Tomato growers in Maharashtra and Karnataka have blamed two different viruses for the loss of yields earlier this year. Farmers in Maharashtra have said their tomato crop was impacted by attacks of the cucumber mosaic virus (CMV), while growers in Karnataka and other South Indian states have blamed the tomato mosaic virus (ToMV) for crop losses.
Over the last three years, growers of tomato have complained of increased infestation with these two viruses, leading to partial to complete crop losses.
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. ToMV belongs to the Virgaviridae family and is closely related to the tobacco mosaic virus (TMV). ToMV hosts include tomato, tobacco, peppers, and certain ornamental plants.
CMV has a much larger host pool that includes cucumber, melon, eggplant, tomato, carrot, lettuce, celery, cucurbits (members of the gourd family, including squash, pumpkin, zucchini, some gourds, etc.), and some ornamentals. CMV was identified in cucumber in 1934, which gave the virus its name.
How do these two viruses spread?
ToMV spreads mainly through infected seeds, saplings, agricultural tools and often, through the hands of nursery workers who have failed to sanitise themselves properly before entering the fields. It would require only a few infected saplings for the virus to take over an entire field in a matter of days.
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, Dr M Krishna Reddy, a former head of crop protection at Indian Institute of Horticulture (IIHR), Bengaluru, said. These conditions were seen in Maharashtra — the late rabi crop (planted in January-February) faced a sudden bout of rain followed by extreme heat.
For ToMV, farmers in Maharashtra have blamed seed manufacturers and nurseries. Tomato growers plant 3-4-inch saplings in their fields, which they buy from nurseries. It is very important to ensure that nurseries maintain bio safety, and restrict entry into the premises. “Seed treatment at the nursery is necessary to prevent future spread of the virus,” Dr Reddy said.
How do the viruses affect the crop?
Both viruses can cause almost 100 per cent crop loss unless properly treated on time. The foliage of plants infected with ToMV shows alternating yellowish and dark green areas, which often appear as blisters on the leaves. Distortion of leaves and twisting of younger leaves are also symptoms. The fruit develops necrotic spots, which leads to overripening. Younger plants are dwarfed, and fruit setting is affected.
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. In cucumber, the virus causes a mosaic-like pattern of alternating yellow and green spots. In tomato, fruit formation is affected, and in some cases the fruit is distorted and small.
While specific effects vary depending on the host, overall, CMV causes stunting and lower production.
How can the viruses be controlled?
Dr Reddy stressed the importance of following biosafety standards in nurseries, and compulsory seed treatment to stop the spread of ToMV. Farmers who buy trays of saplings should check before planting, and discard any visible infected material, he said. They should also look out for signs of infection throughout the cropping cycle, and remove infected plants without allowing it to touch healthy ones.
ToMV can remain dormant in weeds and plant remains around the field, and come back later. Fields must, therefore, be cleared of weeds and plant material before fresh planting. Plants cannot be cured of ToMV, but the infection can be controlled with good agricultural practices. “Earlier we had reports about the virus mostly from Maharashtra, but reports are now coming from Karnataka, Andhra Pradesh, and other states,” Dr Reddy said.
Controlling CMV is more difficult, given the large number of hosts the virus can live on. The best way is to stop the aphids, which can be done by spraying quick acting insecticides or mineral oils on the plants. Irritating the aphids can spread the virus to other fields. An eye must be kept on aphid migration so that measures can be taken while planting the crop.
The foliage of plants infected with ToMV shows alternating yellowish and dark green areas, which often appear as blisters on the leaves. (Photo: Wikimedia Commons) 
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