Three viruses are recorded infecting vegetable brassica crops in Western Australia — turnip mosaic virus, cauliflower mosaic virus and beet western yellows virus (also known as turnip yellows virus). These three viruses occasionally cause significant economic loss but their occurrence is spasmodic. Vegetable brassica crops include cauliflower, broccoli, cabbage, Chinese cabbage, Brussels sprouts, radish, turnips and swedes. These viruses also infect canola.
Turnip mosaic virus (TuMV)
All vegetable brassica crops are susceptible to TuMV (formerly called cabbage black ringspot virus) including broccoli, Brussels sprouts, cauliflower, cabbage and Chinese cabbage. Other susceptible crops include canola, lettuce, mustard and rhubarb.
The virus has a wide natural host range, including many weeds such as wild radish which can act as reservoirs for infection.
TuMV occurs worldwide. In Australia, it has been reported infecting plants in many brassica growing areas. In Western Australia, it infects vegetable brassica and canola crops as well as certain weeds. Its incidence in canola and vegetable brassica crops is low.
Wild radish is the most important weed host identified so far in Western Australia.
Severe mosaic symptoms commonly occur in leaves of infected plants. It also causes chlorotic (yellow) ringspots in young leaves. In older leaves, these ringspots develop into yellow or brownish spots surrounded by circular or irregular necrotic (dead) rings. Necrotic streaks, flecks and patches may also occur and plants can be deformed and stunted (Figures 1 and 2).
Infection occurs initially in the field, but necrotic spots may develop after harvest during storage. These symptoms are on internal leaves, only showing when an infected cabbage is cut open.
TuMV is transmitted by a wide range of aphids including green peach, turnip and cabbage aphids. Aphids pick up the virus from either TuMV-infected crop plants or nearby infected weeds and then spread it to healthy brassica crop plants. Climatic conditions that favour the build-up of aphid populations increase the spread of the virus.
Cauliflower mosaic virus (CaMV)
CaMV infects many vegetable brassica crops including broccoli, Brussels sprouts, cabbage and cauliflower but does not infect non-brassica crops. It also infects some brassicaceous weeds such as wild radish which act as infection reservoirs.
CaMV is found in temperate regions worldwide. It infects vegetable brassica and canola crops in Western Australia.
In cauliflower, symptoms on leaves can be initially hard to see in the field. Later in the season, mosaic patterns are easier to observe and veinal chlorosis (vein clearing) becomes prominent (Figure 3), plants are stunted and heads are small and poor quality. In other vegetable brassicas, CaMV-infected plants often do not show any symptoms.
CaMV is spread by a number of different aphid species such as the green peach and cabbage aphids, both of which colonise vegetable brassicas. As with TuMV, the virus is picked up from infected crop plants or weeds and spread to healthy brassica crop plants.
Beet western yellows virus (BWYV)
BWYV is also known as turnip yellows virus (TuYV). BWYV infects all vegetable brassica crops, canola and brassicaceous weeds. It has a very wide host range, including many other non-brassica crop plants and weeds from a range of plant families.
BWYV is distributed throughout temperate regions of the world. In Western Australia, it commonly infects canola crops but may also be found in vegetable brassica crops. Wild radish is a key host and acts as a reservoir for the virus. Many non-brassica crops and native plants can also become infected, acting as reservoirs for spread to brassica species.
The virus causes reddening of lower leaves and plant stunting, resulting in reduced yield. Its symptoms are easily confused with those of nutritional imbalance, herbicide spray damage, waterlogging or other stress factors. In cabbage, BWYV also induces tip burn in internal leaves, particularly during storage.
BWYV is spread predominantly by the green peach aphid which colonise brassica crops. Once an aphid is infected with BWYV, it transmits the virus for the rest of its life, so it is important to control aphid populations. Most populations of the green peach aphid in Western Australia are resistant to ‘old chemistry’ insecticides.
Control of TuMV, CaMV and BWYV
- Plant healthy brassica seedling transplants.
- Destroy all old crops promptly once finished, as old plants are potent sources of infection for spread to non-infected crops.
- Avoid planting brassica crops sequentially in close proximity.
- Sow non-host barrier crops — cereal perimeter surrounding the brassica crop.
- Rotate brassica crops with non-host crops — tomato, celery — to help break the disease cycle.
- Manipulate planting dates to avoid exposing vulnerable young plants at times of year when peak aphid populations develop.
- Remove all weeds and volunteer crop plants in and around crops that might harbour aphids and virus.
- Remove plants with virus symptoms within the crop as these are an infection source.
- Have a brassica free growing period to break the infection cycle by removing the host for aphids.
For TuMV only
- Use varieties with resistance to TuMV if available. Resistant Chinese cabbage varieties are commercially available.
For BWYV only
- Apply registered ‘new chemistry’ insecticides as required to control green peach aphids. Insecticides from different chemical groups should be used to prevent the aphids becoming resistant.
Important note: Spraying insecticides to control TuMV and CaMV is not a good management technique because insecticides do not act fast enough to prevent the rapid spread of these viruses by aphids.
Always read the label
Users of agricultural or veterinary chemical products must always read the label and any permit before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this web page.
Figures 1 and 2 were supplied by Queensland Department of Agriculture and Fisheries.
Figure 3 was supplied by South Australian Research and Development Institute.
This information is based on earlier DAFWA and Horticulture Innovation Australia (HIA) supported research and extension conducted by Lindrea Latham and Roger Jones.