Aster Yellows

This disease is caused by a virus-like organism called a phytoplasma and is spread by sap sucking insects (notably leafhoppers). It isn’t usually fatal, but reduces vigor and can seriously affect the growth of carrot (most important), celery, lettuce, onion, tomato and a variety or ornamentals.

This disease is most prevalent in cool, wet weather and first shows itself when leaves become pale and chlorotic (that’s why it is called Aster yellows). They may also curl and in some cases small leaves may grow inside the flower! Lettuce may also develop spots.

Strategies for controlling this disease are the same as for viral infections. Remove infected plants promptly to reduce sources of infection, along with weeds that may host the disease (these include dandelion and plantain). You can prevent the insects which transmit the disease from reaching the plants by planting under row covers. You should also rotate crops, avoid planting infected plants and use resistant cultivars when available.

Photo: Whitney Cranshaw, Colorado State University, Bugwood.org