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How to deal with Tobacco Mosaic Virus (TMV) when growing cannabis

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Cultivating cannabis

How to deal with Tobacco Mosaic Virus (TMV) when growing cannabis

By Tori Smith March 29, 2021

Growing and cultivating cannabis has many hazards but Tobacco Mosaic Virus (TMV) can be one of the most damaging. What was originally a plant disease solely affecting tobacco plants has evolved to become one of the most common pathogens attacking a variety of hosts – including cannabis plants. If not prevented outright, TMV can ruin an otherwise bountiful harvest.

TMV is notorious for its resilient characteristics: this virus has no known cure.

TMV 101

In 1889, Dutch botanist Martinus Beijerinck found ‘tobacco mosaic disease’ to be caused by a pathogen able to reproduce and multiply in the host cells of the plant. Beijerinck labelled it a ‘virus’ (from the Latin word meaning poison) to differentiate this version of the disease from versions caused by bacteria. Why? Simply because it was unlike any version seen before.

So how does TMV infect plants? Once attached to the host, this sub-microscopic, rigid, rod-shaped particle attacks the plant cells causing a subpar yield as a result of derailed growth and development. Leaves become blotchy and twisted, sporting a distinct mosaic-like mottled pattern that looks pretty unsightly.

Diagnosis

Specific symptoms may depend on the virus strain, environmental conditions, age and genetic background of the afflicted plant, and it can take up to ten days for the symptoms to make themselves known.

Some plants exhibit no symptoms at all and may only begin to display signs of the infection after it undergoes any form of stress. Stressors include transplanting, pruning, and nutrient problems. The good news is that plants infected with TMV rarely die of the disease, but the bad news is that it can tank the quality and amount of yield the plant can produce.

If you’re worried that your plants could have TMV, here are some typical symptoms:

Stunted development – plants growing at a snail’s pace? TMV-infected plants can suffer from delayed growth or the plants won’t reach their height potential and end up producing small buds with subpar quality.

Strange stems – TMV can cause plant stems to weaken, get twisted or become discolored with red or purple.

Leaf discoloration – leaves of plants infected with TMV often develop purple or black patches with a mottled mosaic pattern. Some may also exhibit yellowing or turn brown and have burnt edges.

Crumpling foliage – affected plants can also exhibit wrinkling, blister-like spots, webbing or a curling under or upwards of leaves.

Compromised immune systems – TMV can increase a plant’s susceptibility to mold attacks and other diseases.

TMV transmission

TMV can be transmitted from one plant to another very easily. Here’s how:

Tobacco

Products made with air-cured tobacco may carry TMV, in contrast with flue-cured tobacco (used in making cigarettes) which when processing is heated repeatedly, thus inactivating most of the TMV. If tobacco is handled or kept in pockets, hands and clothing can become contaminated with TMV and passed to plants.

Mechanical wounds

The virus typically spreads via contaminated hands, clothing or tools such as pruning shears and hoes. TMV is hugely concentrated in most plant cells, so if crops are handled, tiny leaf hairs and outer cells get damaged and leak TMV-imbued sap onto the hands, tools, or clothing.

Infected mother plant

TMV can be carried in seeds from infected plants. Females infected at a younger age are more likely to pass the pathogen to its offspring by contaminating its seed coat.

Insect vectors

When insects (aphid, whitefly, and thrips among them) feed on an infected plant, its mouthparts and saliva also get contaminated. A new host plant will quickly develop an infection as soon as the infected pests attack another crop.

Why is it so pervasive?

Once TMV has made its way into the plant cell, the protein coat of the virus falls away and releases its genetic code. The plant confuses the virus for its own RNA and begins producing more of the viral proteins and spreads to the entire plant.

TMV can be a significant problem because, unlike most other viruses, it does not die when the host plant dies and can infect other plants years after its former host has died. How? While TMV can only multiply inside a living cell, it can remain in a dormant state in dead tissues. The virus may even survive in crop debris on the soil surface before infecting a new crop planted on contaminated soil.

Tips for preventing TMV

Because there is no cure, every effort should be made to keep TMV away from cannabis crops:

  • TMV is overwhelmingly spread by insects, so use preventive measures against pests by covering the plants with floating row covers or aluminum foil mulches to keep insects at bay.
  • Pick hardy cannabis varieties. Certain strains like Northern Lights and White Widow are more resilient to infections, and ensure they’re fed with sufficient nutrients to improve their immune systems.
  • Purchase plants and seeds from reputable sources.
  • Before buying a plant, inspect it first for TMV symptoms.
  • Infected plants should be removed from the grow area and quarantined immediately.
  • Learn to be okay with saying goodbye to a plant if the infection is severe.
  • Clean gardening tools with a 10% solution of household bleach before pruning the plants.
  • Regularly launder gardening clothes.
  • Do not compost the debris or soil of infected plants.
  • Wash hands thoroughly after handling TMV-contaminated plants using antibacterial soap.
  • Avoid working in damp conditions; it’s easier for infections to spread in this environment.

To sum up…

It really is all about prevention – it’s the only way to beat TMV altogether. Preventive measures will decrease the likelihood of contracting the disease and save you a huge headache.