C10 - Viral Diseases

Introduction

Viral diseases are rarely a major problem in protected herb production systems. Umbelliferous herbs, e.g. parsley, coriander, dill and chervil, are susceptible to several aphid-borne viruses but disease occurrence is rare. Examples of diseases caused by aphid-borne viruses include carrot motley dwarf, celery mosaic, cucumber mosaic and parsnip yellow fleck. Viruses that are transmitted by thrips such as Tomato spotted wilt virus, Iris yellow spot virus and related tospoviruses, have not been found to date on UK herbs.

  

Symptoms

Carrot motley dwarf affects parsley, coriander and dill. The severity of symptoms expressed can vary with plant age and environmental conditions. On young plants or in cool weather (e.g. in unheated polytunnels), green leaves become tinged with red and then turn pale yellow. Symptoms may easily be confused with nutritional disorders. Stunting of roots and foliage can also occur in addition to loss of vigour. Under warmer conditions and on plants infected when they are mature, symptoms are less severe. At higher temperatures of 24^oC or more (e.g. in heated glasshouses), infected plants may be symptomless.

Plants of parsley and coriander infected with Celery mosaic virus develop vein clearing and a distinct light green mottling between the veins on young leaves (Figure 1). On mature foliage, leaflets are narrow, twisted and cupped, and leaves may have a shiny appearance. Yellow/green leaf mottling is also characteristic of Cucumber mosaic virus, which can infect mint as well as umbelliferous herbs.

Herbs such as parsley, chervil and coriander may also be affected by parsnip yellow fleck, which is transmitted by the willow-carrot aphid (see Section B.4). Symptoms include stunting, leaf yellowing or reddening and even the death of growing points in crops such as chervil.

The first case of the notifiable virus, Iris yellow spot virus (IYSV) has recently been confirmed in the UK, on cut flower lisianthus.  IYSV is a tospovirus, closely related to Tomato spotted wilt virus (TSWV) and Impatiens necrotic spot virus (INSV). Chive (as well as onions and garlic) is a known host of IYSV in other countries but not yet in the UK. Symptoms have been well described on onion; white to pale yellow elongated lesions develop along the length of the leaf (Figure 2). On seed onions, the bleached lesions often coalesce to girdle the flower stalk, causing the seed head to topple over. To date, the only confirmed case of IYSV in the UK was an isolated occurrence, and the virus was eradicated under PHSI notice. If any symptoms suspected to be IYSV are seen on susceptible host crops, you must immediately inform your local Plant Health and Seeds Inspector (contact details are available on the Defra website at: http://www.defra.gov.uk/planth/senior.htm) or the PHSI HQ, York, tel: 01904 455174. Also see Section A - Principles of IPM.

Disease sources and spread

Methods of transmission from infected to healthy plants vary with viral species and host plant. For viral diseases of umbelliferous herbs, disease spread occurs most frequently by aphid transmission. Weed hosts (e.g. cow parsley) as well as crop hosts (e.g. parsley, dill, or coriander) may provide a source of viral infection. Thrips species are another common virus vector. For example, IYSV that could potentially infect UK chives, is spread by onion thrips (Thrips tabaci).

Spread of certain virus species can occur via seed and during vegetative propagation when plants are grown from cuttings taken from infected stock plants. Some viruses can persist for short periods in plant sap, and may be transmitted in this way, for example on cutting equipment. Viral spread between plants in close contact with one another (mechanical transmission) may occur occasionally.

  

Conditions for infection

For aphid-transmitted viruses, infection of protected herbs crops is largely dependent on conditions favouring aphids. See Section B.4 for further details.

Certain viral diseases require the presence of two or more viruses in order for transmission, infection and symptom expression to occur. For example, carrot motley dwarf disease results from a mixed infection by two unrelated viruses, Carrot redleaf virus and Carrot mottle virus, which are both transmitted by the willow carrot aphid. Aphid transmission of Carrot mottle virus can only take place when the ‘helper' Carrot redleaf virus is also present. Similarly, transmission of Parsnip yellow fleck virus by the willow carrot aphid is facilitated by the presence of a specific ‘helper' virus called Anthriscus yellows virus. Chervil and coriander are hosts of both Parsnip yellow fleck virus and Anthriscus yellows virus.

  

Integrated disease management

Avoidance of viral diseases relies mainly on the control of the relevant vectors, such as aphids on protected herbs.

  

Cultural control

• Avoid taking cuttings from stock plants with viral symptoms, as viruses can be transmitted in this way.
• Maintain strict weed control in and around glasshouses and tunnels.
• If only a few plants are showing symptoms, then these should be rogued out and disposed of carefully.
• Avoid sequential planting of susceptible host crops where there could be aphid migration.
• See Sections B.4 and B.6 for guidelines on use of cultural control for management of potential virus vectors such as aphids and thrips on protected herbs.

  

Biological control

See Sections B.4 and B.6 for guidelines on use of biological control for management of potential virus vectors such as aphids and thrips on protected herbs.

  

Chemical control (protected herbs)

See Sections B.4 and B.6 for guidelines on use of pesticides for management of potential virus vectors such as aphids and thrips on protected herbs.

• Full details for the use of biological control agents and compatibility of pesticides are available from biological control suppliers or consultants.
• Pesticide approval information in this guideline is current at 31 March 2009. 
• Regular changes occur in the approval status of pesticides arising from changes in pesticide legislation or from other reasons. For the most up to date information, please check with a professional supplier or with the Information Office at the Pesticides Safety Directorate (PSD) Tel: 01904 640500; or on their website (http://www.pesticides.gov.uk/).
• Always follow label recommendations or statutory conditions for use on Specific Off-Label (SOLA) notices of approval.
• Always follow instructions for Pesticide Resistance Management guidelines given on the label or SOLA.
• Growers must hold a paper or electronic copy of the current SOLA before using any product under the SOLA arrangements.  Any use of a pesticide with a SOLA is at grower's own risk.  Relevant SOLAs are sent to HDC members by HDC, or are available from PSD (see above) or from consultants.
• Use pesticides safely.

  

Further information

HDC Factsheet 19/08.  Iris yellow spot virus:  A potential threat to the onion industry.

Davis, R.M. & Raid, R.N. (Eds). 2002. Compendium of Umbelliferous Crop Diseases. USA: APS Press.