Question
have a very large pin oak in southeast Kansas that seems to have several limbs dying and leaves turning brown.....i have also noticed round gray patches on the limbs....it has been very dry here for the last 2 yrs....

Answer
This is a disease called hypoxylon canker.

Hypoxylon cankers are prevalent and highly visible disease problems affecting oaks and other hardwoods, Hypoxylon spp. are not considered aggressive killers. Instead, they are usually secondary in that they take advantage of trees suffering from any number of injuries of stresses. Hypoxylon cankers are often the finishing blow to oaks suffering from water stress, root disease, soil compaction, construction damage or other, related injuries.

Hypoxylon infections originate when ascopores of the various pathogens come into contact with injured or severely stressed tissues of susceptible hosts during the late summer, fall or winter. Spores are spread via wind and splashing rain and presumably by certain insects, birds, and rodents. The fungus develops in the bark and wood tissues and the following spring or summer the bark is sloughed, revealing the typical powdery masses of conidia. Conidia apparently play a limited (if any) role in initiating new infections, but perhaps perform some type of sexual function which results in the production of the infective ascospores later in the summer or fall.  
Control: Trees with extensive Hypoxylon infections are usually beyond repair. Removal of severely infected trees to reduce local sources of inoculum (i.e., infectious spores) is recommended. Careful pruning of branches that have localized infections should help prevent advancement of the fungus within the infected trees. Prevention of infections through avoiding wounds, root damage, etc., and providing adequate moisture via irrigation to susceptible trees during prolonged periods of dry weather is the best method of control. The presence of Hypoxylon cankers is usually an indicator of severe stress and often a warning to take precautions to reduce stresses (if they can be identified and reduced) affecting nearby trees of the same species.

I would try fertilizing the tree with 1 lb of fertilizer per inch of trunk diameter scattered around the tree and watered in good. Apply just before a rain storm and you will not need to water. Apply the fertilizer now and again in the spring. This should increase the overall health of the tree. and help the tree fight off the disease.

The canker will not spread to healthy trees so keeping the trees healthy is the key.