There were a lot of gardeners who had concerns about odd things that had happened to their gardens last year. A hot spring that started in March, followed by hot, wet, dry, etc. probably had a lot to do with it. At one of our Master Gardener meetings last fall, Donna Tatting, who writes many articles, mentioned that part of those garden problems could be because of Aster Yellows. Information from the University of Illinois Extension convinced me that that could be one of the problems.
Aster Yellows is caused by one or more strains of a bacterium that lacks a cell wall and therefore is very difficult to culture. This organism is carried primarily to its host plants by the aster or six-spotted leafhopper. It is transmitted during the feeding of the insect and causes severe reduction in the yield and quality of crops. The bacterium can infect over three hundred kinds of plants in just about every family of plants. Aster Yellows is a common and destructive disease worldwide, although it is rare in areas where the air temperatures are above 90 degrees for extended periods. Losses from Aster Yellows vary among different host crops with the greatest losses among carrots and lettuce. Last year, Minnesota’s boutique garlic industry was devastated by Aster Yellows. Because of the mild winter, the garlic sprouted earlier than usual and the leafhoppers migrated earlier than usual. There was little else to feed on at the time they arrived, so they feed on a readily available early sprouting garlic crop.
The leafhopper acquires the bacterium when feeding on winter grain crops in southern states such as Arkansas or Oklahoma. In the spring, as wheat and barley mature and are no longer desirable food sources, the leafhoppers leave the grain fields and are carried north by the wind.
The most common symptom is a yellowing, stunting of the plant, and resetting of the leaves. On plants that produce a cluster of leaves, like lettuce, the older and outer leaves are usually of normal size but may show some purple or read color on the leaf margins. The inner younger leaves are usually dwarfed, yellowed, and may have small brown specks along the margins. On tomatoes and potatoes, the leaves curl and twist and turn purple or yellow. On carrots the taproots develop hairy roots and the roots are tapered, pale, and have a bitter taste. Many of the Master Gardeners feel that the stunting and odd shapes of the onions last season was due to Aster Yellows.
Aster Yellows is difficult to control because of the number and diversity of plants attacked and because of the very efficient insect vector. To control this problem, approach it the same way as you would blight. Destroy overwintering hosts like last year’s plants and weeds and infected plants as soon as they appear. Don’t plant a susceptible crop next to a yellow infected crop. Fast growing crops such as lettuce or valuable plants could be grown under a cloth screen. There are no resistant or immune crop varieties available.
— Jerry Vitalis is a Chisago County Master Gardener