Context: Farmers in Maharashtra have said their tomato crop was impacted by attacks of the cucumber mosaic virus (CMV), while growers in Karnataka blamed the tomato mosaic virus (ToMV).

What are CMV and ToMV?

• The two plant pathogens have similar names and cause similar damage to crops, but they belong to different viral families, and spread differently.

• Impact: Both viruses can cause almost 100 per cent crop loss unless properly treated on time.

Tomato mosaic virus (ToMV)

• Family: It belongs to the Virgaviridae family and is closely related to the tobacco mosaic virus (TMV). ToMV was first reported in the United States in 1909.

• Hosts: Tomato, tobacco, peppers, and certain ornamental plants.

• Spread: ToMV spreads mainly through infected seeds, saplings, agricultural tools and often, through the hands of nursery workers who have failed to sanitize themselves properly.

• Effect: It causes foliage of plants to show alternating yellowish and dark green areas, which often appear as blisters on the leaves. 

• It also causes distortion of leaves and twisting of younger leaves.

• The fruit develops necrotic spots, which leads to overripening. 

• Younger plants are dwarfed, and fruit setting is affected.

Cucumber mosaic virus (CMV)

• Family: It belongs to the Bromoviridae family and was identified in cucumbers in 1934.

• Host: Cucumber, melon, eggplant, tomato, carrot, lettuce, celery, cucurbits (members of the gourd family, including squash, pumpkin, zucchini, some gourds, etc.), and some ornamentals. 

• Spread: CMV is spread by aphids, which are sap-sucking insects. CMV too can spread through human touch, but the chances of that are extremely low.

• Conditions of high temperature followed by intermittent rain, which allow aphids to multiply, are conducive to the spread of CMV.

• Effect: CMV too causes distortion of leaves, but the pattern is different. Often leaves at the top and bottom are distorted while those in the middle remain relatively blemish-free.

• While specific effects vary depending on the host, overall, CMV causes stunting and lower production.

• In cucumber, the virus causes a mosaic-like pattern of alternating yellow and green spots. In tomatoes, fruit formation is affected, and in some cases the fruit is distorted and small.

Ways to Control the Virus Spread

• Following biosafety standards in nurseries, and compulsory seed treatment to stop the spread of ToMV. 

• Fields must be cleared of weeds and plant material before fresh planting as ToMV can remain dormant in weeds and plant remains around the field, and come back later.

• To control CMV the best way is to stop the aphids, by spraying quick acting insecticides or mineral oils on the plants.

• Checking the trays of seedlings  before planting, and discarding any visible infected material.