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Crops > Plantation crops > Banana > Fungi > Use of bleach as a surface sterilant
July 2017. This entry follows from one on the use of bleach to decontaminate banana suckers in case of contamination by Fusarium Tr4. (It was thought to be risky to take suckers from any country where the fungus exists). Plants should be moved a virus-tested tissue cultures. However, a member wrote concerned the potential danger of bleach as some commercial products are concentrated.
This is what was said:
1. Safety concerns
One point with safety when working with bleach or Janola. The active ingredient is sodium hypochlorite. Just beware of industrial strength products, because they are often strong concentrations (cheap but also dangerous). Best to decide on a dilution based on knowing the % sodium hypochlorite in the original product/container. So, at high rates it should be treated just like a poisonous pesticide (wear protective gear when decanting and diluting it, especially wear protective glasses and gloves). At industrial rates it will burn holes in your clothes, and your skin and a splash could easily blind you. We use it to sterilise our insect rearing equipment, and it is very useful because UV (sunlight) degrades the a.i. very quickly, leaving no residues. Low concentrations are okay. We even sterilise the surface of fresh leaves with a very low concentration.
2. Surface sterilant
Following that, another member wanted to know if bleach would be useful for the surface decontamination of seed. It was thought that it should work, especially as the seed was said to have a thick testa. Another method was to flame in alcohol.
Yet another method, and one from Paul Neergaard, Seed Pathology (2 volumes), is to use Paul Neergaard, Seed Pathology (2 volumes). MacMillan 1977, uses calcium carbonate. It makes the host cells tougher so the fungus cannot invade as easily. Calcium is used in cell wall structures, but will only be incorporated when the cells are growing. It can also alter the pH and make the environment less conducive for fungal growth.
One member suggested to use 10% Calcium hypochlorite or 5% Sodium hypochlorite for 30 minutes for seeds, adding washing detergent or 2% Tween 20 to help penetration in crevices and air-pockets. It was recommended to test the treatments on a small number of seed before treating large batches. Seeds differ in their resilience to such treatments.
3. Mixing bleach with pesticides
This discussion caused a question from The Post-Entry Quarantine and Diagnostic station at Ibadan. Was it alright to use bleach along with insecticides and fungicides for seedlings and sucker dips. It was not considered suitable to use sodium hypochlorite (bleach) with pesticides on seedlings. Any use of bleach in contact with live material, whether plant or animal, needs very careful assessment and use. Bleach degrades all organic material. If your dose is a little too strong you will kill everything. Sodium hypochlorite is a very strong oxidant, and is very likely to have an effect on the active ingredient of the pesticide, which in most cases will be an organic molecule.
One exception to degradation, at low levels of chlorine, may be captan. Hypochlorites are surface disinfectants and will seldom enter the plant part being treated. I would prefer calcium hypochlorite to avoid residual problems with sodium residues. The pH greatly affects the biocidal activity of hypochlorites, decreasing with increasing pH. These issues are well documented.
If calcium carbonate is added to a pesticide mix it will raise the pH. Water of high pH is known to inactivate some active ingredients. So,
it should also not be mixed with most pesticides since, if it does not degrade them immediately, residual life will be decreased (coppers excluded). A suitable pH for most water used in pesticide applications is about pH6 and commercial buffers are available to maintain suitable pH levels if required. Alkaline water >pH7 can be a reason for poor performance of many fungicides and insecticides. With coppers, to prevent plant injury and reduce the release of soluble plant damaging coppers, acid waters ( <pH7) must be avoided. Residues of lime on the plant surface to be treated similarly must be avoided with pesticides, other than coppers. 4. Flaming in alcohol
Using ethanol and methylated spirits to sterilise laboratory tools, by flaming them after dipping in these products, is often ineffective in eliminating many bacteria that can survive in their solution, if flaming is inadequate. 99.9% control of organisms, often quoted for these solutions and gels, is not adequate in pathological techniques.