Dear Olefogey, from the pix of the tomato leaves it looks like you have airborne fungal diseases that require dew or rain to infect the plants. These diseases build up rapidly in wet and humid weather and cause dark leaf spots followed by yellowing and defoliation (leaf drop). They may also produce spots on the fruit.

Cut off the affected foliage and put in the trash can, not the compost pile. Unfortunately these fungal infections are difficult to control once established. This fall, pull up and destroy any remaining vines. Because this fungus can over winter in the soil, rotate your tomato plantings every year (plant in the same place only once every 4 years). Mulch the base of the plants with 1-2 inches of straw, newspaper or other organic materials and water the plants from the bottom. Consider spacing the plants farther apart to increase air circulation and use a fungicide as needed.

You have the same issue with your squash plants -- fungusamongus! Same recs apply.

You may be fighting a losing battle if the plants are in an area with little air movement to help the leaves dry after they get wet. I had the same problem with a potted standard rose in a western exposure located left of my front door next the the wall of my house. There was very little air movement there and I got the white fungus on the leaves which caused them to drop. First I moved the plant to a breezier location then I sprayed with a fungicide.

I found the fungicide in my refrigerator and it worked very well. I used 1 cup of skim milk mixed in a gallon of water and throughly sprayed the tops and bottoms of the leaves with it for 3 days. You can drop it to 1/2 cup of milk and still get the same results. Apparently milk has anti-fungal properties. Whole milk can be used too, but the fat in it will go rancid and cause your garden to smell like sour milk. I did not notice this issue with skim milk. Milk also has calcium that the plants will use.

If you are unsure, Google milk fungicide and you will see it discussed on a few web sites. I can't put my finger on the scientific study that was done in regards to this use of milk in the garden, but it has been proven to be effective. Give it a try, it will not hurt anything and you might be satisfied with the results.