Yellow, then death
Everything in my garden started to turn yellow and then die. It started with my bush beans, then seemed to spread across my garden consuming everything. I haven't seen any aphids. I water regularly at least twice a week. My tomatoes were producing greatly when they got it and within 2 weeks they were just sticks. What could this be?
There is not really a fungus, bacteria or virus that attacks all the different types of vegetables in the garden in this way, so I think that you can rule that out.
My gut would say that either the plants are underwatered or overwatered. While you do water 2X a week, if the water is not enough (2-4" per week) or if the water is not penetrating the soil, the plants would yellow and die in this fashion. On the flip side, if you were giving the plants too much water and it was not draining properly (i.e. they were in standing water a significant amount of time), the roots would start to rot, which would also cause the plants to yellow and die.
The other possibility is that something is being sprayed on the plants, likely accidentally. A weedkiller that has been mislabled or if weed killer was sprayed nearby on a windy day and the overspray got on the plants, they would also yellow and die.