You could solarize your soil, next year since the optimum time to do that is June through August. The link below takes you to a paper from UCDavis about solariazing.
The best thing to do now is have a good, reliable soil test done and using these simple soil tests,
1) Structure. From that soil sample put enough of the rest to make a 4 inch level in a clear 1 quart jar, with a tight fitting lid. Fill that jar with water and replace the lid, tightly. Shake the jar vigorously and then let it stand for 24 hours. Your soil will settle out according to soil particle size and weight. A good loam will have about 1-3/4 inch (about 45%) of sand on the bottom. about 1 inch (about 25%) of silt next, about 1 inch (25%) of clay above that, and about 1/4 inch (about 5%) of organic matter on the top.

2) Drainage. Dig a hole 1 foot square and 1 foot deep and fill that with water. After that water drains away refill the hole with more water and time how long it takes that to drain away. Anything less than 2 hours and your soil drains too quickly and needs more organic matter to slow that drainage down. Anything over 6 hours and the soil drains too slowly and needs lots of organic matter to speed it up.

3) Tilth. Take a handful of your slightly damp soil and squeeze it tightly. When the pressure is released the soil should hold together in that clump, but when poked with a finger that clump should fall apart.

4) Smell. What does your soil smell like? A pleasant, rich earthy odor? Putrid, offensive, repugnant odor? The more organic matter in your soil the more active the soil bacteria will be and the nicer you soil will smell.

5) Life. How many earthworms per shovel full were there? 5 or more indicates a pretty healthy soil. Fewer than 5, according to the Natural Resources Conservation Service, indicates a soil that is not healthy.
and using them as a guide to determine what your soil needs to become a good, healthy soil that will grow strong and healthy plants that are less susceptible to diseases.

Here is a link that might be useful: Soil Solarization

Kimmrs advice sounds good. I tried solarization once to kill weeds and failed miserably but others seem to make it work. A lot of the organic gardening books and articles I read suggest innoculating your soil with lots of good micro beasties via compost and tea, etc. to help combat the diseases in addition to crop rotation for food plants and annuals. Here in the east it has been a bad year for tomatoes with some sort of disease running around. Hope you don't have the problem that is affecting them here where you are. A change in varieties might also help. Have any conditions significantly changed over the last few years like light, moisture, etc.?

I have similar problems fleemo and after a dismal tomato year I am considering solarization. It just gets worse every year. I do have more shade than I used to, but even the tomatoes in the sunny part of the garden grew slowly and have terrible blight. The taters get blight and croak in July. Don't even get me started on the vine crops.

Thanks for the link kimmsr.