Aster Yellows

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

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

Aster yellows phytoplasma (AYP) is a disease transmitted by leafhoppers and root-knot nematodes. It gets its name from the family of plants affected (carrots and sunflowers), the symptoms (yellowing), and the bacteria that causes it (phytoplasma).

Aster yellows can affect all Aster family members, including celery, onions, coriander, caraway, and lettuce. Purple coneflower, marigolds, and coreopsis can also be infected.

Losses of 25 to 80% occur in commercial crops due to aster yellows. Quarantining new plants is the easiest way to prevent aster yellows from occurring in your garden or greenhouse. (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 the phloem of susceptible plants. These bacteria help leafhoppers and nematodes live longer, but the opposite is true for our plants. 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 

Stunted and twisting leaves are common symptoms of infection with aster yellows. Leaves 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 called witches’-broom. 

If you see these symptoms, pull the plant and throw it in the trash. You do not want this disease spreading through your garden or landscape via infected compost. Carrots infected with aster yellows are prone to soft rots and taste bad.

Phytoplasmas control

There is no known cure for aster yellows, so we have to look at the disease vectors: beet 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. 

Bacillus thuringiensis (Bt) can help control these disease carriers. If an area becomes infected, avoid planting carrots there for two or three seasons. Severe infestations require soil solarization, but that’s pretty drastic since it kills everything in the soil, including beneficial soil microbes.

If you have deformed carrots, I hope it is caused by rocks or compacted soil, as those problems are much easier to fix. And, hey, some of those carrots can look pretty amazing!

Dancing carrots