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Ministry of Agriculture, Food and Rural Affairs

VIRUSES

Early yellowing from Raspberry Bushy Dwarf Virus in spring Raspberry Bushy Dwarf Virus Viruses Viruses Leaf mottling
Click to enlarge.

Beginner

Identification

  • A virus is a tiny microbe made up of protein particles
  • Viruses require a living host in order to grow and multiply.
  • Viruses move systemically in the phloem. Once a plant is infected, the whole plant can carry the virus particles. 
  • There are no chemical controls for viruses.
  • Symptoms of virus include leaf curl, pucker, yellow-mottled discolouration, small crumbly berries, low yields, reduced vigour.
  • Symptoms vary with cultivar, weather conditions and the type of virus or combination of viruses in the plant.  
  • Sometimes infected plants do not show symptoms. Symptoms are usually more severe if the plant is infected with multiple viruses.
  • Viruses can be spread by different vectors, such as aphids, nematodes, pollen, whitefly, and thrips.  Viruses can also be moved from farm to farm on infected planting material.
  • Diagnosing viruses can be difficult, and requires services of a diagnostic lab.
  • Common virus diseases in Ontario include Tomato Ringspot Virus (ToRSV), Raspberry Bushy Dwarf Virus (RBDV), and a group of viruses known as Potyvirus.

Often Confused With
Herbicide Injury and Nutrient Deficiency
Drought
Poor Pollination and Tarnished Plant Bug Injury

Period of Activity
Virus diseases are spread when vectors are present and active. Thus, transmission rates are usually higher during the growing periods. However, once a plant has become infected, it will harbour the virus for the remainder of its life. Symptoms are best observed during spring as leaves are growing and expanding. Hotter weather can suppress virus disease symptoms.

Scouting Notes
Leaves of virus-infected plants may not always show virus-like symptoms. Symptoms may appear in the spring and disappear again. The existence of symptoms does not indicate the presence of a virus. To increase the chance of detecting the virus, sample young leaves and shoot tips in the spring and fall when plants are actively growing and the weather is cooler. Diagnosing viruses is difficult. Rule out other potential causes first. The Pest Diagnostic Clinic at the University of Guelph can diagnose some virus diseases using ELISA test kits.

Thresholds
None established.

 

Advanced

Identification
A virus is a tiny microbe made up of protein particles that can only be seen with an electron microscope. Viruses require a living host in order to grow and multiply. They move systemically in the phloem. Once a plant is infected, the whole plant and all plant parts can carry the virus particles.  Unlike other plant pathogens, there are no chemical controls for viruses.

Viruses disrupt normal cell processes in infected plants and cause various symptoms.  These include leaf curl, pucker, yellow-mottled discolouration, small crumbly berries, low yields, reduced vigour. Symptoms vary with cultivar, weather conditions and the type of virus or combination of viruses in the plant.  Sometimes infected plants do not show symptoms. Symptoms are usually more severe if the plant is infected with multiple viruses.

Viruses can be spread by different vectors, such as aphids, nematodes, pollen, whitefly, and thrips.  Viruses can also be moved from farm to farm on infected planting material.

Diagnosing viruses can be difficult. Many labs carry ELISA test kits, which will detect the presence of virus in plant tissue. However, the antibodies used in the ELISA test kits have only been developed for certain viruses.
New technology can be used to diagnose viruses by detecting specific patterns in their genetic makeup. These diagnostic tools (eg. RT-PCR) are still quite expensive and available at only a few labs. However, technology in this field is changing quickly and better diagnostics will be available in the future.

Other highly specialized techniques can be used to detect viruses in clean plant programs and quarantine situations.  Viruses can be detected by grafting or mechanically inoculating infected plants onto indicator plants. These indicator plants are known to show symptoms when infected with certain viruses. 

Common virus diseases in Ontario include Tomato Ringspot Virus (ToRSV), Raspberry Bushy Dwarf Virus (RBDV), and a group of viruses known as Potyvirus.

Often Confused With

Herbicide Injury and Nutrient Deficiency
Herbicide injury, nutrient deficiency and viruses can result in weak plantings and yellowed or curled leaves.  Laboratory diagnosis is required to confirm the presence of virus after other factors have been ruled out.  

Drought
Both drought and virus infections can cause poor vigour and yellowing or curled leaves.

Poor Pollination and Tarnished Plant Bug Injury
Poor pollination, insect injury and virus infections can result in crumbly berries and low harvest yields. If the problem is due to virus infection, the same plants will show symptoms every year and the symptoms will spread slowly up and down the row.

Biology

Tomato Ringspot Virus (ToRSV)
Tomato Ringspot Virus occurs primarily in red raspberry and some blackberry varieties, but does not infect black raspberry. It is also found on some common weeds, such as dandelion and chickweed. ToRSV is seed-borne and infections will be latent in new plants that arise from ToRSV-infected seed.
Dagger nematodes, Xiphinema americanum, transmit ToRSV and may harbour the virus for several months before infecting new plant tissue. (For more information on dagger nematodes, refer to the Nematode infosheet in the Diseases and Disorders Section).

Infected plants are usually found in circular or oval patches in the field, which expand outwards over time due to the movement of nematode vectors. In spring, raspberry leaves exhibit yellow rings or line patterns and vein chlorosis. Infections are most visible in plants along the outer edges of these patches. As leaves expand during the summer, symptoms fade and plants will often appear normal, except for their slightly stunted growth and lack of vigour. Fruit produced by infected plants is crumbly and yields are low.

Raspberry Bushy Dwarf Virus (RBDV)
This virus primarily affects red and black raspberries, however, many cultivars remain symptomless. Often, the most obvious symptoms are evident in early spring, when plant growth is slow and weather is cool.  Some cultivars, (eg. Autumn Britten, Caroline) will develop  severe yellowing of foliage in the spring, which fades or becomes less obvious as the plant continues to grow. Other cultivars ( eg. Qualicum) develop distinct pale or white bleached areas between on the veins. When RBDV infects a plant in combination with other viruses, symptoms tend to be more severe. When RBDV and black raspberry necrosis virus (BRNV) infect a plant simultaneously, the affected plant will exhibit stunted growth and a bushy appearance due to an increased number of shoots.

This virus is carried from infected plants to healthy ones on pollen moved by wind, or by bees.  Pollination is required for infection, therefore only flowering plants can be infected. If RBDV-infected pollen is transferred to an already RBDV-infected plant, the resultant seed can become infected. Thus, the virus can over-winter in raspberry seeds as well as other plant tissues.

Period of Activity
Virus diseases are spread when vectors are present and active. Thus, transmission rates are usually higher during the growing periods. However, once a plant has become infected, it will harbour the virus for the remainder of its life. Symptoms are best observed during spring as leaves are growing and expanding. Hotter weather can suppress virus disease symptoms.

Scouting Notes
Leaves of virus-infected plants may not always show virus-like symptoms. Symptoms may appear in the spring and disappear again. The existence of symptoms does not indicate the presence of a virus. To increase the chance of detecting the virus, sample young leaves and shoot tips in the spring and fall when plants are actively growing and the weather is cooler. Diagnosing viruses is difficult. Rule out other potential causes first. The Pest Diagnostic Clinic at the University of Guelph can diagnose some virus diseases using ELISA test kits.

Thresholds

None established.

Management Notes

The following steps help minimize the spread of -viruses:

  • Use virus-tested, disease-free planting stock from an accredited plant propagation program.
  • Destroy all nearby wild raspberries and blackberries.
  • Set new plantings at least 16 m from older plantings to reduce spread of aphids.
  • Black and purple raspberries are very susceptible, especially black raspberry variety Jewel. Do not plant red raspberries close to purple and black raspberries.  
  • Rogue out and completely destroy all infected plants as soon as symptoms of mosaic or leaf curl appear.
  • Prior to establishing a new raspberry planting, test the soil for dagger nematodes. Fumigate with a nematicide if nematodes are present. Refer to OMAFRA Publication 360: Fruit Crop Protection Guide (Chapter 3 Berry Crops) for information on soil fumigation.
  • Remove broadleaf weeds as they may serve as alternate hosts to ToRSV and their seeds may spread the virus to previously healthy raspberry plantings.
  • For some viruses, insecticides to control aphids will reduce the spread of virus infections, but will not completely protect plantings. See OMAFRA Publication 360: Fruit Crop Protection Guide (Chapter 3 Berry Crops) for more information on aphid control.