What causes a powdery gray sediment in the toilet sink and tub of one bathroom but nowhere else in an older house?
It's probably the original bath and the steel pipes are corroded and leaving mineral deposits in that bath. Other pipes may have been replaced.
Amorphous sediment in urine are things like crystal phosphates, uric acid stones or calcium stones that are found in the urine. Certain amorphous sediment found in urine is normal. The cause of amorphous sediment depends on the type of sediment. For instance, uric acid stones can be caused from not drinking enough liquids.
What disease causes white powdery dust to appear on leaves and the lower leaves and stem to turn grayish?
Powdery mildew is the disease which causes powder-white dust to appear on leaves and lower leaves and stems to turn grey. The disease in question reflects the work of fungi within the Erysiphales order. Its control and prevention generally require concerted efforts with fungicides, resistant varieties, sanitation, and ventilation.
There is no current research showing that incinerating the powdery mildew spore and inhaling the smoke from the combusted material causes health problems. It's completely possible that once the spore has been incinerated it dies and will not have any role in affecting your biological system. This being said, it's still mold and you're smoking a living creature.