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Diseases of white clover - 1: virus diseases | AG0731 |
Rod Clarke, Knoxfield
November, 1999 |  |
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White clover mosaic virus (WCMV)
Importance:
WCMV was first recorded in Victoria at Maffra in 1959. Outbreaks since have been sporadic and vary between years and districts. This disease is rarely present in newly sown stands but common in stands of greater than 4 years. Glasshouse studies have recorded dry matter losses of up to 24 % due to WCMV. An additional effect of WCMV is that it reduces the plant’s ability to compete in a pasture sward, and loss of legume numbers means a reduction in nitrogen fixation.
Symptoms:
WCMV appears as yellow markings along the veins and in interveinal areas. Symptoms seldom occur on all leaves and are more noticeable on actively growing plants in the spring and autumn. Recently grazed leaves will show no symptoms. Affected plants lack vigour but rarely die.
Biology:
Survival: The virus survives in live plants and alternate hosts.
Transmission: WCMV is transmitted by mechanical means such as grazing livestock, tractors and mowers. Aphids are not known to be vectors of this virus.
Host Range: WCMV occurs on white clover, red clover, strawberry clover, subterranean clover, broad bean and sweet pea.
Control:
At present there are no effective control measures, and there are no tolerant or resistant cultivars.
Alfalfa mosaic virus (AMV)
This virus can reduce yields and is one of several virus diseases that causes mosaics in white clover. Twenty percent or more of plants infected in stands older than 2 years is not uncommon. AMV may also predispose plants to drought injury.
Symptoms:
AMV causes an interveinal light green or yellow mottle on the leaves and a stunting of the plant. 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.
Biology:
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 favour 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.
Control:
At present there are no effective control measures, and there are no tolerant or resistant cultivars.
Clover phyllody
Importance:
This disease is caused by a mycoplasma, a virus like organism. Although clover phyllody is widespread, its incidence in white clover is higher when the clover is grown as a pure stand or where it forms the major component of the stand.
Symptoms:
Affected plants are stunted, with yellowing of the veins and bronzing of the older leaves. The flowers are transformed into clusters of green leaves. When infection occurs at flowering, individual flower stalks can extend to three times their normal length. The flowers remain white but are much smaller than those of unaffected plants. No seed is set from diseased plants.
Biology:
Survival: The clover phyllody mycoplasma is carried over on alternative hosts. It is not known to be seed-borne.
Environmental conditions: Mild winters and hot dry summers favour the build-up of vector numbers within the stand and thus the spread of this disease.
Transmission: The clover phyllody mycoplasma is transmitted by leaf-hoppers.
Host Range: Clover phyllody occurs on white clover, red clover, alsike clover, strawberry clover and a number of annual weeds.
Control:
There are no effective control measures for this disease.
Spraying to control leaf-hoppers is uneconomical and impractical on a large scale.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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