I have a beautiful and valuable 400-600 year old white oak tree in my backyard. I planted grass underneath that is growing beautifully even with the speckled sun. But someone told me that by watering the grass, I can activate an "oak tree fungus" that could kill my tree in 10-20 years. Is there any truth to that?
I'd probably post your question over on the Beginner gardening or Beginner Landscaping forum though rather than here, more people will see it there (this forum is for discussing the Landscaping.com website and not a ton of people make it over here)
That being said, I know that things like the coast live oaks are very sensitive to moisture and really shouldn't get much (if any) water in the summer for just that reason, but I'm not sure that white oaks are native to CA so they may have more tolerance for moisture if they're native to somewhere that gets summer rain. But even if it has some tolerance for summer water, being in a lawn is probably not the best situation for it since the lawn undoubtedly has higher water needs than the tree. One of two things will likely happen (or maybe both)...either the tree will wind up with too much water and have problems, or often grass has a hard time really getting going under trees because it's competing with the tree for water and nutrients, so it could well be a lose-lose situation.