This disease is caused by a microorganism, a mycoplasma-like organism (MLO), which is spread by the 6-spotted leafhopper, aka aster leafhopper(Macrosteles quadrilineatus). The leafhoppers pick it up while sucking the sap of diseased plants.
This aster disease hits in early summer and affects about 200 species of ornamentals, vegetables, and non-woody fruit plants. It’s a common disease of many annuals, and overwatering increases the chance to get the disease, particularly in heavy soils.
There’s a wide range of abnormalities
that are shown in this disease, especially in new growth:
- Veins may turn slightly yellow while the rest of the leaf stays green (China Asters, Callistephus). The leaf then turns pale, losing almost all of its color, with brown edges.
- Yellow, spindly new leaves, with the petiole elongated and the blades dwarfed.
- Six or more leaves may unfold where there usually is only two
- stunted overall plant growth
- flower buds may not open at all, and those that do, are often distorted or dwarfed and sickly looking with virescence
- delphiniums are often given a short, stocky look, because of short internodes and many secondary shoots
- seeds are made sterile
- vegetables get warty skins and lack flavor, as well as ripen early
- older leaves may be purplish
After the mycoplasm is sucked up by the leafhopper, it multiplies for 10-days, then it is injected into other plants as the leafhopper feeds. Aphids may also spread MLO, but it’s not carried in seeds and doesn’t live for long without its host, for example, when it’s in the soil. However, if it’s in a perennial host, such as daisies and plantains (weeds or plants) then it can overwinter.
Beans, peas, and other legumes are not susceptible to MLO.
There is no cure; diseased plants should be destroyed, and garden tools should be sterilized to avoid spreading the disease. The mycoplasm remains infectious for 3 months or more.