Tomato yellow leaf curl virus

In March 2006, tomato leaf curl disease was found in cherry tomato crops in the south and west periphery of Brisbane. The disease has been found in many crops, with infection levels ranging from 5 to 100% of plants.

Losses in severely affected crops have been very high and the disease is a major threat to tomato production. In April 2006, infected plants were also found around Bundaberg. By June 2007, the virus was present in the Lockyer Valley, Fassifern Valley, Esk, Caboolture and Redlands areas. Since 2009 it has become a serious production constraint around Bundaberg. In February 2011, it was found in backyard tomato plants in Mareeba on the Atherton Tablelands.

Tomato leaf curl disease is caused by viruses in the Geminivirus family of plant viruses, and is spread by whiteflies.

The virus causing this disease is tomato yellow leaf curl virus (TYLCV). This virus is distinct from tomato leaf curl Australia virus (TLCV), which occurs in the Northern Territory and at several locations on Cape York Peninsula.

Symptoms

TYLCV can be confused with several other tomato conditions such as tomato big bud, tomato yellow top, physiological leaf roll and phosphate and magnesium deficiency.

Tomato yellow leaf curl virus (TYLCV)

• Plants are stunted or dwarfed
• New growth only produced after infection is reduced in size
• Leaflets are rolled upwards and inwards
• Leaves are often bent downwards, stiff, thicker than normal, have a leathery texture, show interveinal chlorosis and are wrinkled
• Young leaves are slightly chlorotic (yellowish)
• Flowers appear normal
• Fruit, if produced at all, are small, dry and unsaleable
• Affected plants tend to be distributed randomly or in patches

Tomato big bud

• Produces enlarged green flowers
• Affected plants tend to be distributed randomly or in patches

Tomato yellow top virus

• Leaflets are reduced in size and become rounded with yellowish, down-curled or up-curled margins
• Affected plants tend to be distributed randomly or in patches
• Can be difficult to distinguish from TYLCV

Physiological leaf roll

• Due to water stress, does not stunt plants
• Young expanding leaf tissue is soft rather than rigid

Phosphate deficiency

• Stiff, stunted plants with a purplish tinge
• All parts of the plant are reduced in size
• More or less evenly distributed throughout a planting

Magnesium deficiency

• Yellowing of the interveinal areas of the middle and lower leaves
• More or less evenly distributed throughout a planting

How it spreads

Tomato leaf curl disease is not transmitted in seed, soil or from plant to plant by handling. It is harboured in infected host plants, some of which may be hosts that do not show symptoms. The virus causing tomato leaf curl disease is spread from plant to plant by silverleaf whitefly (SLW) (the biotype B of Bemisia tabaci). SLW is a serious pest in tomatoes and other vegetable crops in the coastal and some inland areas of Queensland and New South Wales. It is an established pest in Western Australia and cotton production systems in Queensland.

Although the nymphal stages of SLW can acquire virus from infected plants, it is the active adult insects that are responsible for almost all virus spread into and within crops.

SLW adults acquire the virus while feeding, using their piercing-sucking mouthpart to pierce plant cells and suck sap through a stylet. The virus persists in the insect which can then transmit the virus throughout its life. SLW need to feed on infected plants for at least 15 minutes to acquire the virus and then feed on another host plant for 15 to 30 minutes to transmit the virus. Transmission efficiency increases as the duration of the feeding times increases.

Although the transmission efficiency of individual insects may be low, the enormous populations of SLW moving within and between crops can result in rapid spread and high disease levels. Research results are inconclusive, but TYLCV is probably not carried from generation to generation through the SLW egg. This virus is not spread by other sap-sucking insects such as aphids or leafhoppers nor by leaf-eating pests such as grasshoppers, heliothis larvae or beetles.

Host plants

Tomato is the major host of TYLCV, however, many other species are also TYLCV hosts. Some other host plants are:

• Boerhavia erecta (erect tar vine or erect spider vine)
• Capsicum annuum (capsicum or chilli pepper)
• Capsicum chinense
• Cleome viscose (tick weed)
• Croton lobatus (lobed croton)
• Cyanchum acutum
• Datura stramonium (common thorn apple)
• Euphorbia spp.
• Eustoma grandiflora (lisianthus)
• Lycopersicon esculentum (tomato)
• Macroptilium spp.
• Malva parviflora
• other Malva spp. (small flowered mallow)
• Mercurialis ambigua
• Phaseolus vulgaris (beans)
• Physalis spp.
• Polygonum spp. (knot weed or wire weed)
• Sida spp.
• Solanum nigrum (blackberry nightshade)
• Solanum luteum, Solanum villosum (hairy nightshade)
• Wissadula spp.

Control

There are two key points to managing the spread of TYLCV:

• do not move infected or host plants or seedlings, or infected SLW
• control SLW on farms, surrounding vegetation and seedling nurseries.

If moving plants and fruit that host TYLCV to markets:

• within Queensland – plants and fruit may be moved without restriction.
• to interstate markets – movement of fruit is not restricted except tomato fruit attached to its truss. This is not permitted entry to Western Australia.

Good farm management and farm hygiene practices will help manage the spread of TYLCV:

• use seedling plants produced in an area free from virus and whiteflies
• destroy old crops as soon as possible after final harvest
• control SLW adults before destroying crops to reduce the migration of SLW to other crops
• plant new crops as far away as practicable from existing crops which may harbour the virus and its carrier, silverleaf whitefly
• control silverleaf whiteflies using appropriate chemicals, application methods and IPM strategies
• maintain a high standard of weed control within and around crops to reduce hosts of both the virus and silverleaf whitefly.

Some tomato cultivars with resistance to TYLCV are commercially available, but these must be used with good farm management and hygiene practices to keep the resistance.