Virus diseases are spread to healthy blueberry plants by vectors that include primarily aphids, nematodes, leafhoppers, and occasionally honeybees, which can spread virus-infected pollen. Virus diseases also are spread by diseased plants from infected nursery stock. Once a bush is infected with a virus, it remains infected for the life of the plant.
Virus diseases cannot be controlled like a fungal or bacterial disease with chemicals. Prevention and sanitation measures are the best control for virus diseases. Suggested control practices include planting virus-free clean stock plants in clean soil, destroying alternate hosts such as wild blueberries that may harbor viruses, removing and destroying plants that are diseased or suspected of having virus infections, and controlling insect and nematode vectors.
Blueberry Scorch Virus
Blueberry scorch virus can cause severe flower and leaf browning in highbush blueberries. All varieties of highbush blueberry are considered susceptible.
This disease is spread by aphids, with transmission from infected to uninfected plants taking place in a matter of minutes or hours. Aphid control is the best method available to stop the infection of the entire field. The virus spreads outward from the first plants infected.
The symptoms of blueberry scorch first appear during bloom in late April to early May. Symptoms in some varieties consist primarily of blossom blight with a few brown leaves near the blighted flower clusters and some marginal yellowing of leaves produced on older wood. The blighted blossoms often are retained throughout the summer but fail to develop into fruit. Affected bushes develop symptoms every year. Initially, only one or a few branches are affected. Bushes appear to recover as the season progresses; however, yield is reduced or eliminated. Symptoms reappear in following years with more branches affected. Plants can be killed in 3 to 6 years, with all plants eventually infected. Tolerant varieties may not show symptoms but still serve as sources of inoculum.
Blueberry scorch can spread rapidly. The best method of control is to plant virus-free stock. The spread of the virus has been recorded only over short distances. If no known blueberry scorch exists in close proximity to a grower's field, scorch should not become a problem. The problem occurs when a neighbor has tolerant varieties that are infected with this virus--these will be a constant source of potential new vector-spread infections. If an infection is observed early--when only a few plants are showing symptoms--then an aphid-control program combined with removing and burning diseased bushes over a 3-year period should prevent further spread of this virus. The Blueray, Bluetta, Duke, Chanticleer, Elliott, and Weymouth varieties are susceptible. Jersey is tolerant, and Bluecrop is intermediate.
The blueberry aphid spreads shoestring virus. There is a latent period of 4 years between infection of the plant and expression of symptoms. Shoestring-infected wild blueberries also have been found in the wooded areas.
The most prominent symptoms are elongated reddish streaks about 1/8 inch wide by 1/2 to 3/4 inch long on current-year and 1-year-old stems, especially on the side exposed to the sun. During blossoming, flowers of infected bushes exhibit pinkish to reddish petals. Infected leaves often are straplike, hence the name "shoestring." Many leaves on a bush might appear this way, although in some cases just a few clumps near the crown will show this symptom. A few leaves may show red-vein banding or reddish streaking along the midrib of the leaf. In some cases, an "oak leaf" pattern will show on the leaf blade. Other leaves may be crescent shaped and partially or totally reddened. Infected stems may appear crooked, especially the tip-end half.
Aphid control is critical to preventing the spread of shoestring virus. The first insecticide application should begin when aphids first appear on the terminals of the stems, usually by late May or early June. Two or three sprays may be required throughout the growing season to keep aphid levels low. The long latent period makes identifying infected bushes before they serve as sources of inoculum impossible, so roguing is not feasible or effective. Clean planting stock is critical. Bluecrop shows resistance. Also, diseased wood used for propagation is another way to spread the virus from one field to another.
Tomato Ringspot Virus Disease
Tomato ringspot virus is vectored by the dagger nematode. This virus can infect many different species of plants, including other fruit crops such as apples, peaches, and raspberries, and weeds such as chickweed and dandelion. Infection spreads slowly.
Symptoms include leaves that are malformed and have circular chlorotic spots on them, 1/16 to 1/8 inch in diameter. In addition, stems, twigs, and branches may exhibit circular, brownish necrotic spots of similar size. Younger terminal leaves tend to be strap shaped and have a mottled pattern (alternating yellowish to greenish stippling). Fruit production may be reduced and infected plants may eventually die. One indication that the disease is spread by the dagger nematode is that symptoms spread slowly in a circular pattern at a rate of about 3 feet per year in all directions.
The best control for this virus is to test the soil for nematodes before planting and avoid following with fruit crops. Weeds, especially dandelion, serve as a reservoir for the nematode and should be controlled. Plant only virus-tested clean stock.
Red Ringspot Disease
The cause of red ringspot virus is unknown. Mealybugs, however, may be involved in transmitting this virus.
Infected stems at least 1 year old often exhibit reddish-brown spots with green centers. The spots, 1/8 to 1/4 inch in diameter, also develop on the upper surfaces of older leaves in mid- to late summer. The powdery mildew fungus can cause similar symptoms on both sides of the leaf. Plants with this disease exhibit a loss of crop; the amount varies with variety. Because the vector has not yet been identified, the primary form of control is the use of virus-free stock and removal of infected bushes. Blueray and Bluetta are especially susceptible.
Stunt is caused by a phytoplasma not a virus. Viruses and phytoplasmas are quite different, but they are often grouped together in discussions of plant pathogens.
Viruses consist only of protein and genetic material (DNA or RNA) and cannot replicate (reproduce) on their own, instead needing to infect cells to complete the process. Phytoplasmas are essentially a type of bacteria without cell walls. Both become systemic throughout the plant.
Stunt is a very important disease of blueberry throughout the United States and eastern Canada. Most varieties of highbush blueberry are susceptible. Stunt can be found in wild highbush and lowbush in the woods. No yield data are available on the losses caused by stunt, but symptomatic bushes are usually less than half the size of healthy bushes, and crop yields vary from very light to none.
Overall dwarfing of the bush is the primary symptom, hence the name "stunt." Small leaves that are cupped downward or puckered are characteristic symptoms. Leaves of infected bushes are often yellow, with yellowing most pronounced along leaf margins and between lateral veins. Midribs and lateral veins usually retain normal green coloration. Yellow areas often turn a brilliant red in the late summer. Stem internodes become shortened, and growth of normally dormant buds causes twiggy branching.
Stunt is actively spread in the field by the sharpnosed leafhopper. The pattern of stunt disease spread appears random. Leafhoppers are strong fliers and may come into a field from a great distance. Insecticides applied on a timely basis to control the leafhopper help keep the disease in check. Also use virus-tested planting stock when establishing a new field.