Aster Yellows

Problem type: Disease

Name of problem: Aster yellows

Plant name(s): Vegetables, herbs, field crops, strawberries, annual and perennial herbaceous ornamentals

Symptoms / Characteristics:
Symptoms of aster yellows may vary according to the respective plant host. Essentially all plants infected with aster yellows will exhibit yellow, reddish or purplish leaf discolouration as well as stunted and deformed growth. Flower parts are often deformed and non-functional. In many cases, they are replaced by sterile, leafy structures. Seed production is rare.

In carrot (a very common host on the prairies), yellow discolouration of younger leaves is the first symptom to occur, followed by a flush of weakened or unhealthy shoots from the crown. Older leaves first take on a whitish color and later become a distinct bronze or red. Leaves and leaf stems become twisted and deformed. These parts are weakened and often break off. A proliferation of deformed, fibrous rootlets usually appears along the taproot. In celery, yellowed leaves are common as well as stunted, deformed and entangled growth. Eventually infected plants turn completely yellow or even white. Lettuce is another vegetable crop that can be severely infected with aster yellows. Yellow, stunted and deformed center leaves appear first and these symptoms may spread to the entire plant, depending on the severity of the infection. The presence of a brownish latex substance on the leaf underside is a characteristic symptom. This substance is usually found near the midrib. Infected onion leaves tend to be flattened, yellow and streaked. Infected potato plants exhibit curled leaves with yellow or purple discolouration.

Aster yellows is caused by a virus-like organism, known as a phytoplasma. Once it is transmitted to a healthy plant, it resides in a part of the plant vascular system that is responsible for transporting nutrients. Sap-sucking leafhoppers transmit the organism from plant to plant. The range of host plants that become infected in a given area reflects the diet of the respective leafhopper species. There are over 350 reported hosts for the disease.
The aster leafhopper, Macrosteles quadrilineatus, is primarily responsible for transmitting the aster yellows pathogen. It is yellowish-green in color with a dark grey abdomen and six dark spots on the head. Foods of choice include carrots, lettuce and celery. The actual feeding activity is relatively harmless, per se. It is the transmission of the pathogen that can be detrimental to the plant. Insect populations can fluctuate according to environmental temperature. Extremely hot and dry conditions can hinder or delay the development of the insects and subsequent disease transmission. Periods of high moisture lead to the production of lush, succulent plant growth that is particularly appealing to the leafhoppers. Under such conditions, you can expect disease transmission to escalate.

Control / Preventions:

The phytoplasma can overwinter in any perennial host, including weeds such as dandelion and sowthistle. Remove and destroy any perennial weeds or crop residues (including roots) that might harbor the pathogen. Do not use these plant materials in compost. In smaller plantings, early detection and manual removal of infected plants will effectively reduce further disease spread. Controlling the insect vector via organic or chemical insecticide may reduce future infections. There are no resistant varieties but infection may be avoided by planting species that are not particularly favoured by the leafhopper. Interplanting with yarrow, tansy or mint has been suggested to help deter the insect vector. There is no method of control once the pathogen has colonized the plant vascular system.