Are you finding flowers in your garden that are strangely colored or deformed? With petals that are green or greenish, short or even absent, or there are leaflike growths instead of normal petals? Sometimes small green secondary flowers pop out of the main one, like little green sputniks orbiting a green sun. Is this a mutation?
Probably not. What you’re seeing is most likely aster yellows, a disease caused by a phytoplasma called aster yellows phytoplasma (AYP), a microorganism similar to a bacteria. It causes various forms of abnormal growth, including phyllody (when petals are transformed into leaves) and floral virescence (when a flower turns green), but also chlorosis (yellowing of the foliage), flower sterility, stunted growth, or even witches brooms (dense, massed growth).
You’re more likely to see the aster yellows on plants in the daisy family (Asteraceae), including echinaceas, gaillardias, asters, marigolds, rudbeckias, and coreopsis, but it also infests onions, carrots, snapdragons, and celery, all of which are from other families. In fact, the disease has been found in more than 300 species in 38 families of flowering plants.
Aster yellows does not kill its host plant, but certainly reduces its productivity and makes it less attractive.
Aster yellows is transmitted by the aster leafhopper (Macrosteles quadrilineatus), a small sucking insect common in fields and gardens throughout much of the world. When it pierces the leaves or stems of a plant in order to suck up its sap, it injects, if it is contaminated, a few phytoplasma cells which then reproduce and spread throughout the plant via its sap, causing the changes noted. If a phytoplasma-free leafhopper then bites into the plant, it can pick up the disease and carry it to a new host.
What to Do?
Ideally, you need to discourage leafhoppers from ever reaching your plants.
There are certain ways to do this. For example, avoid applying too much nitrogen-rich fertilizer (the first number on the fertilizer label indicates the percentage of nitrogen), as leafhoppers prefer the tender green growth that results from excess nitrogen applications.
Also, leafhoppers seem to dislike mulch, so adding mulch may help.
Encourage beneficial animals in your garden, too. Spiders are particularly effective, and studies show that leafhoppers tend to avoid plants where they see a spider web.
Finally, some sources claim pelargoniums and petunias will repel leafhoppers. I’ve never had much luck using supposedly repellant plants (and indeed, petunias are among the many plants can suffer from aster yellows!), but who knows? This might be the exception!
Another method of treatment is using pesticides to control leafhoppers early in their cycle, before the damage is done, thus nipping the infestation in the bud. But leafhoppers are pretty discrete insects, largely hiding among foliage where they can rarely be seen. Thus they may already be present in large numbers before you even notice them.
Even when you have an insecticide on hand, leafhoppers can be hard to reach. They are very alert little pests and will jump or fly out of harm’s way given the slightest chance. Among the products you could try that should be relatively effective are neem (no longer available in Canada), insecticidal soap and pyrethrum. Remember though that by spraying your entire garden with an insecticide to prevent leafhoppers, which is pretty much what you’d have to do, you’ll be killing beneficial insects too. If you do go this route, at least make sure to apply the insecticide early in the morning before pollinating insects are active.
That said, not many gardeners are proactive when it comes to preventing leafhoppers. Usually they run into symptoms of the disease rather than the insect that transports it, far too late for insecticides. What to do then?
The Ultimate Solution
Essentially the only treatment for aster yellows that is totally effective is to remove and destroy the diseased plant. There is no known cure for aster yellows. An infested plant will often live for several years and remain a source of disease that each year’s new generation of leafhoppers can spread to plants nearby.
Culling a favorite plant deformed by aster yellows can be painful, but it will do your other plants a world of good. And the sooner you do it, the safer your other plants will be.