James wrote:
Try not growing the same plants in the same spot year after year.
Disease and insect population survive and multiply year after year
specially when you plant their food of choice there all the time.

Try solarizing the garden by covering it a few months with clear
plastic sheet. This will kill many of the pathogens in the soil.

Use another patch of ground as it may not be infected.

Plant varieties resistant to insects and diseases. For example I tried
husky gold tomatoes for a couple years and all the fruits had big fat
juice worms. Different vrieties next to them were worm free.

Thanks, I dug a new garden as soon as I read your advice.

Now that you pointed me in the right direction, I think nematodes are a
good explanation. Each year as my first tomatoes get big, the nematode
population reaches the point where it takes the vitality from the plants.