It is one hundred percent NOT powdery mildew, nor can I think of any other fungal disorder that this resembles.

Powdery Mildew appears as a whiteish dust on the leafs surface and it can be wiped off. Those leaves are demonstrating Chlorosis, yellowing, for some reason and that is mostly the result of a nutrient imbalance. Some may tell you this is the result of a Nitrogen or Iron deficiency which might possibly be or it might be because of a wrong soil pH that is preventing proper utilization of some nutrtient or it may be that there is too much of something that keeps the plant from properly utilizing some other nutrient.
Excess Phosphorus in the soil can keep a plant from utilizing Zinc, Iron, and Copper while excess Potash can inhibit utilization of Nitrogen. Excess Magnesium can inhibit utilization og Calcium and vice versa, and both are needed for a plant to produce Chlorofil which is what makes leaves green.
What is your soils pH?
What is the balance of Ca to MG?
How much organic matter is in that soil?
How well does that soil retain moisture?
How well does that soil drain?
What kind of life is in that soil?

i guess i need to get out the soil test kit i have never used. soil drains well, raised bed also housing Derby beans, California Wonder sweet peppers, and some marigolds. everything else is thriving with nice, green growth and large fruit. thanks for your info, tho. i will get on it today. what would be a good fertilizer for this? is the Miracle-Gro insufficient to correct this?

as long as it has all the macro and micro nutrients in it. new growth is iron problem and old growth is nitrogen problem.

nitrogen and phosphorus are low, pH is a tad high. i have sulfur i can use for the pH but probably won't do that until the fall. anything i can use to raise the nitrogen and phosphorus immediately?

Chlorosis appears when there are any one of several nutrient problems.
Nitrogen - Lower leaves yellow, overall plant light green, growth stunted.
Potassium - Tips and edges of leaves yellow, then brown, stems weak.
Magnesium - Interveinal chlorosis, growth stunted.
Zinc - Interveinal chlorosis, leaves thickened, growth stunted.
Iron - Interveinal chlorosisl, growth stunted.
Sulfur - Young leaves light green overall, growth stunted.
Boron - Young leaves pale green at base and twisted. buds die.
Copper - Young leaves pale and wilted with brown tips.
Manganese - Intervinal chlorosis on young leaves with brown spots scattered through the leaf.
Molybdenum - Interveinal chlorosis, growth stunted.
From the Rodale Encylopedia of organic gardening.

As long as your plant is producing a good crop and the fruits are normal and the new growth is green and the white leaves are on the lowest part of the plant, they may just be senile, old leaves.

the use of senile cracks me up ..

i would call them overly mature.. and the plant is shedding them .. it happens all the time on a lot of annuals.. the oldest leaves are just 'spent' ...

ken

Senile has become a perjorative word. IOW it originally mean 'overly mature', but got stolen by masses to refer to dementia. In zoology and biology and especially medicine referring to something as senile has nothing to do with its mental status, but that it has progressed past its prime.

Helpful list, Kim, and interesting factoid, Calliope.