Fungicides and Herbicides.
Fungicides:
These are used to check the growth of fungi. Fungi are plants without chlorophyll. So they cannot use solar energy to convert CO2 and H2O into carbohydrates (photosynthesis). They live as saprophytes on decaying organic matter or as parasites at the expense of living organisms. Fungicides play a vital role in controlling the growth of fungi. Broadly speaking, fungicides are of two types.
Systemic Fungicides: These fungicides, when applied to the plant, rapidly distribute itself in various plant tissues. Some examples are:
Non Systemic Fungicides: These protect only that parts of the plant where it is applied. Some examples are:
Effect of Fungicides on Environment:
Fungicides have the following effect on the environment.
In the soil when fungi are eliminated by the use of fungicides, the bacterial population increases. In this way the plants become susceptible to bacterial
Certain fungicides are toxic to soil arthropods. For example, captan at a higher dose reduce the population of the useful soil invertbrates like springtails, collembola, earthworm and millipedes.
Mercural fungicides, e.g. phenyle mercury cyanide used in seed treatment cause destruction of the seed eating birds like bobwhite quaal and pigeons. These insecticides also impair the egg hatching in pheasants, resulting in reduction of population of such species.
Certain fungicides e.g. captan inhibit the growth of fresh water algae even in very low concentrations.
Continued use of pesticides makes the pests (that are desired to be eliminated) resistant to them.
Mercural fungicides are also responsible for human poisoning and deaths. This happens when ingestion of flour and wheat seeds treated with organomercury fungicides leads to mercury poisoning. Such poisonings have been recorded in Iraq in 1956, 1960 and 1972, in guatemala in 1965 and in Pakistan 1969.
Redressal of the Effects of Fungicides:
The environmental effects of fungicides can be minimized by using them selectively in low concentrations. Attempts should be made to develop new fungicides, which should be more effective, less toxic and more environmental friendly. One such fungicide is a derivative of β methoxyacrylic acid.
It is likely to be a very useful anti-fungal agent, since it is effective against a wide range of fungal pathogens.
Herbicides:
Herbicides are chemicals that kill unwanted weeds or interfere with their growth. Weeds are unwanted plants which grow at places where they are not required, e.g. in agricultural crops and deplete the nutrients which are meant for the betterment of the crops. The weeds retard the growth of the main crops. Herbicides are of two types:
Contact Herbicides: These herbicides kill the weeds by direct contact and are quick acting. Some examples include. Contact herbicides are ineffective against those weeds whose roots produce new shoots at a fast rate.
Systemic Herbicides: These diffuse into the plant’s body and are very useful. Examples include the following. Systemic herbicides are effective at low concentrations.
Herbicides act as growth inhibitors for the weeds. For example, the contact herbicide, pentachlorophenol acts as mitotic poison. Some herbicides (e.g. diuron act as a photosynthetic inhibitor. Finally some herbicides (e.g. picloram) act as a respiratory inhibitor
Effects of Herbicides on Environment:
Like fungicides, herbicides also are extremely important for ainicultura: production. However, these have the following effects on environment.
- Residues of herbicides in soil increase fungi count resulting in fungi:
- Hyerbicides like 2, 4-D and 2, 4, 5-T when sprayed of the trees, etc., are harmful to millards, quails and partridges.
- Herbicides are hazardous to live stock which eat weeds exposed to sub lethal doses of 2, 4-D and 2, 4, 5-T.
- The production of fresh water algae is reduced due to 2, 4-D and 2, 4, 5-T.
Herbicides are extremely harmful to the health and survival of human race. On being exposed to 2, 4-D and 2, 4, 5-T which are used to defoliate forest areas (as in the case of Vietnam War) produce birth defects, e.g. children being born with deformed or missing limbs, etc. This is attributed to damage of male sperm cells.
The weeds develop resistance to the conventional herbicides by their continued application. The best way to solve this problem is to develop new and novel herbicides. One such herbicides is 8-aminolevulinic acid. It uses solar energy to produce toxic components that kill the weeds. On exposure to sun light 8-aminolevulinic acid is converted into magnesium tetrapyrrole, which is precursor to chlorophyll synthesis.
The magnesium tetrapyrrole absorbs light energy and undergoes photochemical reaction producing singlet oxygen, a highly reactive form of oxygen. The liberated free radical kills the weeds. The left over 8-arninolevulinic acid or magnesium tetrapyrrole is biodegradable and so does not cause any problems.
