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Hops are susceptible to a number of diseases and based on the timing of your problem and the description of disease problems here: http://ipm.wsu.edu/field/pdf/HopHandbook2009.pdf, the cause might be downy mildew, though I'd need more information to be sure.  Are the spots on the leaves contained within the veins?  Are the leaves cupped up or down?  Is this this first year that you have had the problem?

Also, in terms of the plant's environment, is it in full sun?  A shadier spot will be more conducive to fungal growth. 

Please take a look at other potential culprits in the link that I sent and answer the questions posed above, and you can likely find the culprit of the problem.

Please let me know if you need any further questions.

Weston