Is this yellow aster? Several of my coneflowers just had brown cones; they didn’t bloom. Can I cut down the plants now or do I need to take them completely out? They always seem to struggle with powdery mildew, too.

If I take them out, can I plant anything there? We are on drip, so removing will leave a spot with a drip line for a new plant. I don’t know if I need soil there to heal before I plant something else.

It’s hard to say from your photo whether this is aster yellows because I can’t see enough of your plant to check for additional symptoms on the foliage. This could be aster yellows but there is also an eriophyid mite, the coneflower rosette mite, that causes similar symptoms on the flowers. How can you tell which problem your echinacea (coneflowers) have?

Plants with aster yellows may have stunted or twisted growth and yellowed and/or deformed leaves. The petals on the coneflowers often lack their classic rich colors and may be stunted or missing. The flowers on infected coneflowers have odd or deformed growth as well. The plants may be purplish or red.

If echinacea are infested with coneflower rosette mites, the symptoms are limited to the flowers, and the foliage and general growth will be normal. These very tiny mites can be found inside the flowers before the blooms are fully developed and their feeding at the base of the flowers can cause tufts of galls and distorted growths on the cones of the coneflowers.

If mites are the problem with your flowers, you have a few management options.

• Coneflower rosette mite damage is mostly cosmetic, so you could choose to do nothing, but you’d lose some of the seed production and reseeding you usually see with echinacea.
• Deadheading is another strategy. Carefully removing affected flowers and disposing of them can reduce the mite population. Don’t compost the infested plant material. Deadhead carefully; you can secure a bag around the blossoms to prevent mites from dropping off as you remove flower heads.
• You can treat the flowers with insecticidal soap or horticultural oil. Both are effective against mites with good timing and thorough coverage. Spray one to one and a half weeks before bud break and then again at bud break.

No matter which option you choose, good sanitation is important. Diligent weed control, cutting plants back to the ground at the end of the season and thorough fall cleanup are important.

Unfortunately, if the problem is aster yellows, there isn’t a good solution. This is a systemic disease (the entire plant is infected, not just the flower head) without a cure. Hundreds of species of broadleaf herbaceous plants, grasses and bedding plants are susceptible to this disease.

Aster yellows is caused by a phytoplasma, an organism similar to a bacterium. The phytoplasma is carried from plant to plant by infected leafhoppers and other sap-feeding insects. The phytoplasma live in the phloem, the food-transport tissues of the plant, so they are found throughout an infected plant. The infection isn’t typically fatal, so the phytoplasma can be present in the flowerbed for as long as the infected plants are present.

In addition to being stunted, twisted or yellowing with deformed flowers, infected plants may sometimes be purplish or red.

There is no cure, so plants infected with aster yellows should be dug out and completely removed, not just cut back.

The phytoplasma can only survive in infected plant tissue or in insects that have fed on infected plants, so there is nothing you should do to “heal” the soil.

You can replant the area with woody plants (shrubs and small trees) and other plants that aren’t in the aster family and aren’t listed as susceptible to aster yellows.