Question
I have growing at the base of one of my trees up into the bark a few inchs and out into the lawn what seems to be fungus or mold, It is white approx. 5/8" thick kinda looks like califlower, it is spreading and covers about 6 sq. feet, all of it is connected with no open spots. The site is in the bay area of california, hot in the summer, damp and foggy in the winter, with ground staying wet. Type of grass is perinal rye and tall fescue mix, I'm not sure of type of tree (large leaves that drop every year) This problem really took off when the cooler weather and rains started.

Answer
Good Afternoon Robert:

That sounds like one big mushroom! I have a hunch there are thousands of mushrooms coming together making it look like one large one. The cauliflower texture is a good indication of that.
Since mushroom and puffball fungi grow on decaying organic matter, they are most likely to form on tree stumps, tree roots, or in other spots in the lawn with high organic debris. Although they will eventually disappear naturally after the organic food base is exhausted, this can take up to 10 years or more for a large stump or root. It would appear that the root zone of the tree is dying on one side providing food for the mushrooms. The cause could be a girdling root that has cut off nutrients to the tree. You might find that on the side of the tree where the mushrooms appear there is no flare at the base of the tree trunk. That would be a good indication of a girdling root. Because of the size of the tree, there is not much that can be done unless you want to dig out the girdling root and that would not garantee the tree will live over a few more years. Another cause of root rot would be lack of drainage in that particular area.
Look at Armillaria Root Rot below:
http://ppathw3.cals.cornell.edu/trees/Armillaria.html
http://www.extension.umn.edu/projects/yardandgarden/ygbriefs/p415armillaria.html
It is difficult to totally eradicate nuisance mushrooms. Adequate control usually can be accomplished by digging up and destroying rotting stumps, roots, or other underground sources of organic debris. In that instance you would have to replace the soil you dug out with new soil.
Although I have not tried this product, I have heard that it shows promise. Go to the web site:
http://www.galaxymall.com/product/landscaping/
WARNING: Do not eat any mushrooms or puffballs found in the lawn without first having them identified by a competent authority.
I hope I have been of some help to you. Have a good lawn!
Floyd McMahon