All these ideas are good. I agree with Ken that your zinnias don't like their roots disturbed. I start mine in six packs and transplant them as gently as possible without disturbing their roots. If you were pricking them out into a bigger pot, that could well be your trouble. Fortunately, they are easy to start from seed and take off fast once the weather warms. Even if you have to start over, there's still time.

ritug, If nothing ever went wrong with our gardening activities, we wouldn't learn anything. I have had the exact same problem you had and, in my case, the problem was nutrient deficiencies. Lower leaves that can't get enough light will also die, especially if the plant wilts for lack of water. It is best to use a soluble urea-free nutrient formula in a sterile medium like Miracle-Gro Potting Mix, because there are no soil microbes present to break the urea down into usable nitrate and ammonium ions. In the absence of the soil microbes, the urea can accumulate, possibly to harmful levels. Plants can't use urea directly through their roots, although apparently it can be used in a foliar spray. I used to use the urea-free Peters African Violet food for my indoor plants, but apparently it has been discontinued and I now use Better-Gro Orchid Plus diluted to about one-half teaspoon per gallon. You can get it at several online stores, including Amazon, or if you have a Lowe's Home Store handy, they usually have the Better-Gro products in their garden section. Some garden centers also have it. You can detect an early onset of nutrient deficiencies by watching the zinnia seed leaves (cotyledons). You can see one "peeking out" in this picture. It is still fairly healthy even though the plant is fairly well advanced. As long as the zinnia plant has plenty of nutrients, the seed leaves will remain green and healthy even though the plant has grown considerably. The plant will "steal" nutrients from the seed leaves if it experiences a nutrient shortage, and they will shrivel and die. If the condition persists, the plant will begin stealing nutrients from the lower leaves to feed the newer leaves. My Mother used to refer to that sort of thing as "robbing Peter to pay Paul". ZM

The lady beetles will help because they do dine on many small insects other than aphids. To help some you can use a sharp water spray to knock these wee buggers off the plants or you could, with due care, spray them with an insecticidal soap. I'd suspect that these are also aphids which come in a wide variety of colors ranging from green to orange to brown somewhat depending on the plant.

Nothing wrong. Horticultural Zinnias are strong-growing annuals that put all their energy into tops and flowers. By the time they start flowering the bottoms always look raggedy and dried-up. At least mine do. I always plant them very thickly which hides that to some extent, and they don't seem to mind crowding each other. I notice others do the same around here.