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Aster Yellows

September 26, 2012

Aster yellows is a phytoplasma disease caused by a mycoplasma-like organism that is often mistaken for plant viruses or bacterial diseases.  Phytoplasmas are single-celled microorganisms (intermediate between bacteria and viruses) that live as parasites in the phloem of plants.  Aster yellows is most prominent in outside production systems, but can also be observed inside greenhouses, particularly when the starting materials were previously grown outside, such as when planting perennials from bareroot.  

Aster yellows appears on many plant species, particularly herbaceous plants such as herbs (such as Dill, Coriander, and Parsnip), perennials (including Aster, Echinacea, Gaillardia, Rudbeckia, Scabiosa, and Veronica), and weeds (Chickweed, Common Groundsel, Horseweed, Purslane, and Common Sowthistle to name a few).  Aster yellows has been documented on 40 plant families; infecting over 200 species of plants worldwide.

Aster yellows, also referred to as witches broom, is an irreversible viral-like disease that causes plants to appear and grow very abnormally, often forming a massed appearance or brush-like development of numerous weak shoots.  Symptoms vary with the host plant. Plants may appear stunted, have chlorotic foliage, and develop flower parts with curious abnormalities. Some plants, such as Echinacea, develop flower parts that revert to leaf forms (deformed, yellowish-green flower heads) after they are infected with Aster yellows. Additionally, Aster yellows may be expressed with upright leaves or witches’ broom in some plants.

Aster yellows is vectored from plant to plant by the aster leafhopper (Macrosteles quadrilineatus) and certain other leafhoppers.  There are numerous plants that harbor Aster yellows phytoplasma, but annual and perennial weeds are the most common source.  Leafhoppers can carry this disease over very long distances. In fact, Aster yellows observed in the northern United States often originates from the South and is moved north with migrating leafhoppers.

Once leafhoppers have acquired the phytoplasma, it takes several weeks of incubation before they can transmit Aster yellows to susceptible host plants.  It takes several weeks for symptoms to appear after a leafhopper has vectored the phytoplasma.  Once inside the plant, there is no cure, an infected plant will remain infected even though symptoms may not be expressed.  It is also not uncommon with perennials and perennial weeds for symptoms to be over-wintered and expressed the following year.

The best strategy is to prevent Aster yellows from occurring is to control the leafhopper populations and keep them from reaching excessive levels.  This can be challenging as leafhoppers move into crop areas from adjoining fields, weeded areas, and with air currents. By they time leafhoppers are detected and controlled, it is very possible that they have already transmitted the phytoplasma.  Plants with Aster yellows should be removed from the production site and destroyed.

Management strategies should include regular scouting and using sticky cards to detect the presence of leafhoppers.  It is important to control weeds in the vicinity of the production area to eliminate host plants from harboring the disease.  Many weeds are symptom-less hosts of Aster yellows which can easily get introduced into crop areas by movement of leafhoppers and their subsequent feeding activities. As soon as Aster yellows symptoms appear, immediately rogue out and destroy infected plants to prevent the risk of spreading the disease to uninfected plants nearby.  It may be advantageous to produce susceptible high value crops, such as Echinacea, in a screened facility which prevents leafhoppers from entering the crop area.  

If leafhoppers are present, it is important to keep their population from reaching excessive levels.  Growers can obtain some control using contact insecticides, such as bifenthrin, carbaryl, horticultural oil, and insecticidal soaps.  Controlling leafhoppers can be difficult as leafhoppers are very mobile and new leafhoppers often enter the treated area after the sprays have dried.  Several systemic products containing the active ingredients imidacloprid, thiamethoxam, and acetamiprid are effective and offer extended control with less frequent applications.

Paul Pilon
Perennial Solutions Consulting
paul@perennialsolutions.com

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