Aster yellows (AY) is beginning to show up in many crops this season, however reports indicate that the leafhopper populations that transmits the pathogen from plant to plant appears to be low. Two years ago, in 2012, the incidence of AY in many vegetable crops grown in Ontario was very high. Aster yellows is a disease that can infect over 300 species of plants including many agriculture crops such as carrots, celery, onions, garlic, and cucurbit vegetables. The disease is caused by a bacteria-like organism that does not have a cell wall called a phytoplasma. The AY phytoplasma lives and reproduces inside the nutrient rich conducting cells or tubes (phloem) in the vasculature of host plants or in the leaf hopper vectors.

The pathogen is vectored by leaf hoppers particularly the Aster leaf hopper (ALH), Macrosteles phytoplasma. The ALH is found in most states and provinces in North America. In colder climates such as Ontario, this insect overwinters in the egg stage in the tissues of winter wheat, rye and grasses. In the spring, the eggs hatch and develop into nymphs that feed locally in these overwintering grain crops and grasses. As the grain crops mature and the leafhoppers develop into adults, they disperse to feed on other plants such as weeds and vegetable crops. If a leafhopper feeds on an overwintering weed that was infected the previous year with AY , it will pick up the phytoplasma pathogen with its piercing-sucking mouth parts.

The AYphytoplasma must first incubate and multiply within the ALH for 2- 3 weeks before it can be transmitted to other plants. The leafhopper can then transmit the pathogen for the rest of its life following the acquisition and incubation period. It takes time for the leafhoppers to hatch from eggs, develop from nymphs into winged adults, and to acquire the pathogen from infected weeds. For this reason, it is often later in the season that growers in Ontario see this disease.

In most years, the incidence of AY in Ontario vegetable crops is low and sporadic. However, adult ALHthat overwinter in the southern US can be carried northward with storms, arriving long before local populations have had a chance to develop and build up. If the migrant leaf hoppers are carrying the AY pathogen early in the growing season, they can transmit the phytoplasma quickly upon arrival, resulting in a higher incidence of this disease.

The incidence of AY in any year depends upon the population of the ALH vector and the percentage of the vector population that carries the phytoplasma. In some years the population of leafhoppers are low but a high percentage may be carrying the AY pathogen, resulting in a high incidence of the disease. In other years, the leafhopper population is high but a low percentage is carrying the pathogen, resulting in low to moderate incidence of AY. When both the ALH population is high and a high percentage is carrying the pathogen, the incidence and damage from AY can be very high.. Many of the adult leafhoppers that moved northward on storms from the south during the spring of 2012 carried the AY phytoplasma and AY was very high in many susceptible crops that year.

Symptoms of AY varies depending upon the host crop. In carrots the leaves initially turn yellow (Figure 1) and plants appear stunted, however, infected plants often turn bright red. Celery plants infected with the AY pathogen appear stunted with yellow leaves that eventually take on a bleached appearance with green veins (Figure 2). Infected onions also appear stunted with the center leaves appearing light or pale green (Figure 3). In garlic, symptoms include stunting and severe leaf yellowing (figure 4). Some varieties of garlic also turn red or produce some red pigments in the lower leaves of infected plants. In fact, aster yellows almost wiped out the entire garlic crop in Minnesota during 2012.

Controlling AY is very difficult in many crops. Planting resistant varieties of certain vegetables is the best strategy. Unfortunately many susceptible vegetable crops do not have resistance to AY and management relies on timely applications of insecticides to control the leaf hopper vectors and good weed control in and around susceptible vegetable crops.

Figure 1. Carrot field with high incidence of Aster yellows during 2012.

Figure 2. Aster Yellows symptoms in celery plants include stunting and yellow or beached leaves with green veins.

Figure 3. Onion plants infected with Aster Yellows appear stunted with light green or yellow inner leaves.

Figure 4. Aster yellows infected garlic display bright yellow leaves often starting in the middle of the plant.