We need more information in order to help you.

When you say "wilting" do you mean the whole plant, just the leaves or maybe the flower stems? Hanging like a wet towel? Collapsing? Discolored?

If you can, we'd like to see photos of your plants. You can attach up to three at a time, directly to this reply, and we will take a look.

Christine

We think that this looks like heat stress or the need for water.

Have you noticed that it gets better in cooler evenings? Even with the rain, plants can be stressed by the high heat we've had. Really the only way to know is to put a screwdriver down to the root area and then feel it with your finger. Ideally it should feel moist and cool. If not, give them a nice deep drink and check again. Hopefully it has not spread.

The cut and treat with alcohol we would not recommend. Not sure how that may show up.

Christine

The main risk from the alcohol is phytotoxicity, or the damage to tissues from a chemical exposure. Despite being an oft-recommended home remedy, alcohol isn't always safe to use on plants. Sometimes gardeners get lucky and avoid damage, but it can "burn" leaves, leaving the treated tissues vulnerable to sunburn, desiccation, or pest or disease attack. While it is sometimes useful for carefully swabbing pests off of houseplants, it should be used with caution (and generally not outdoors, especially given the heat). Tissue damage from alcohol or other phytotoxic exposures may also impede the "healing" process (plant's don't heal like humans do, but the process' goals of preventing further tissue damage are similar).

Home remedies of other sorts run equal risks, since they are not regulated pesticides that have undergone rigorous testing for safe and effective use on plants and the target pest. Legal use of a pesticide is limited to information given on its label or, in a special circumstance, in a provision granted by the EPA. Homemade pesticides may also include secondary ingredients that are not inert in the environment or harmless to the target plant. Penn State has a good discussion of this topic on this webpage. Because of this, household hydrogen peroxide is not recommended for use on plants, as this is not its intended purpose.

Tips for avoiding damping-off, the sudden wilting and collapse of seedlings, are given on here. Essentially, maintaining excellent air circulation (especially indoors) and avoiding over-watering are key.

Removing some or most suckers on tomatoes can be beneficial. Here is more information about suckers under the final bullet point for "Growing and Care of Tomatoes" on this page: https://extension.umd.edu/resource/tomatoes

Miri

We will have our plant pathologist take a look at the new photo when he returns to the office later today or tomorrow, but there are two signs you can look for in the meantime to see if that narrows-down potential causes.

Southern Blight is a disease that causes similar rapid-collapse symptoms. You may find evidence of spores on the soil surface around the base of the affected Coneflowers. More information and a sample photo is available on this page: https://extension.umd.edu/resource/southern-blight-disease-flowers

Clip off one of the wilted stems at the base and try to split/cut it along its length so you can see core inside. Look for an insect larva or signs of tunneling (though some plant stems are naturally hollow) with either holes chewed to the outside of the stem or remnants of frass (this is insect poop, though it's often mixed with wood shavings or pulpy plant leftovers).
https://extension.umd.edu/resource/borer-insects-flowers

Miri

Thank you for the new photos. While we can't see any specific diagnostic symptoms, between their appearance and your lack of discovery of stem borers, this does suggest some sort of root or crown rot or wilt (such as Verticillium Wilt). Several pathogens cause these disorders, so it's hard to determine which exact one this may be. (It may not matter much, though, since control is essentially the same - prevention through minimizing waterlogging of the soil - and fungicides are impractical or ineffective at this stage.)

Coneflower is native to prairie habitats and as such has pretty good drought tolerance. They should only be watered when the soil several inches deep is becoming fairly dry (and even then, they should handle that well to avoid being watered too often). While it's good that the bed you have them planted in drains well, perhaps they need less manual watering and can rely more on rainfall for the time being.

All you can do currently about the suspected rot is to remove what plant parts collapse and make sure mulch is not touching the plant crown (where the stems meet the roots). Air circulation around a plant's base can help avoid fungal issues, though it isn't a guarantee.

Miri

You're welcome.