Has Aster Yellows Turned Your Plants Into A Pack Of Mutant Monsters?

Do your echinacea (coneflowers) look like mutant, green monsters from an alien universe? Mine do. And that’s because a disease called aster yellows is running rampant in Minnesota this year. The disease is caused by a phytoplasma, a microscopic bacteria-like organism that makes its home in the vascular system of plants.

Aster yellows causes unearthly distortions in many plants, including coneflowers and phlox.

Not all plants can become infected with aster yellows, but many annuals, perennials, vegetables and weeds are affected by it, including asters, carrots, chrysanthemums, coreopsis, cosmos, daisies, dandelion, marigolds, onions, petunias, potatoes, thistle and tomatoes. Aster yellows isn’t a new disease. It’s just worse this year than it normally is for reasons experts are still pondering. Hot weather seems to have something to do with it.

Aster leafhoppers are the reason why the disease travels through gardens so quickly. They transmit the disease from plant to plant when they feed on infected plants and suck up sap that contains the phytoplasma. After a short incubation period in their tiny bug bodies (the leafhoppers are not harmed), the microorganisms multiply and the insects spread the disease further as they feed.

It’s hard to do, but you really should rip infected plants out and throw them away.

The symptoms of aster yellows vary from plant to plant. But most of the time you’ll notice that infected plants look stunted and distorted in weird ways. Foliage can be yellow and flowers often look yellow or a spooky shade of green. Seeds and fruit don’t develop. You might also see spindly stems and flower stalks. It’s not a pretty sight.

The biggest bummer, though, is that once a plant has aster yellows it can’t be cured. You’ve got to rip the whole plant out and throw it away. I know, I know. I don’t do that either. I just cut off the infected part of my coneflowers and let the rest of the plant that looks good stay. But that is a bad idea. Yes, I get to enjoy the relatively normal-looking parts of my alien, mutant coneflowers. But by allowing those plants to stay in the garden, I’m ensuring that aster leafhoppers will continue to spread the disease to other susceptible plants in my yard and my neighbors’ yards.

It’s really the same tough reality crew members on spaceships in sci-fi films often face. You remember the plot. They all know that their friend was infected by some horrible monster creature thing down on the planet they just visited without wearing proper protective gear (what’s up with that?). And they all know it’s only a matter of time before a baby monster creature thing bursts out of their friend’s chest and tries to infect them all. But, hey, for the moment the friend seems mostly fine, so why not let the poor sot live, right? We know how that story ends.

So let’s all vow to rip out plants infected with aster yellows—at least by the end of the season when we cut down plants for fall. It’s fine to throw these infected plants in the compost because the phytoplasma dies when the plant dies. Not all plant diseases work that way. Unfortunately, this harsh step won’t guarantee that aster yellows won’t come back again next year because infected leafhoppers are likely to still be around or you could inadvertently bring an infected plant home from the garden center. I think I’m finally going to give up on coneflowers all together. I like them, but I’m tired of knowing there’s a mutant, green monster lurking behind those pretty purple flowers.

Note: This post ran this week as my Everyday Gardener column in Minneapolis’ Southwest Journal.