If you've had a bad growing season because of any number of pests or diseases, you can turn back the clock on your soil by organically sterilizing it. Use the sun to "cook" your soil into shape. Here's how:
Measure the area that you need to sterilize and cover it with plastic sheeting. Either thin or heavy plastic will do. This doesn't need to be gardening plastic, or an expensive investment, painter's drop cloths wrapped and taped together will do just fine.
The goal is to heat the soil enough to kill any destructive organisms and seeds living in it. A sunny spot is best because it will retain the most heat. Repeated high interior temperatures (under the plastic) of between 98 and 135 degrees F for a sustained period will do the trick. The soil will heat better if it is moistened before applying the plastic.
After application, the plastic should be sealed as thoroughly as possible to the soil, leaving few if any gaps. Bricks, pots, stones, and wood all work well to anchor and seal the plastic as close to the ground as possible. Leave the plastic in place for at least a month, two if you can swing it, the longer the better. I know that this means most of a growing season in many areas.
The advantage of this method is that it doesn't require chemicals. My experience has been that once a spot starts exhibiting problems, chemical solutions don't always provide a permanent fix. One fallow season with a sterilizing sheet works wonders.
If this means that your vegetable patch is a no-go this year, try gardening in pots.
Container gardening on a deck or patio is an interesting experience; one not to be missed.