Aster yellows is a plant disease that occurs throughout North America, affecting a wide range of plants, including many important crops and ornamental plants. Home gardeners are most likely to notice its effects on purple coneflower and some other daisy-like flowers, as the symptoms on those flowers tend to be more dramatic than on other types of flowers or the foliar symptoms.

This disease is caused by a phytoplasma, a bacterium-like organism similar to a mycoplasma (so formerly called a mycoplasma-like organism or MLO), but which infects plants, and causes virus-like symptoms. In plants is it found only in phloem sieve cells. Its host range includes more than 300 plants species in 38 plant families of broad-leaved herbaceous plants as well as a number of grain crops such as wheat and barley.

Some of the most important vegetable crops it can infect are lettuce, carrots, and celery (and this disease was partially responsible for the demise of the commercial production of those vegetable crops in Wisconsin), and its symptoms are commonly seen in many ornamental plants in the aster or sunflower family (Asteraceae / Compositae) such as purple coneflower, China aster (Callistephus chinensis), marigolds, and zinnias.

Some weeds commonly affected include ragweed, marestail, thistles, and pineappleweed. It overwinters in many species of perennial hosts (both native and introduced) or in annuals growing in mild climates in the southern part of the continent.

There is a huge range of characteristic symptoms which vary with the host plant species, temperature, age, and/or size of the plant when it was infected, and other factors.

The symptoms on young plants and foliage can be quite subtle. Symptoms tend to be more severe in hot weather; in cool weather infected plants may not show symptoms. Plants infected when small tend to be stunted, may develop rosetting of leaves, and affected leaves are often narrower than healthy leaves.

Affected leaves may be chlorotic (yellowed), may have vein clearing, or may develop reddish, bronze or purple tinges. On potatoes the disease is often called “purple top” as the leaves of this crop frequently turn purple. On many plants the leaves are twisted or curled – resembling herbicide damage – or witches brooms (abnormally numerous shoots) develop. In carrots, the tops can become bunched with lots of short shoots and the foliage may be yellow, reddish or stunted, while underground numerous hairy rootlets develop in characteristic bunches on the taproot and the root has an unpleasant flavor and is more susceptible to rotting.

The most dramatic symptoms occur on reproductive parts of the plant. The flowers are frequently deformed, with under-developed petals or weird, extra growths, or a change of color.

Plants exhibiting virescence don’t develop the normal color of the petals, but are green (but don’t confuse this with the normal green color of some newer cultivars, such as ‘Green Envy’!). Plants with phyllody have tufts of pale, deformed leaves, often inside the flower or in place of the petals. On coneflowers secondary flower heads are often produced on top of the primary flowers. The pathogen induces sterility, so seeds often are not produced, and those that are produced are generally not viable.

There is another problem only on Echinacea that might be confused with aster yellows because of its similar symptoms, caused by eriophyid mites. These nearly microscopic spindle-shaped mites have only 4 legs at one end of the body (all other mites have 8 legs as adults). The feeding damage of these coneflower rosette mites – which don’t have a scientific name yet – results in tufts of small, distorted flower parts sprouting from the cone, but doesn’t affect the ray flowers.

The plant otherwise grows normally, but the mite damage can reduce seed production. Cut off and dispose of any affected flower heads to limit infestation on other flowers and the following year.

Aster yellows can be fatal to young plants, but typically does not kill perennial host plants. There is no cure for aster yellows, so the only thing that can be done once a plant is infected is to rogue out the symptomatic plant so that it cannot serve as inoculum to infect other plants.

Unlike most pathogens that infect plants directly, the aster yellows phytoplasma requires an insect vector to move it from plant to plant (it can also be transmitted through grafting).  The aster leafhopper, Macrosteles quadrilineatus (=fascifrons), is the only significant vector for this disease. This insect and phytoplasma have evolved together with a very specialized relationship.

The phloem-feeding leafhopper acquires the pathogen while feeding on infected plants, but instead of being able to transmit the phytoplasma immediately to another plant, there is an incubation period during which the pathogen multiplies within the leafhopper and then moves to the salivary glands of the insect. Only then is the leafhopper capable of transmitting the pathogen to another plant when it is feeding. This means there is a length of time – the latent period – during which an insect can be infected with the pathogen, yet not able to transmit it to cause disease. This latent period lasts 2-3 weeks, depending on temperature. It takes another 10 days to three weeks or more (depending on temperature and plant species) for symptoms to show on the plant. High temperatures (88°F for 10 to 12 days) inactivate the pathogen in insect vectors and plants, so the disease is rare or absent in hot areas of the world, and is reduced during hot spells.

The aster leafhopper can move around a lot as an adult – including vast distances across North America under just the right weather conditions. The aster leafhopper does overwinter (in the egg stage) in the upper Midwest, but frequently adults are found far before the local populations have hatched and developed. This insect also overwinters in the southern parts of the US, particularly the Gulf States. It begins developing down south early in the spring, often building up large populations in the grain fields of Missouri, Arkansas and other states. The adults will migrate on the prevailing winds and jet streams that frequently move northward in the spring, flying into the upper atmosphere to be blown hundreds of miles north before falling out onto new host plants in an area far from where they took off. These insects have the potential to have already acquired the pathogen along the migration route or while still in the South and completed the latent period, so in some springs they show up in Wisconsin and adjacent states ready to transmit the disease before the local leafhoppers have even hatched! But in other years, weather patterns are not conducive for their movement into our area, and only the local populations are important in disease transmission.

This ability to move long distances, the relatively long latent period during which they are not infective, and their wide host range of both pathogen and vector complicates control of aster yellows. In commercial vegetable production control is focused on managing the insect vector to prevent infection, usually by spraying insecticides when infective insects are present (based on monitoring leafhopper numbers and infectivity levels of migrants and local populations). This isn’t a practical approach for most gardeners, and in the home garden control is focused on recognizing symptoms and eliminating infected plants, including weeds which can harbor the disease. When buying perennials that are susceptible to the aster yellows, inspect the plants carefully to be sure they don’t have any symptoms at the time of purchase. Monitor garden plants frequently for symptoms and destroy any that appear to be diseased. Early detection and prompt removal of infected plants may help reduce the spread of the disease. (But remember that not all abnormal or distorted plant growth is a symptom of this disease; damage from herbicides (including Weed ‘n’ Feed) can be mistaken for aster yellows.) If aster yellows is a consistent problem, try planting things that are not as susceptible, such as celosia, geraniums (Pelargonium), salvias, and verbena, as well as most woody ornamentals. There are differences in susceptibility of many vegetable cultivars, but there are no resistant cultivars. Susceptible vegetables, such as lettuce plants, can be grown under floating row cover to exclude the vector leafhopper.

– Susan Mahr, University of Wisconsin – Madison

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