Diseases of lucerne - 1: bacterial and viral diseases	AG0726
Rod Clarke, Knoxfield November, 1999

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Importance:
Bacterial wilt, caused by the bacterium Corynebacterium insidiosum was a major disease of Hunter River and other susceptible cultivars. This disease causes a steady decline of a stand, through plants dying, over a number of years. Eventually the stand becomes uneconomical for hay or seed production. The introduction of resistant cultivars in the mid 1980's saw a dramatic drop in the incidence of this disease.

Symptoms:
The first infected plants may appear scattered throughout the stand, 2 to 3 years after sowing. These plants are stunted, yellow-green and may show a marginal leaf scorch.

Bacterial Wilt can be diagnosed in the field by removing the outer bark on the roots to expose the brown discoloured surface of the woody cylinder. In cross-section a dark ring is apparent under the hard layer, and this discolouration may eventually extend through-out the root. As the disease progresses, the bacteria in the roots spreads to all parts of the plant, eventually killing it.

Survival: The bacterium can remain viable in hay for up to 10 years, and in the roots and crowns of infected plants. Some seed transmission has been demonstrated.

Environmental conditions: The bacterium may enter the plant via wounds on the roots, crowns or stems. Symptoms are most obvious in regrowth after cutting, in late summer and autumn.

Dispersal: Local spread may be by surface water, insects, animals and machinery.

Host Range: Corynebacterium insidiosum occurs on lucerne, other Medicago spp. and Bokhara clover.

Control:
Use resistant cultivars.
Avoid injuries to roots and crowns with machinery and livestock.
Maintain high soil fertility.
Cut stand for hay when dry to avoid transfer of water-borne bacteria. Cut healthy stands before infected stands to avoid contamination.


Witches' broom

Importance:
This disease is caused by a mycoplasma, a virus like organism and is usually found in stands of at least 4 to 5 years old. Witches' broom can thin stands and significantly reduce both herbage and seed yields. It is confined mainly to the northern areas of the state.

Symptoms:
Infected plants are severely dwarfed and hunched due to the proliferation of short, spindly shoots originating from the crown and auxiliary buds of older stems.
The leaves are small, rounded, often with yellow edges, and young leaves may be puckered or twisted. Infected plants produce very few flowers, become more stunted with each cutting and may die during the first winter. Maximum symptom expression is in late summer and early autumn, with some masking of symptoms in spring.

Survival: The Witches' broom mycoplasma survives on living infected plants and alternative hosts.

Environmental conditions: Severe symptoms are associated with hot dry conditions and mild symptoms with high rainfall. The incidence of this disease appears to be lower when stands are grown under irrigation.

Transmission: The mycoplasma is transmitted between legumes by the leaf-hopper (Orosius argentatus).

Host Range: The Witches’ broom mycoplasma occurs mainly in lucerne and a few nonleguminous plants.

Control:
No control measures are available.
Removal of diseased plants, reseeding and good management will help prevent the spread of this disease.


Lucerne yellows

Importance:
The cause of this disease is unknown, but it is thought to be by a mycoplasma, a virus like organism and is usually found in stands of at least 2 years old.

Symptoms:
During summer the leaves become yellow to red-purple. More than 50% of plants in the stand may be affected with many dying. A distinctive brown colour can be seen on the underside of the root bark, while the woody cylinder remains a healthy white (thus differing from a root affected by Bacterial Wilt). In some cases spindly regrowth may develop from the crown after cutting.

Survival: The mycoplasma survives on living infected plants and alternative hosts.

Environmental conditions: Symptoms usually found under hot conditions.

Transmission: The mycoplasma is transmitted by leafhopper.

Host Range: Unknown.

Control: No control measures are available.


Alfalfa mosaic virus (AMV)

Importance:
This virus can reduce yields and occurs in most lucerne stands. Twenty percent or more plants infected in stands older than 2 years is not uncommon. AMV may also predispose plants to drought injury.

Symptoms:
AMV symptoms vary from light, and almost unnoticeable, to severe, causing plants to die, due to the diversity of strains of the virus, the cultivar and environmental conditions. Symptoms may be masked by warm weather. AMV causes an interveinal light green or yellow mottle on the leaves and a stunting of the plant.

Survival: Infected seed is the most likely primary source of infection. AMV carries-over on live plants and alternative hosts.

Environmental conditions: Symptoms are most likely to become apparent in spring because of cool temperatures, high light and lush growth. Spread within a stand is dependant on local build up of aphids. This occurs in years with summer and autumn rains which favours plant growth and thus an increase in aphid numbers, and dry springs which favours 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: AMV is transmitted by the pea aphid, Acyrthosiphon pisum, the green peach aphid, Myzus persicae and at least 11 other aphid species.

Host Range: AMV occurs on lucerne, red clover, white clover, French beans and peas.

Control:
Use seed that is known to be free of AMV.
Aphid spraying is expensive, and aphid population movements are difficult to predict.



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