Sick plants! How come you have sick plants? The bottom line (without mincing words) is that you're stressing them somehow and usually one of the first signals is yellow leaves.
1) Too much water
2) Not enough water
3) Not enough light - common for indoor plants
4) Too much light (common in the South)
5) Pushing for growth by high fertilizer levels
Yellowing leaves are one of the primary symptoms of light problems as well as watering problems.
And the remedy here is to understand the light level your plant requires and give it that.
Perhaps provide indoor lighting on stretched out, yellow seedlings but shade in the South for perennials that "melt" away.
Pushing chemical feed on plants is like treating your plants as a drug-addict; they always want more and their health goes downhill as a result.
And then, when the insects attack because the plant is sweeter or has a “different” hormonal value than it should, you spray a chemical to “cure” the problem.
The chemical fungicide or insecticide knocks the bad guys back for a short time and it also kills off the good guys that eat the bad guys. But then the bad guys reappear (bad guys always seem to have higher rates or reproduction than good guys) and re-infest. So you spray again.
All these sprays get washed into the ground from dew or rains or overspray where they also kill off good guys living in the soil.
You now have another problem and another set of bad guys invades.
You’ve create an environment where the bad guys have an advantage both in the soil and on the plants.
Then the bad guys mutate and become resistant to your sprays. Now, you have an even more serious problem.
How much simpler it would have been to admit that you’re going to lose a few plants or blooms to the bad guys in any case.
Use compost and compost tea or other natural products to control extreme invasions and problems and have a healthy balance in the garden where both pests and controls exist.
Hmmm. Well, there's a ton of science now on compost and compost teas that points out the benefits of this ancient gardening technique.
It is really only since the second world war that chemical agriculture really got going - and then only because the explosives industry had to find something to do with all their manufacturing capacity.
The chemical industry with their huge advertising and p.r. spending likes to try to convince us that they have the answer. It's like old joke - "Before the use of chemical fertilizers and insect controls, we lost a huge 30% of our crops to insects. Now that we have chemicals to control pests and weeds, we only lose 30% to insects."
I'm sure that a biological pesticide (you don't need chemical ones - there's enough biologicals to control darn near every pest you'll see) is a good idea under some conditions but again, I prefer to set up conditions where the natural garden controls its own problems and I can live within that world
Get the watering right - use the finger test as I suggest in the video.
Get the light levels right. There's no one size fits all here - you have to read the tag or other info to know where to plant.
Get the feeding right. Using compost in the early spring and fish emulsion or other organic food is all the garden really needs.