> .7*.5 = 0.35
No Disease & Good ==> .7*.5 = 0.35
"Find the probability that, when the harvest is observed to be poorer than average, the disease D is present."
(Disease&Bad)/((Disease&Bad)+(NoDisease&Bad)) = 0.24/(0.24+0.35)