Is this perennial ryegrass? I am not sure any seed variety claims resistance to pythium...but...a top quality perennial rye, that claims in general "disease resistant" is a good idea. Be sure to get a variety that is listed among the top ten for New Jersey in the NTEP tests. Be sure to get a newer variety that was released recently--say nothing earlier than 2008.
Cooler weather--and without the excess moisture is my only other suggestion. Probably mid-September when the temps come down below 80.