Blueberry Scorch Virus Prevention: Beyond Chemicals

Blueberry Scorch Virus is a serious disease that threatens blueberries throughout British Columbia. Currently, there is no cure for the virus, so prevention is the only defense. A proactive strategy is critical as plants typically do not show symptoms until 1-2 years following initial infection.

Symptoms are most easily observed during bloom: blossoms and leaves blight and dry up rapidly. Sometimes only blossoms are infected, or a few infected shoots may be visible. Even with minimal symptoms, the entire plant will be infected and never regain healthy productivity. Plant decline occurs over several years, and even healthy-appearing new growth will be infected. From the very start of infection, plants remain a reservoir for the virus whether they appear symptomatic or not.

The BC Ministry of Agriculture will test plants for presence of the virus, and samples can be submitted in person, or by mail. Find more info here. When testing plant material for this virus, consider sampling from plants that appear healthy as well. Due to the long incubation period, and varietal differences in the severity of visible symptoms, plants that appear healthy may also carry the virus. A wider scope of sampling will give a more accurate picture of the area infected.

Once a plant is infected, it can spread the virus to healthy plants through mechanical transmission and insect vectoring.

Steps to minimize risk of infection:

• Only plant new stock material that is proven virus-free.
• Quickly remove any infected plants, including as much of the root mass as possible.
• Be diligent with cleaning and sanitation to prevent the spread of infected insects or plant material.
• Control aphid populations to reduce chances of insect vectoring.

Aphids may be controlled using a chemical spray program.

There are also options for organic growers or those who want reduced chemical applications. The most prevalent species of aphid in BC blueberries can be controlled using the parasitic wasp, Aphidius ervi. This wasp needs the aphid to complete its life cycle and is very effective at finding even individual aphids. Since parasitic wasps can fly, very few release points are needed with this biocontrol agent (BCA). As with most beneficial insects that are released preventatively, repeated releases will be needed to ensure continuous protection.

The minute pirate bug, Orius, is another great option for preventing aphid establishment. It is a generalist predator that hunts a variety of soft-bodied insect pests, making them very useful in any biocontrol program. Orius are quite mobile and need relatively few release points per acre, however, Orius may enter diapause when daylength is less than 12 hours per day. Due to their wider range of prey, and ability to feed on pollen as well as insects, they can establish a long-term population more readily than some BCAs but may still require multiple releases before establishment is achieved.

Consider implementing a regularly scheduled release of one or both beneficial insects to get all the benefits of a preventative program. Preventative releases are a more effective strategy to limit populations of virus-spreading aphids from building in your crop, though they can also be used as part of a more reactive treatment plan.

In cases where aphid population have established, a different strategy may be used. First, use a “gentle” knock back spray such as insecticidal soap to reduce the aphid population. As aphids reproduce rapidly and beneficial insects are a special-order item, this will allow time to place the order and receive the insects for your treatment. The soap does not have a long residual, and therefore has minimal impact on beneficial insects. Soaps are an indiscriminate insecticide and can impact beneficial insects that contact the product at time of application. It is therefore recommended they be used for spot sprays and be applied well before the beneficial insect release so that it has time to dry and will pose no risk.

There are many beneficial insects available and choosing the best one depends on your unique circumstances. The BCAs used in the preventative program may be released at a higher rate to help combat an active infestation, or other predators may be released in hot spots as well.

Aphidoletes are predators that feed only on aphids and are not limited by type of aphid species. Lacewings and ladybugs may also be released for hot spot treatment, both of which are generalists that will feed on many species of aphids and other soft-bodied insects in high numbers. Any of these three may establish populations in the field, but are highly mobile as adults, and may move out of the area once they grow from their ravenous larval stage to their flying adult stage. Since larval stages of these BCAs crawl rather than fly, these would need to be introduced directly on to the plants requiring treatment. Repeated applications are recommended until either the aphid populations are below the action threshold, or the beneficial population is successfully established in the field. The methods used to determine if the beneficial insect has established successfully vary by species and can be monitored as part of the regular scouting schedule.

Talk to your local TerraLink field advisor for help on identifying the best Blueberry Scorch Virus control strategy for your operation.