Every year, especially around this time, I am constantly asking myself this question.
In the southeast, we essentially live in a petri dish. Between the heat and the humidity, fungal and bacterial diseases proliferate. And while disease and rot are worse with excessive moisture, they also spread by way of the vast quantities of bad-guy bugs, which are always faster at populating than the good guys that control them.
Even if their reproductive cycle is generally slower than the "bad guys", I'm always planting beneficial attracting flowers, letting cilantro and dill bolt (go to seed/flower), and companion planting in the beds that I'm harvesting from. They attract the "good guys" that help control "bad guy" populations. (More on that in another note.)
But before you go looking for a disease or become an entomologist, look at the soil. Most of what people bring me as a mystery ailment turns out to be one of three fertility problems.

Nitrogen tied up. Too much wood chip worked into the soil, or used as a mulch that gets turned in, and the microbes breaking it down borrow the available nitrogen to do it. Your plants go pale and sit there. The chips are not bad for the soil. They are just not finished yet.
Nitrogen burn. The opposite, and usually from compost that was not finished before it went on. Fresh compost is hot both physically and chemically. It will scorch roots and stunt or kill a young plant, and it is the single most common way a well-meaning gardener damages a bed.
Micro and macronutrient deficiencies, from asking one patch of ground to grow several crops back to back without ever giving anything back. This is an even bigger issue in raised beds that leach nutrients more quickly than in-ground beds, especially with the accelerated microbial activity in warm, wet climates.

The fix for all three is the same and it is boring: rest the ground and/or feed it. Ideally you grow a crop, then put that area into a cover crop, a mix of nutrient accumulators, biomass generators and deep-rooted plants that break up compaction and heavy clay. Then mow it down, tarp it for a while, and plant again.
If your garden at home is similar to my farm, you mostly cannot afford to spend the time doing that. Similar to my small farm, the "real estate" in the garden is limited, so resting ground that long would cut production significantly. I am constantly flipping beds, seeding, and planting right after removing a previous crop. And I pay for it in fertility, requiring more compost and composted poultry manure, more kelp, algae, and fish emulsion foliar applications, etc.
If you do have the luxury of time and space, I highly recommend taking a bed or two out of production, and planting a cover crop for a few months, tarping it, and planting again next season.
Soil and ecological health are the main priority here and the biggest factor in healthy, productive plants. Nearly every question that starts "what's wrong with my plant" ends up back in the dirt.
