IDENTIFICATION — Deformed flowers, yellow foliage and stunted growth are classic signs of this disease. Though symptoms vary according to the temperature and the age and type of plant, the first thing most gardeners notice is green, twisted flowers with clusters of extra leaf growth in the center. However, if you’re really paying attention, yellowing of new leaves, including the veins, usually happens first. (If the veins remain green, the plant probably has a nutrient deficiency). Growth may also be lopsided, with one side of the plant looking normal while the other side is stunted.
DAMAGE — Aster yellows is caused by a tiny phytoplasma, an organism smaller than a bacteria but larger than a virus, that infects the food-conducting cells of the plant. Hundreds of different plants are susceptible, including aster, zinnia, coneflower, canna, carrot, lettuce, tomato and many others. Weeds, such as dandelion, thistle and plantain, can also carry this persistent disease.
The phytoplasma is carried from plant to plant by hungry leafhoppers, known as six-spotted leafhoppers (Macrosteles quanrilineatus). These tiny, winged insects live on the underside of leaves and feed on the sap, spreading the disease as they travel from plant to plant. Cool, wet summer weather favors both leaphopper and phytoplasma spread.
CONTROL — There is no cure for aster yellows, but it rarely kills a plant outright. It’s important to remove and destroy any infected ones. If you leave these plants in the garden, leafhoppers will continue to visit them and spread the disease. To prevent aster yellows, keep your garden clear of dead foliage and debris in late fall to eliminate overwintering sites for leafhopper adults and eggs. You can also spray the tops and undersides of leaves with insecticidal soap or insecticides like pyrethrin or neem to get rid of leafhoppers.