This could be an inhalant allergy as it seems to be seasonal or it could be a reaction to insect bites including fleas or biting flies etc.
With an inhalant allergy the spots could be a secondary infection with yeast or staph due to the allergic response.
It might also be a contact allergy from sitting on grass or running through vegetation etc.
You can read about allergies here
http://www.lbah.com/allergy.htm
You should talk to your vet about possible use of an antihistamine with your dog and maybe an antibiotic shampoo such as a chlorhexaderm shampoo.
Hope this helps you!
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