We are hearing many reports from gardeners about aster yellows, a highly contagious plant disease spread by aster leafhoppers. Plants show more symptoms during periods of hot weather. The symptoms can be dramatic and often cause gardeners to wonder what their plant is doing.

This photo is a classic expression of aster yellows on a purple coneflower. The plant’s center cone has green, leaf-like sprouts popping out of it. While asters and coneflowers are plants best-known for being susceptible to aster yellows, the disease affects a wide array of ornamental and edible plants, including coreopsis, cosmos, marigold, petunia and phlox as well as carrots, onions, potatoes, even dandelions!

Here’s how the University of Minnesota describes the disease’s lifecycle and spread:

“The cause of aster yellows is a microscopic organism called a phytoplasma (formerly called a mycoplasma-like organism or MLO). Phytoplasmas are most similar in size and composition to bacteria. Aster yellows is vectored (transmitted) by insects called leafhoppers, and through grafting. The phytoplasma survives winter in perennial and biennial plants. In the spring, leafhoppers feed on infected plants. During feeding, plant sap containing the phytoplasma is sucked into the leafhopper’s body where multiplication of the microorganism takes place. Following an incubation period, the phytoplasma is transferred to healthy plants during leafhopper feeding.“

There is no cure for aster yellows, only prevention. Gardeners are advised to remove infected plants and throw them away, weed carefully (weeds often carry the disease) and monitor for leafhoppers.