Both blueberry stunt (phytoplasma) and viruses (red ringspot, mosaic, necrotic ringspot and shoestring) are caused by microorganisms that cannot be grown in artificial culture. They only survive and reproduce inside the host plant or inside an insect vector (carrier). This makes these diseases difficult to diagnose because the actual disease organism is never seen -- there are no spores, mold or bacterial ooze as seen with diseases caused by fungi and bacteria.
Phytoplasmas and viruses do share a general class of symptoms, and these are the basis for the names of the diseases. For instance, blueberry stunt causes stunting, ringspot viruses produce rings on foliage, blueberry shoestring virus causes long, thin, strap-shaped leaves. They also share characteristics of transmission -- they are carried by insect vectors and can also be spread by propagation if cuttings are taken from infected plants.
Stunt is the most important of these diseases in North Carolina. The viruses listed are those that have been observed in NC fields; however, they do not seem to spread here as they do in blueberry growing areas further north. In most cases, viruses appear only when infected planting stock has been imported from northern nurseries. Perhaps we lack the insect vectors necessary to transmit viruses, or warmer temperatures in NC prevent survival of the virus particles.
- Typical symptoms of blueberry stunt.
- Close-up of stunted leaves.
- Young plants may carry stunt from infected cutting wood.
The stunt organism overwinters in the vascular tissue of infected stems and roots. When leaf symptoms become apparent in early spring, the phytoplasma bodies increase in number in the sieve tubes of the leaves and cause yellowing along the margins and between lateral veins and reduced plant vigor. Leaf cupping, interveinal chlorosis and shortened internodes give the young branches a bushy appearance. In late summer or early fall, the chlorotic areas in leaves redden brilliantly. Fruit on infected plants never develop normal sugar content and ripen late or not at all. The causal agent is transmitted in the field by the sharpnosed leafhopper and by cuttings taken from stunt-infected plants.
Red Ringspot (Virus) is of minor importance in North Carolina but it is a problem in New Jersey and has spread to other areas on diseased plants. Symptoms of red ringspot are distinct red spots and rings, and oak-leaf patterns that first develop on older leaves. Symptoms occur only on the upper leaf surface, and may or may not develop on the stem. Differences in cultivar susceptibility range from apparent immunity (Jersey) to very susceptible. Berries produced on infected cultivars are small, often deformed and ripen late. On some cultivars (Darrow), the berries may develop blotches and rings before stem and leaf symptoms appear.
Mosaic (Virus) occurs in many commercial blueberry plantings. Affected leaves are mottled with yellow and yellow-green areas. The degree of mottling will vary with cultivar and leaf position on the plant. Lower leaves may be mottled while the upper leaves show no symptoms.
Necrotic Ringspot (Virus) symptoms include stunting, twig dieback and chlorotic spots and rings on new leaves. Severe leaf infections may result in reduced leaf size and distortion. This virus is thought to be transmitted by a nearly microscopic worm called a nematode. Nematodes live in the soil, and those that feed on plants can transmit viruses.
Shoestring (Virus) affects leaves, twigs, flower and fruit. Affected leaves are narrow, wavy and often distorted, and are usually found at the base of the bush. Red streaks or blotches may be present on affected twigs. Flowers on affected twigs may be deformed and immature fruit develop a red to purple cast on part or all of the upper half of the berry. Shoestring is a serious virus disease of blueberry in Michigan. The disease is transmitted by the blueberry aphid.
Eradication or roguing consists of immediate removal and destruction of any plant showing symptoms of the disease. This is a critical part of blueberry stunt control and the only control measure available for viruses once symptoms appear in the field. In the case of stunt, it is probably a good idea to spray the infected bush with an insecticide before disturbing it to kill any leafhoppers that may be present. Otherwise when you cut down the infected bush they will fly off, carrying the disease to adjacent healthy bushes.
Vector control consists of a regular spray schedule designed to coincide with peaks in the insect vector population. The sharpnosed leafhopper that carries (vectors) blueberry stunt disease has three generations each year, peaking in May, July and October. Consult the blueberry spray schedule or the NC Agricultural Chemicals Manual for appropriate insecticides and rates of application.
Virus-and Stunt-free propagation depends on the use of tested, true-to-variety cuttings. Growers who do their own propagating should establish nursery areas that are maintained free of disease through continuous roguing and insect vector control. Scouting prospective fields the previous season ensures that no symptomatic bushes will be used as a source of cuttings.