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The lentil crop is affected by a number of viral diseases in different regions. Although their present economic significance is much less than that of the fungal diseases, some viruses have the potential to adversely affect lentil seed yield and quality. One example is the lentil yellows disease, which in recent years has been found to be widespread in the major lentil-growing regions of the world. Here, five virus diseases of lentil are described. Control methods are briefly discussed at the end.
Lentil yellows
Lentil yellows is the most widespread virus disease of lentil, occurring worldwide. In the West Asia and North Africa region it has been observed in Syria, Algeria, Egypt and Ethiopia.
The disease is caused by a number of related luteoviruses such as bean leaf roll virus (BLRV), beet western yellows virus (BWYV), or subterranean clover red leaf virus (SCRLV). The causal virus is not transmitted through sap or seed, but in a persistent manner by a number of aphid species such as Acyrthosiphon pisum, Aphis craccivora and Myzus persicae. Epidemic spread of this disease is always associated with high aphid vector populations.
Symptoms: Leaves of infected plants are reduced in size and show mild systemic interveinal chlorosis which intensifies with time until the whole leaf becomes yellow (Fig. 26).
Plants infected early will be severely stunted (Fig. 27) and will bear few or no pods.
Bean yellow mosaic
The disease is worldwide in distribution and affects many other legume and nonlegume species.
The disease is caused by the bean yellow mosaic virus (BYMV) which is flexuous, rod-shaped and belongs to the potyvirus group. The virus is transmitted through sap and by aphids in the nonpersistent manner. More than 20 aphid species are reported to transmit the virus including Acyrthosiphon pisum, Aphis fabae, Aphis craccivora and Myzus persicae. Transmission of the virus in lentil seed is not reported.
Symptoms: Leaves of infected plants show mild mosaic followed by narrowing. The new growth from leaf axils has leaves that are narrow, elongated and light green (Fig. 28). Early infections adversely affect plant growth and yield.
Broad bean stain
The disease is known to occur in Europe, North Africa, West Asia and Australia. It also naturally affects faba bean, dry pea and vetch.
The disease is caused by the broad bean stain virus (BBSV). The aspherical virus, belonging to the comovirus group, is transmitted through sap and seed (up to 27%) and by weevils, Apion vorax and Sitona spp.
Symptoms: The disease is characterized by a mild mottling on the leaves, which is not easily recognizable because of the small size of the lentil leaf.
The affected plants are reduced in growth, which is easily recognized, especially when they are compared with healthy plants (Fig. 29a). Yield reductions of 15-16% could occur in some different cultivars. Seeds from infected plants occasionally show dark staining on the seed (Fig. 29b).
Pea seedborne mosaic
The disease occurs in Asia, Australia, Europe, New Zealand, North Africa and North America. Besides lentil, it also affects dry pea and faba bean.
The disease is caused by the pea seedborne mosaic virus (PSbMV) which belongs to the potyvirus group and consists of flexuous particles. The virus is seed borne in lentil (up to 44%) and also in faba bean and dry pea. It is transmitted in a nonpersistant manner by the aphids Acyrthosiphon pisum, Aphis craccivora and Aphis fabae. This results in further spread of the seed-borne infection under field conditions.
Symptoms: The disease is characterized by a mild mosaic and malformation of leaves. The affected plants show a little stunting and twisting of stems. Seeds from the affected plants are smaller than normal and misshapen.
Cucumber mosaic
The disease is worldwide in distribution. It has a very wide host range and affects many legume and nonlegume species.
The disease is caused by the cucumber mosaic virus (CMV), which is spherical in shape and belongs to the cucumovirus group. The virus is transmitted through sap and by more than 60 aphid species in a nonpersistent manner. The virus also infects members of Solanacea, Cruciferae and Compositae.
Symptoms: Vein clearing followed by mild systemic mottle are characteristic symptoms of the disease. Some affected genotypes remain symptomless.
The affected plants show stunting as the disease advances.
Control: For the three seed-borne viruses (BBSV, PSbMV and CMV) it is important to avoid the use of virus-infected seed lots.
Rogue out early infected plants to prevent further disease spread.
For the aphid-transmitted viruses, spray systemic aphidicides, especially in lentil nurseries or seed multiplication plots. This should be considered in situations where winged forms of aphids begin to appear. Spraying will help suppress aphid populations in lentil fields.
For BBSV, which is transmitted by weevils, treat seed with promet (12 ml/kg seed) to help reduce the vector and hence disease spread.
All these viruses have a wide host range including other legumes such as dry peas, phaseolus bean, faba bean, alfalfa and clover. Therefore it is advisable to plant lentil away from these cultivated legumes to reduce possible spread of these viruses to lentils.
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