Maybe it's slugs or aphids or whiteflies make leaves turn yellow and slugs love to eat holes in leaves . You could try sluggo never worked for me though. lightning bugs eat slugs and you could buy Diatomaceous Earth or copper barriers here's a link Here is a link that might be useful: slug control

Too much water, perhaps? Salvia don't like wet feet. They need good drainage. Water only when the top couple of inches of soil feels dry.( test by sticking a finger into the soil around the base of the plants) They need plenty of sunlight, too.

The best wood conditioner in my experience is what is called a one pound cut of blond(white) shellac---that is one pound of shellac flakes dissolved in one gallon of denatured alcohol and then dewaxed(let stang so the wax settles out and is removedI have tried ready made conditioners and found them severly lacking. Minwax is a nationally known company and I use many of their products, but!!!!----most of their recommendations are generic and do not cover individual cases---like your chair. Conditioners are designed to limit the stains ability to penetrate wood. If a variety of wood does not usually require a conditioner---oak, for instance---using one can be detrimental. Basically, to only woods I use a conditioner with are pine/spruce/fir, maple and cherry. Some varieties of mahoghany might need it, but I have not stained woods other than maple, the soft wood family, cherry, oak, ash, and walnut. Sanding too fine----over 150 grit----can also create blotching by burnishing the harder parts of the wood. Try sanding the chair by hand with 150 grit sandpaper and reapply the stain on the underside of the seat---that should indicate whether you have done any good.

Not leaf miner; miners work inside the leaf. This is the result of something chewing the top surface of the leaf, allowing the sun to bleach the inside.