Verifying viral virus truths: Tomatoes, tiranga and TRPs

Diseases that emerge from viruses are a threat to profitable tomato cultivation. In the absence of anti-viral products, management strategies rely chiefly on genetic resistance and hygienic practices to prevent the spread of viruses and the destruction of diseased crops.

Increasing international travel, along with seed trading, have enhanced risks. The consequent spread of newly introduced viruses and / or associated sap-sucking insect vectors — coupled with the changing climate — are potential threats.

The tomato is the most economically important vegetable crop across the world.

While India has become surplus in most agri-commodities, farmers often fail to get reasonable prices for their produce due to poor farm-infrastructures such as cold storage, processing, inadequate marketing support in domestic and export sectors.

Farmers are often victimised when there is bumper production, particularly for perishable commodities like tomatoes.

While growing the vegetable, it is a challenge for farmers to curb viruses. Hundreds of viruses infect tomato crops as new viruses keep emerging.

A new controversy emerged over a so-called Tiranga virus that allegedly invaded Maharashtra’s vast tomato belt in the past few months. The media wrongly claimed a correlation between this virus and the novel coronavirus disease (COVID-19).

Host-switching to humans and other animals from plant viruses has not yet been established. A clarification based on scientific scrutiny by the media must be expected in such cases.

The virus attacking tomato crops in Maharashtra has generated curiosity and panic: Between April and mid-May 2020, there were several news reports over the mysterious virus.

The nomenclature tiranga (tricolour) emerged after farmers alleged infected tomatoes had three colour stripes: Red, green, white and yellow in patches.

There is, however, no scientific acceptance for this claim yet.

The Indian Agricultural Research Institute’s regional office in Pune received a few samples of infected tomatoes. The varieties they received were: Abhinav, Ansul and Ansul-83 from hybrid seed companies Syngenta, Seminis and Clause respectively.

The samples were from farmers in Kolwadi and Alandi areas of Pune district.

The samples were tested for seven viruses:

• Cucumber Mosaic Virus (CMV)
• Groundnut Bud Necrosis Virus (GBNV)
• Tobacco Mosaic Virus (TMV)
• Tomato Mosaic Virus (ToMV)
• Tobacco Streak Virus (TSV)
• Pepper Mottle Virus (PMoV)
• Potato Virus Y (PVY)

Virus tests were done through enzyme-linked immunosorbent assay tests and by polymerise chain reaction for four viruses (CMV, GBNV, TMV and TSV). Results were positive for five viruses: CMV, GBNV, ToMV, PMoV, and PVY.

Pepper Mottle Virus Photo: GK Mahapatro

Tobacco Streak Virus Photo: GK Mahapatro

Groundnut Bud Necrosis Virus (GBNV) Photo: GK Mahapatro

There are a set of good agricultural practices (GAP) that should be followed to curb the spread of viruses in tomatoes.

Seed source

• Procure quality seeds from reputed sources, adopt resistant / tolerant varieties if available
• Procure good quality seedlings grown in insect-proof net nursery, from reputed vendors only

Seed treatment

• Treat with 0.3 per cent trisodium phosphate for 48 hours, wash properly, shade dry and follow up with suitable fungicidal treatment
• Expose dry seeds to high temperature (70 degrees Celsius) for two to four days. Using 10 per cent trisodium phosphate (minimum 15 minutes) is alternatively suggested
• Skim milk wash can be used also as treatment (to be validated scientifically)

Nursery

• Nursery bed, raised 15 centimetres high, soil treated with 25 grams / three square metre bed Thimet (to be banned with effect from December 2020), admixed with sand or soil for ease in delivery.
• Use Insect-proof net (40-60 mesh)
• Remove symptomatic plants whenever possible to slow the virus spread
• Use balanced fertilisers and manures
• Use label claimed and registered products only
• Prior-transplanting, seedlings are to be treated with 0.05 per cent imidacloprid solution

Main field management

• Adopt floating row cover if feasible, particularly in insect vector prone areas (aphids, whiteflies, thrips)
• Use silver mulch to conserve moisture and insect-vector distraction
• Install yellow and blue sticky traps for aphids and fleas
• Few diseases spread via contacts (root and foliage), so spatial distancing avoiding physical contacts is suggested
• Weeding of alternate hosts in and around the field is advocated as they often harbor insect-vectors, and act as virus reservoirs in off-season
• Follow alternate application of neem oil (0.2 per cent) and imidacloprid (0.05 per cent) till flowering, at 10-12 days interval
• Avoid cross-infection from tobacco field or production system
• Dispose of infested tomatoes properly: Do not scatter them around
• Destroy old standing crop soon after harvesting, as it carries virus carry-over inoculums
• If insect-vectors like whiteflies survive in old harvested crop stand, treat them with appropriate toxic insecticide to prevent their spread to newer crops / neighbouring areas
• Take proper care in the post-harvest stage, wash equipment carefully, use storage bins and avoid physical contact
• Farm labourers should not enter storage areas unnecessarily without washing exposed body parts, aprons, gloves and proper hygiene are a must

Label claimed pesticides (to be used for GAP)

Source: Gazette of India, No. 537, December 24, 2018. Maximum residue levels for these pesticides given in the gazette

Care for your cultivation

A vast array of viruses — more than a hundred — invade tomato crops. At least a dozen, however, are of serious concern. International travel and trade make it worse: When the climate is conducive on several occasions, the viruses infect simultaneously and cause complex symptoms and problems.

They are mostly transmitted by insect-vectors, whiteflies, thrips and aphids. They may also be caused by mechanical means — contact, farm equipment — and in some cases, via seeds. Proper care should, thus, be taken for crop hygiene. Like social-distancing that is a must in times of COVID-19, plant spatial-distancing in crops is also needed to avoid some virus diseases.

Rumour mongers somehow manage to spread fake news. Instead, consult agriculture institutes nearby and follow the advice of the Union and state government’s agriculture professionals. Rumours can also spoil the export potential of tomatoes.

Pesticide dealers and plant protection consultants often advise farmers to apply chemicals to combat viruses.

These chemicals are fungicides or others, often without registration and label claims. There is no antivirus agent. These are merely aggressive marketing strategies carried out unscrupulously.

Be careful, when using pesticides: See if they are for insect-vector control or for fungal problems. Always follow labels strictly.

Gaps between governmental agencies and vulnerable farmers must be bridged as well to build on self-reliance.

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