After about a week of cooler, rainy weather, I noticed today in the mulch around my potted lemon tree, there is a white powdery- looking substance. It is even starting to growi up a wood brace. Please help. This is the first year out of approximately an 8 or 9 year old tree that has a bumper crop of lemons. Maybe 30 lemons or more. Help me to save the lemons and tree please!

It appears you have a powdery mildew which is a fungus. Remove as much as possible and treat the area with baking soda mixed with horticultural oil obtained at your garden centre. Not being able to see what you have, this is the nearest diagnosis I can come up with. You could scrape some up and take it to your nearest garden centre to see what they decide.