Leafhoppers and Aster Yellows

Bugs are Bad Enough

Bugs that feed on your cut flowers are bad enough, but when you mix bugs with disease you have a real disaster potential. This is the way it is with  leahoppers and a phytoplasma-like organism such as aster yellows. The leafhopper is like the computer-guided cruise missile, carrying the payload to the site. The mycoplasma-like aster yellows organism is the explosive charge that the missile (leafhopper)  carries to your plant. Yes, I have been reading too much about warfare lately, but I thought the missile and explosive explanation would help bring home the message.
Aster yellows is an unfortunately common plant disease that affects a wide range of flowers, vegetables and weeds. Susceptible cut flowers include asters, cockscomb, chrysanthemum, cosmos, echinacea, dianthus, gladiolus, marigolds and petunias. Weeds such as dandelions, plantain and others listed in this article are susceptible and can serve as sources of the disease in cut flower plots.

What Should You Watch For?

The first symptom of the disease is loss of chlorophyll or green pigment in the leaf veins. This is followed by yellowing of newly formed leaves, sporadic bushy growth, erect growing habit, and stunting. Stems and flower stalks may be numerous and spindly. Flowers often remain green and become distorted. Seeds and fruit do not develop. Specific symptoms vary with the kind of plant.
Gladiolus that become infected may have thin, weak, yellow leaves, and the flower spikes may be twisted and deformed, while the flowers remain green. The whole plant is generally stunted and spindly, and the top is often killed.
Infected asters will be stunted, and have stiff yellow growth with many secondary shoots. In heat of the summer the symptoms are more severe, and appear more quickly. If you grow asters in a cool greenhouse at 55 degrees or lower, aster plants may be infected without the symptoms being obvious until the greenhouse heats up. Affected leaves are somewhat narrower than healthy leaves. Old leaves may develop a slightly reddish, brownish, or purplish tinge in the late stages. The main branches will be shortened. Flower parts may develop into leafy structures. The weird stuff that diseases do to plants is just amazing.
Aster yellows is caused by a tiny organism known as a phytoplasma and is spread from plant to plant by leafhopper feeding. In most areas in the East Coast and Midwest, aster yellows often follows an outbreak of the six-spotted or aster leafhopper, Macrosteles quadrilineatus. This leafhopper is found throughout the Northeast, Midwest and central part of Canada.  The direct feeding of the leafhopper is usually not significant.  Leafhoppers feed and inject saliva into plant tissue as they feed. After the aster leafhopper has fed on a plant infected with aster yellows it transmit its saliva to the plant it feeds on next. The disease does have to incubate for 10 -20 days inside the insect, but once this time period is reached the leafhopper can transmit the disease for the rest of its life.  Once the disease is in the plant the result is distortion and death of the plant. No cures are available.
If you are in the West or South are you free of  this problem? Not really. In the West and southeastern U.S. there is a leafhopper called Scaphytopius irroratus that is active and capable of transmitting the phytoplasma strains that produce aster yellows. S. acutus is another leafhopper species that has been reported to transmit aster yellows in the Northeast.

When to Watch for these Bugs?

Adult aster leafhoppers do not overwinter well in the North but have been recorded to survive as eggs in New England and Canada. Winged adults do overwinter in the Gulf area and in the southern Great Plains. In the spring they are caught in updrafts and carried north to infest cut flowers growing in  fields, high tunnels and greenhouses. Expect see most of them migrate during May and June.
The ability of leafhoppers to transmit the organism is reduced when temperature is over 90F. This is one good part of a hot summer. Symptoms show in plants in 10 to 40 days after insect feeding. The disease can be serious when dry weather forces leafhoppers to migrate from wild weeds to irrigated fields of susceptible plants. So watch out in hot, dry summers for migration of leafhoppers to your irrigated cut flower fields.
The adults will lay eggs in leaves and stems of plants and the eggs hatch in abut a week. Dandelion and plantain can be infected with aster yellows and serve as disease reservoirs that will haunt cut flower fields. Try to control dandelion and plantain near your cut flowers. It can also feed on cinquefoil, daisy fleabane, dandelion, horseweed, plantain, ragweed, thistle, wild carrot, and wild lettuce, so control these weeds as well.
The adult aster leafhopper are pale green, up to 1/8 inch long, and feed on the underside of leaves. Disturbing a plant will cause them to rapidly fly away  The nymphs will run around the other side of a stem or leap off the plant.
Once infected, a plant will not recover even if you do your best celebration dance around it. Learn to accept: THERE IS NO CURE. Diseased plants should be promptly removed and discarded to reduce further spread.

Cultural Control

•Obtain healthy seed, cuttings and plants.
•Control weeds surrounding field.
•Avoid rotations where one susceptible crop follows another.
•Destroy volunteer overwintering plants and avoid planting near established diseased crops.
•Destroy affected plants in small areas as soon as they appear to be diseased.

Chemical Control

This in not an all-inclusive list but here are some of the products labeled for leafhopper control:
•Acephate (Orthene)—applied to foliage give 7 -14 days of control.
•Imidacloprid (several brand names)—applied as soil drench gives season-long control.
•Acetamiprid (Tristar)—can be applied as a foliar spray.
•Azadirachtin (Azatin, Aza-Direct)—applied as foliar application. Will require repeated applications.

Stanton Gill

Extension Specialist

Stanton Gill is an extension specialist (professor-ranked principal agent) in IPM and entomology with the University of Maryland Extension, based at the Central Maryland Research and Education Center in Ellicott City. He is also a professor in the Landscape Technology Program at the Germantown Campus of Montgomery College. Contact him at [email protected]