Subterranean clover: Virus diseases
Note Number: AG0724
Published: November 1999
SCRLV is a relatively new virus, first found in irrigated areas of northern Victoria in 1965, which can render pastures totally unproductive, especially those locked up for hay or seed production. The incidence of this virus varies from year to year, as the distribution depends on the number of infective aphids entering a stand each season.
The first signs occur around August or September. A mild yellowing occurs between the veins of the oldest leaves, then a reddening of the leaf margin.
The red colour extends across the leaf in 2 to 3 days, and later only a small area at the junction of the leaflets remains unaffected.
The leaf stalks (petioles) and runners do not show any symptoms of SCRLV. After 2 to 3 weeks the reddened leaves turn brown and die, and the plants collapse to the ground. Infected plants may be stunted and fail to set seed.
The subterranean clover cultivar Woogenellup develops yellow rather than red leaves.
Survival: SCRLV carries-over on live plants and alternative hosts.
Environmental conditions: Severity and frequency of SCRLV are dependant on the number of infected aphids and their distribution. Increases in aphid populations are favoured by years with summer and autumn rains which promote plant growth and thus an increase in aphid numbers. Dry springs favour aphid activity and the development of symptoms. Aphid numbers are kept down in years with cold wet weather or the presence of aphid parasites.
Transmission: SCRLV is transmitted by the common potato aphid, Acyrthosiphon solani. This aphid takes in the virus while feeding on infected white clover during the summer and autumn, before moving onto the subterranean clover. The large, green potato aphid can often be found feeding on subterranean clover as early as June. The number of aphids entering a stand is usually small, but each can infect a large number of plants, because the virus can persist within the infective aphid throughout its life span of several weeks. There is no evidence of seed or mechanical transmission of this virus.
Host Range: SCRLV occurs on subterranean clover, white clover, red clover, strawberry clover, medics and French beans. Medics develop only marginal yellowing of the leaves without turning red, however white clover, red clover and strawberry clover do not show any symptoms.
There are no control measures available for SCRLV. Aphid spraying is expensive, and aphid population movements are difficult to predict.
Some subterranean clovers are more tolerant than others, but none are resistant.
Subterranean clover stunt virus (SCSV)
Total losses may occur if the initial infection produced by aphids is spread to all plants by a local build up of aphids. Affected stands can often recover due to the growth of healthy plants that escape infection. Although up to 90% of plants can be infected, the average is usually only 5%.
The main symptoms are severe stunting of the plant, and a distortion and yellowing of new growth. The youngest leaves in the centre of the plant may be small and cup-shaped, while the older leaves turn red.
Survival: SCRLV carries-over on live plants and alternative hosts.
Environmental conditions: Severity and frequency of SCSV are dependant on the number of infected aphids and their distribution. Increases in aphid populations are favoured by years with summer and autumn rains with favour plant growth and thus an increase in aphid numbers. Dry springs favour aphid activity and the development of symptoms. Aphid numbers are kept down in years with cold wet weather or the presence of aphid parasites.
Transmission: SCSV is transmitted by the green peach aphid, Myzus persicae and the cowpea aphid, in autumn and early winter. There is no evidence of that this virus is seed-borne.
Host Range: SCSV occurs on subterranean clover, crimson clover, suckling clover, snail clover, button clover, medics, broad bean, French bean and peas.
Severe outbreaks of SCSV occur infrequently, and control measures are not generally recommended.
Some of the older outclassed cultivars such as Tallarook and Howard are resistant.
Cucumber mosaic virus (CMV)
This virus is capable of reducing both seed and herbage production plus longevity of the stand. Virus infected plants are more susceptible to root rot and produce fewer and less effective nodules.
CMV causes dwarfing, a curling down of the leaflets and a mild leaf mottle. Occasionally yellowing may develop.
Survival: CMV carries-over on seed and alternative hosts.
Environmental conditions: As CMV is mainly spread by aphids, seasonal conditions that favour aphid breeding and flights will result in higher incidence of the disease. These conditions include mild, wet autumns and springs.
Transmission: CMV can he transmitted by up to 60 species of aphids, including green peach aphid, Myzus persicae. The aphids acquire the virus on their mouth parts after only a few seconds feeding, but rapidly lose it during subsequent feeding.
Host Range: CMV occurs on subterranean clover, red clover, white clover. lupins, fumitory, stagger weed, king island melilot, wild radish and capeweed.
Control: The subterranean clover cultivars, Larissa and Trikkala have shown some tolerance.
Use seed from CMV free crops. Seed lines can be tested for levels of virus infection.
Good weed control could help minimise infections and losses.
Bean yellow mosaic virus (BYMV)
BYMV is a common virus of pasture legumes with the potential to severely reduce yields, however the incidence and severity are usually light.
On subterranean clover there is a vein clearing which gradually develops into a dark and light green mottle. The leaflets remain small and are puckered.
The leaf stalks are shorter than usual, giving the plants a dwarfed rosette appearance.
The main symptom in red clover is a mottling. Vein yellowing, distortion, vein necrosis and systemic necrosis also may develop.
Survival: BYMV carries-over on a wide range of legumes, especially in irrigation areas. Seed carry-over is rare.
Environmental conditions: Conditions that suit widespread outbreaks of BYMV are those that favour a large scale increase in aphid populations. Warm moist summers and autumns, followed by mild winters would lead to a build-up in aphid numbers in spring.
Transmission: BYMV is spread by several species of aphid including, the Cowpea Aphid Acyrthosiphon pisum, the Green Peach Aphid Myzus persicae and the Black Bean Aphid Aphis fabae. The virus does not persist for long in the aphid. To spread the virus the aphids must acquire it from an infected plant in a short feeding time (minutes) and transfer it to a healthy plant within one hour.
Host Range: BYMV occurs on subterranean clover, white clover, red clover, medics, lupins, beans and a number of native legumes.
Control: No effective control measures have been developed as yet.
There are no resistant cultivars.
This Information note was developed by Rod Clarke, November 1999.