Aster Yellows Phytoplasma

Furry carrots? Twisted roots? It might be aster yellows phytoplasma!

Carrot roots showing symptoms of aster yellows phytoplasma (Whitney Cranshaw, CSU, Bugwood.org) CC BY 3.0

Aster yellows phytoplasma is a disease transmitted by leafhoppers and root knot nematodes. The name, aster yellows phytoplasma (AYP), is actually the name of the disease, aster yellows, plus the bacteria that causes it, phytoplasma.


Aster yellows can affect any member of the Aster family, including marigolds, purple coneflower,  and coreopsis. Carrots, caraway, celery, onions, coriander, and lettuce are the major food crops affected by aster yellows. In commercial agriculture, aster yellows can be responsible for crop losses of 25 to 80%. Aster yellows can also occur in nursery stock, grown in greenhouses, which is why it is such a good idea to place newly acquired plants in quarantine before adding them to the garden or landscape. (I know, I know, it’s hard to wait ~ but some diseases never go away…)


​The bacteria that cause this disease reproduce in leafhoppers, root knot nematodes, and in the phloem of susceptible plants. These bacteria help leafhoppers and nematodes to live longer, but in our plants, the opposite it true. As bacterial populations grow, they block the flow of sap, water, plant hormones, and nutrients within our plants, causing chlorosis (yellowing) and distortion.

Witches’-broom of infected carrot (Whitney Cranshaw) CC BY 3.0

Symptoms of phytoplasma infection 

The leaves of infected plants are stunted, twisted, and can turn reddish-purple or yellow. Leaflets may look more like scales than leaves, and flowers may look more like leaves (phyllody). Flowers become severely distorted. Petals that should be white, turn green (virescence), and the flowers themselves turn into leafy umbels (umbrella shapes). The root  is significantly smaller, becomes woody, and sends out many lateral roots that make it look furry. Infected plants commonly send up clusters of dwarfed, deformed, chlorotic shoots in a growth behavior called witches’-broom. If these characteristics are seen - pull the plant and throw it in the trash. This is not a disease you want spreading through your garden or landscape via infected compost. Carrots infected with aster yellows are prone to soft rots and they taste bad.


​Phytoplasmas control

There is no known cure for aster yellows, so we have to look at the disease vectors: leafhoppers and root knot nematodes. Since leafhoppers can overwinter in weeds and perennial ornamentals, such as thistle, dandelion, black-eyed Susan, and wild carrot, keep these plants trimmed back from carrot planting areas. It’s probably a good idea to plant your beets somewhere else, too, since beet leafhoppers can be carriers. Bacillus thuringiensis (Bt) can be used to control these disease carriers. If an area becomes infected, avoid planting carrots there for a couple of seasons. Severe infestations can be dealt with using soil solarization, but that’s pretty drastic, since it kills everything in the soil, including beneficial soil microbes.


If you have deformed carrots, let’s just hope that is is caused by rocks or compacted soil, as those problems are much easier to fix. And, hey, those carrots can look pretty amazing!