By Danielle Stephens, M.Sc. PAg, IPM Agrologist
Aster yellows is a common disease in Saskatchewan field crops, however levels are usually low and barely noticed. Rarely are the incidence levels high enough to make an impact on yield. Our last year with high incidence of aster yellows was in 2012 and, before that, in 2007, 2000 and 1957.
The aster yellows phytoplasma is carried from plant to plant by leaf hoppers, primarily the aster leaf hopper, Macrosteles quadrilineatus. Adult leafhoppers are quite small (3.5 to 4 mm) and are relatively poor fliers. However, they can migrate from the United States on air currents in greater numbers, and the migrant population usually poses a greater risk of carrying the aster yellows phytoplasma. Not all aster leaf hoppers are infected with the phytoplasma; to be infected, they need to have fed on an infected plant. However, once they have become infected they will remain so for the remainder of their lives (one to three months). The leafhopper feeding itself is not an economic threat to crops.
Aster yellows can affect 300 species of plants in 48 plant families, including broadleaf crops, vegetables, ornamentals, weeds and cereal crops. In Saskatchewan, the crops that we may see symptoms on include canola, camelina, mustard, flax, sunflower, caraway, coriander, alfalfa, potato, carrot, tomato, pea, wheat and barley. Symptoms of aster yellows vary with the species of plant infected.
Canola shows very obvious symptoms of infection, including malformed flowers and pods and the few infected plants sticking out taller than the surrounding crop. The flowers are replaced by green leaf-like structures, and pods can become deformed and bladder-like in appearance.
In this year’s flax, aster yellows-infected plants have been seen with pale striping and yellowing, often just at the top of the plant. Infected plants may be showing deformed flowers at this stage.
Aster leaf hoppers are most often found in cereals. Cereal crops are actually more commonly infected than other crops, however they rarely show symptoms. Symptoms in cereals are the same as those of the Barley Yellow Dwarf Virus (BYDV): yellowing leaves and shrivelled heads. However, many cereal plants can have aster yellows and not show any symptoms.
Other crops in Saskatchewan have different symptoms when infected: Check out the aster yellows symptoms found in infected camelina, caraway, potato (purple top) and carrot (red top) plants.
Symptoms are usually delayed from when the plant is infected and, therefore, appear later in our season, causing minimal damage to a few plants. In most years, actual losses from aster yellows are quite small in field crops such as canola, flax and cereals. Camelina has shown greater susceptibility to aster yellows, and the disease can cause losses in herb and spice crops and in vegetable production in high infection years.