Blueberry Scorch (Blueberry scorch virus)

Damage

Blueberry scorch, caused by Blueberry scorch virus (BlScV), is a serious disease that was first detected in BC in 2000 and has since become widespread in all blueberry production regions. BlScV can spread by as much as 5% annually in infected fields. Damage caused by BlScV can be severe, resulting in significant yield losses on infected plants. Plants do not recover.

Symptoms

Symptoms of BlScV infection may not develop for one or more years following infection (latent period). Symptoms usually appear during bloom, and may include blighting of blossoms and young leaves, leaf chlorosis and dieback. While blossoms and/or leaves may be blighted, the shoot tissue remains green. On some varieties, blighted blossoms may be retained on the bush into the following year. Subtle symptoms may be observed on some varieties, most notably ‘Bluecrop’. These symptoms can include yellowing of leaf margins or leaf mottling, overall pale color, low number of blossoms and a ‘twiggy’ appearance to the bush. Sometimes red line patterns may develop on the leaves of infected cultivars in the fall. Infected plants have decreased or no fruit production.

There are believed to be at least 5 different strains of BlScV present in BC. These strains are closely related to the East Coast and Northwest strains and more research is needed in evaluating their impacts on different blueberry varieties.

Disease Cycle

In BlScV-infected plants virus levels are highest in the buds during the spring and numbers decrease rapidly as the plant grows. BlScV is transmitted non-persistently by aphids. The virus does not replicate in the aphid and it must be transmitted fairly quickly to new plants in order to survive. The resident aphid of highbush blueberry (the Blueberry aphid, Ericaphis fimbriata) and migrant aphids (aphid species that feed briefly on blueberry while moving from one crop to another) have both been shown to transmit BlScV.

BlScV can also be transmitted through infected planting stock. A cutting taken from an infected plant will be infected with BlScV. Since propagation materials are widely distributed, this mode of virus transmission can be very serious, resulting in long-distance spread of BlScV.

Monitoring

Fields should be closely monitored for symptoms of BlScV starting during bloom. Laboratory testing is needed to confirm the presence of BlScV. If any suspicious symptoms are observed, submit a sample to the BCAGRI Plant Diagnostic Laboratory consisting of about 10 healthy green leaves from around the symptomatic area. Flag the sampled bushes so they can be located again should removal be necessary. Samples can be collected from plants showing suspicious symptoms from bloom until late-August.

Fields should be monitored for aphids. See Aphids in this section.

Management

Cultural Control

Only use virus-free planting stock that has been grown according to an accepted propagation protocol includes virus testing.

Manage aphids. See Aphids in this section.

Remove infected plants including the root system. Infected plants will act as a source of new infection for surrounding healthy plants, as long as they remain in the field.

Biological control

None.

Chemical control

None.

Blueberry Premature Fruit Drop

Damage

This disorder was first described in BC in 2000. It causes partially developed fruit to drop prematurely resulting in virtually no fruit production on affected plants.

Symptoms

At bloom, red streaking appears on new leaves and developing flowers. As the season progresses the red colouration of the leaves disappears and by harvest time no visible foliar symptoms can be observed. Fruit initially develops normally, but only attains a size of 3-5 mm and usually drops several weeks prior to harvest. Because there is no fruit load, affected plants are much more vigorous and upright than healthy plants and can be easily identified at this time. Any fruit that is retained on affected bushes is often misshapen. Symptoms recur every year on affected plants. This condition has been observed only in the variety Bluecrop.

Disease Cycle

This disorder spreads from year to year within affected fields. The causal agent has not been determined yet. Research is underway to identify the causal agent of the disorder.

Monitoring

Inspect fields at bloom for red streaking on the leaves and/or blossoms. Following fruit set look for plants which drop immature berries. At harvest watch for individual plants which are particularly vigourous, upright and stand above the field canopy.

Management

Cultural Control

Remove affected plants

Blueberry Shock (Blueberry shock virus)

Damage

Shock virus was first detected in BC in 2001. It has since been found in many fields throughout the Fraser Valley.

Symptoms

Blossoms and new shoots blight at early bloom. At this stage, scorch virus and shock virus symptoms may look the same although shock symptoms usually occur earlier in the season. Plants recover and later in the season produce a second flush of growth.Infected bushes appear normal by late summer, although there will be little fruit. All varieties appear to be susceptible, but Berkeley, Bluegold, and Bluetta are very susceptible.

These symptoms occur for 1 to 4 years and then the plant appears to recover. A good crop can be produced in well-managed fields although recovered plants are still infected with the virus. Pollen produced from these plants is contaminated with the virus. Thus, the virus can spread to nearby healthy bushes during pollination.

Disease Cycle

It is spread by infected planting stock and by insects carrying infected pollen to healthy plants.

Monitoring

Watch for symptoms of the virus at early bloom. Laboratory testing is necessary for correct identification. If any suspicious symptoms are observed, submit a sample consisting of 10 healthy green leaves from around the symptomatic area to the BCAGRI Plant Diagnostic Laboratory. Flag the sampled bushes, should control be necessary. Samples can be collected from plants showing suspicious symptoms from bloom until late-August.

Management

Cultural control

Only use virus-free planting stock that has been grown according to an accepted propagation protocol includes virus testing.

There are no known resistant varieties.

Do not establish new plantings next to virus-infected fields.

Biological control

None.

Chemical control

There are no chemical controls.

Blueberry Mosaic (Blueberry mosaic virus)

Damage

Mosaic is becoming more common in BC blueberry fields. Losses due to this condition are difficult to assess, but reports from other regions show a 15% yield reduction on mosaic-infected Bluecrop.

Symptoms

Affected leaves show mild to bright mottle and mosaic patterns of chrome yellow, yellow, and yellow green. Infected leaves may also show areas of pink colouration. Mottle and mosaic symptoms may vary depending on the blueberry variety. Mosaic symptoms typically appear on basal shoots only. However, in some cases, symptoms can be seen on the whole bush. Detection is difficult as infected bushes do not always have symptoms each year. Fruits on diseased bushes ripen late and are poor quality.

Disease Cycle

The disease cycle of blueberry mosaic virus is not fully understood. Blueberry aphids may play an important role as a vector in virus transmission and spread in the field. The virus may be carried on planting stock as well.

Monitoring

Pay close attention to and do periodic scouting for symptoms on leaves. Monitor and manage blueberry aphid populations. Mosaic virus is difficult to confirm and can be confused with other disorders. Graft-transmission is the only test available to confirm the presence of mosaic. This is an expensive procedure and not frequently used.

Management

Cultural control

Use virus-free planting stock. Prune and fertilize to maintain good vigour Removing infected plants is the only control but may not be cost-effective.

Biological control

None.

Chemical control

None.