Matthew Debacco has created an excellent guide with pictures to help identify pumpkin plant problems.  I have posted his guide here:  Pumpkin diseases and plant problems

 

Information about pumpkin diseases can be found here:

Pumpkin Disease Information
Additional Pumpkin Disease Information

Insects that do damage:

The cucumber beetle is my number one enemy each year.  You can find information about this insect and how to control it here:

http://vegetablemdonline.ppath.cornell.edu/factsheets/Cucurbit_Beetles.htm
http://www.attra.org/attra-pub/cucumberbeetle.html
http://www.bhg.com/gardening/pests/insects-diseases-weeds/stop-cucumber-beetles/

Information about the vine borer can be found here:

http://www.attra.org/attra-pub/squash_pest.html