A lot of gardeners had odd things happening in their gardens last year. A hot spring that started in March, followed by hot, wet, and then dry weather probably had a lot to do with it. Aster Yellows could be another cause.
Aster Yellows is caused by a phytoplasma, similar to bacteria but lacking a cell wall and therefore difficult to culture.
This organism is carried to host plants by leaf hoppers. It is transmitted during feeding and causes severe reduction in the yield and quality of crops.
The phytoplasma can infect over 300 kinds of plants in just about every plant family. Aster Yellows is a common and destructive disease world-wide, although it is rare in areas where air temperatures are above 90° for extended periods.
The damage depends on the host crop, with the greatest losses among carrots and lettuce.
In 2012 Minnesota’s boutique garlic industry was devastated by Aster Yellows. Because of the mild winter, the garlic sprouted early and the leafhoppers migrated early. When they arrived there was little else to feed on.
The leafhopper acquires the bacterium when eating winter grain crops in southern states such as Arkansas and Oklahoma. As the wheat and barley mature, they are no longer desirable food sources. So the leafhoppers leave the grain fields and are carried north by the wind.
The most common symptoms of Yellow Asters are yellowing, stunting of the plant and resetting of the leaves. On plants that produce a cluster of leaves, like lettuce, the older outer leaves are usually of normal size but may show purple or red discoloring on the leaf margins. The inner, younger leaves are usually dwarfed and yellowed, and they may have small brown specks along the margins.
On tomatoes and potatoes, the leaves curl and twist and turn purple or yellow. Carrots develop hairy roots that are tapered, pale, and bitter. Many Master Gardeners think the stunting and odd shapes of onions last season were caused by Aster Yellows.
Aster Yellows is difficult to control because of the number and diversity of plants attacked and because of the very efficient transmission by insects. Destroy infected plants as soon as they appear, and remove overwintering hosts like last year’s plants and weeds.
Do not plant a susceptible crop next to an infected crop. Fast-growing crops such as lettuce and valuable plants could be grown under a cloth screen. There are no resistant or immune crop varieties available.