I hope one of my guesses gives you a helpful idea !
>> How do I go about sterilizing with chlorine bleach?
I meant that cleaning pots only removes most of the soil, so many people soak them in bleach after cleaning as far as practical.
The recipe I heard was to soak pots for 10 minutes in 1 part supermarket "chlorine" bleach plus nine parts water.
But I don't know what concentration that assumes about supermarket bleach. Look for "hypochlorite" or "chlorine bleach", not "oxygen bleach". The newer "oxygen bleach" doesn't kill microbes as well.
>> Doesn't it alter the ph?
I would use chlorine bleach to sterilize pots or tools but definitely not on soil.
I think the only way to keep a bed relatively free from cumulative soil diseases is to use rotations or discover what soil conditions favor that disease and somehow change the conditions. Improved sandy soil sounds like fast drainage and probably good aeration.
I was wondering about ocean salt water getting into your water table. Like, in the soil near the roots, since you wouldn't taste it there. Experimenting with town water might show something, especially in pots or deep raised beds.
A plant disease like blight sounds likely. I guess long rotations wouldn't help much if spores blow on the wind.
It might have gotten worse suddenly just in your yard or throughout your neighborhood, or county-wide. Asking some neighbors if they used to be able to grow tomatoes might turn something up.
Your local cooperative extension office probably has websites that would discuss your regions' most common diseases of each crop. (There's a link to a coop-finder site in my signature block.)
