Copper-bottomed protection for tobacco

tobaccoTobacco remains an important cash crop in many countries throughout the world. (Image source: Terry Mabbett)Tobacco plants are attacked by a wide range of fungal pathogens from seedling stage in the nursery bed to mature plants in the field and copper fungicides can help fight the plant pathogenic bacteria

Copper and tobacco are two of the world’s traditional commodities with the former playing a crucial role in the successful production of the latter. The divalent copper ion (Cu2+), which is the active principle of copper containing fungicides, is virtually unique in possessing this dual fungicidal and bactericidal action.

Often overlooked is copper’s role as a micronutrient or trace element in plant nutrition. Trace elements are required in minute amounts in the tobacco plant’s metabolism as vital ‘co-factors’ for the functioning of specific enzymes. For instance, copper is the co-factor for an enzyme called polyphenol oxidase (PPO).

This enzyme is known to play a role in photosynthesis with high PPO concentrations linked to disease resistance in crop plants. Copper as a micronutrient is only required in minute or trace amounts but without it tobacco plants develop symptoms of copper-deficiency and may even die.

Direct control of fungal and bacterial plant pathogens and the maintenance of plant health through these discrete nutritional actions at plant cell and tissue level are down to the divalent copper ion (Cu2+). This positively charged particle (cat-ion) is formed when metallic copper dissolves in aqueous media and increasingly so with more acid conditions (decreasing pH).

Fixed copper fungicides are ‘insoluble’ (sparingly soluble) salts of metallic copper which are sprayed onto the tobacco leaves to form protectant deposits that gradually release the fungicidal and bactericidal Cu2+ to provide long term protection of the leaves from infection by fungal spores and bacterial cells. Fixed copper fungicides are usually formulated as a wettable powder or a wettable granule, both of which are easily dispersed and suspended in water for spaying onto the tobacco crop.

Fixed copper fungicide in the wettable powder form may also be used a fungicide seed dressing to prevent pathogenic fungi and bacteria, either on the surface of the tobacco seed or in the soil of the seedbed, from infecting and killing the germinating seed or the rooted tobacco seedling. The three mainstream particulate, fixed copper fungicides are cuprous oxide, copper hydroxide and copper oxychloride.

Global spread for tobacco

Tobacco remains an important cash crop in many countries throughout the world where climate and weather, soil and fuel resources are compatible with the growing and curing needs of Flue cured Virginia (FCV) tobacco and the light, air-cured Burley tobacco leaf. The following countries figure most prominently amongst the tobacco producing/exporting countries of the world.

Flue-cured Virginia tobacco: India, Zimbabwe, Brazil and the USA are the four big producers with annual productions of at least 200mn kg of tobacco per annum. Other significant producer/exporter nations with 50 to 100mn kg per annum include Indonesia, the Philippines, Bangladesh, Pakistan, Tanzania and Argentina.

Light air-cured Burley tobacco: Biggest producer of Burley tobacco is Malawi followed by Brazil and USA all with productions of at least 90mn kg per annum. Other significant Burley producer/exporter nations include Thailand, the Philippines, Bangladesh and Argentina all with productions of 20 to 30mn kg per annum.

— By Dr Terry Mabbett

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