Photo?

Sorry for some reason I was having issues attaching in OP

Yes that is damp-off and while it is most common in young plants, it can develop at any stage of growth. But no it does not mean the "entire garden area is infected and unable to take tomato plants". The various fungi that can cause damp-off are common in every soil and even in the air around us. But they require the right conditions to go active - overly wet, poorly draining soil. Those factors are what you need to address.

Dave

Sure looks like it. The shrinkage of the stem into something almost threadlike is the evidence. You might apply fungicide, and avoid keeping the soil surface moist for long periods of time. This is a soil-moisture, poor ventilation problem. But do I see similar stem shrinkage way up at the top of the picture? that would be unusual. I've only seen the problem at ground level.

I've avoided damping off in seedlings by only using heat-sterilized seedling soil.