Stumbled across this guide which identifies common hop troubles. Written for commercial hop growers but there might be of some use for the home grower.
http://www.usahops.org/userfiles/image/1457308044_HopFieldGuide3rdEdition-Full.pdf
http://www.usahops.org/userfiles/image/1457308044_HopFieldGuide3rdEdition-Full.pdf