Viral diseases

Viral diseases – an overview:

This page provides an overview of viral diseases in vegetable crops. The related tools listed at the end of the page provided detailed information about the identification, symptoms, and management of viral diseases. It is important to have a plant diagnostics laboratory confirm the pathogen causing any diseases in a crop so that the disease can be appropriately managed.

Viruses cause major damage to many Australian vegetable crops. They are immobile and are usually transmitted from one plant to another by a living organism called a vector or carrier. The most significant vectors of plant viruses include aphids, whiteflies, thrips, and leafhoppers, which have piercing sucking mouthparts that allow the insects to access and feed on the contents of the plant cells. Viruses can also be transmitted by other insects, mites, nematodes, fungi, infected pollen or vegetative propagating material, contact between plants, and infected or contaminated seeds.

The virus is transmitted by sap-sucking insects in two ways: persistent transmission and non-persistent transmission, which relates to the time taken by an insect to acquire and transmit the virus

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Viruses, crops affected, and damage caused:

Means of transmission: Aphid

Virus	Host plants	Primary damage
Bean common mosaic virus	Beans.	Mottling, curling, and malformation of leaves and a general stunting of the plant.
Turnip mosaic virus	Brassicas.	Mottling and black necrotic spots in cabbage, cauliflower, and Brussels sprouts; mosaic with leaf distortion and stunting in turnip, radish, and Chinese cabbage.
Cucumber mosaic and potato mosaic virus	Capsicum; tomato; potato; celery.	Chlorosis and blistering mottle of leaves; plants are stunted.
Carrot virus Y	Carrot.	Severe root symptoms in carrots including shortened roots, knobbiness and severe distortion.
Papaya ringspot; Watermelon mosaic virus; Zucchini yellow mosaic virus	Cucurbits.
Johnson grass mosaic virus	Sweet corn.
Celery mosaic virus	Coriander; celery; parsley; parsnip.	Plants stunted with severe clearing on leaves, leaf-up curling and chlorosis.
Sweetpotato feathery mottle virus	Sweetpotato; peas.
Subterranean clover stunt	Beans.
Beet western yellow virus	Brassicas.
Potato leafroll virus	Potato.	Stunted plants; lower leaves roll upwards at the margins, develop leathery texture and die prematurely.

Means of transmission: Thrips

Virus	Specific vector	Host plants	Primary damage
Tomato Spotted Wilt Virus (TSWV)	WFT, tomato thrips, and onion thrips.	Capsicum; tomato; eggplant; lettuce; celery; peas;potatoes; sweet basil.	Ringspots, line patterns, mottling, and chlorotic blotches on leaves.
Iris Yellow Spotted Virus (IYSV)	Only by onion thrips.	Onions; garlic; leeks; spring onions; herbs.	Eye-like or diamond-shaped spots on leaves and seed-stalk in onions; extensive chlorosis or yellowing.
Capsicum Chlorosis Virus (CaCV)	Melon thrips and tomato thrips.	Capsicum; tomato; chillies.	In capsicum: yellowing on leaf margins and between veins of young leaves; In tomato: chlorotic spots and blotches on leaves that become mottled.

Means of transmission: Whitefly

Virus	Specific vector	Host plants	Primary damage
Begamoviruses - Tomato yellow leaf curl virus (TYLCV)	Silverleaf whitefly	French beans; cucurbits; capsicum; several weed species.	Affected plants stunted' interveinal chlorosis develops; leaves bent downwards and stiffened; fruit quality reduced.
Beet pseudoyellows virus (BPYV)	Greenhouse whitefly	Lettuce; beet; endive; cucumber; common weeds.	Chlorosis or yellowing between veins in older cucumber leaves, with symptoms spreading to younger leaves; severely affected plants stunted.
Tomato torrado virus (ToTV)	Both Greenhouse and Silverleaf whitefly, with the former responsible for spread in Australia.	Capsicum; eggplant; weeds including Amaranthus, Atriplex, Chenopodium, and Malva.	Early symptoms include necrotic or dead spots; affected areas may fall out, leaving shot holes in the leaflets; necrosis and mottling also extends to the remainder of the leaves.

Means of transmission: Tobamoviruses

	Virus	Host plants	Primary damage
Tobamoviruses: Not transmitted by insects, are highly infectious and very stable in the environment. Can survive on implements, contaminated clothing, crop, root debris, and also on seed.	Tobacco mosaic virus	Crop plants and weeds: eggplant; tomato; bok choy; choy sum; bitter melon; Chinese mustard; long melon; snake bean; Chinese cabbage.	Mosaic; mottling; leaf distortion; and sometimes leaf death and defoliation.
	Tomato mosaic virus	Tomato; capsicum.
	Pepper mild mottle virus	Capsicum, including chillies.	A mild mosaic or mottle.

Management:

Often the intricate relationship between the virus, host plants, and the vector or the carrier, creates problems in developing effective management systems. However, by using a combination of management options, or an Integrated Pest Management (IPM) approach, disease control can be successfully implemented.

Exclusion or avoidance

quarantine; grow crops in regions where the virus seldom occurs or during periods when the virus or its vector are at a low activity level; and use virus-free seedling transplants.

Reduction in virus spreading sources

control weeds and other virus hosts and insect vectors; destroy old crops promptly; separate new crops from maturing crops; and avoid overlapping crops, particularly year-round cropping.

Protection of the host plant

plant virus-resistant varieties; use barrier crops to reduce insect vector activity in the crop; use insecticides to protect plants; and use highly reflective mulches and oil sprays to deter insects.

A key aspect of virus disease management is to accurately identify the virus causing the disease and then implement appropriate management strategies. Monitor and trap insect pests which transmit viruses.