Ask-a-Dave's-Gardener: Why vegetable plants are getting yellow in color & not bear, 1 by ktr944

ktr944 wrote:
I've tried to grow vegetables in my backyard, but this year the results have been very disappointing (such as tomato, gourd plants). The plants are growing to a point where they flowering, but not bearing any fruits. The leaves are getting discolored (yellowish). Please see the attached pictures.

I'm pretty confident that its not due to under-watering as I've drip system running wherein these plants get adequate amount of water every alternate day.

Any ideas what could be causing this ? Really appreciate for responses.