How Fungicides Work

Fungi have no chlorophyll so cannot photosynthesise, and have no vascular system so cannot move water through 'body'; therefore they must live in very close association with plant, usually in a symbiotic or parasitic manner. Here are some brief notes:

What is a Fungicide?

An agent which will kill, repel or prevent, or otherwise mitigate a fungus.
It may be PREVENTATIVE or CURATIVE in nature
It may be naturally derived or synthetic

BIOLOGICAL VS SYNTHETIC

Biological
Soil contains beneficial fungi and bacteria which afford biological control by attacking and destroying pathogenic (disease-causing) fungi.

Synthetic
These are comprised of various chemical treatments concocted by technicians in a laboratory.

HOW DO FUNGICIDES WORK?

• Direct competition
• Antibiosis
• Predation or parasitism
• Induced resistance of host plant
  

PROTECTANT VS CURATIVE

Protectant
Must be present BEFORE infection. Includes Carbamates (includes Mancozeb)

Curative
Tend to work on specific metabloic process. Blocks a part of pathway. Therefore more limited in action. And because they only work at one point, fungus more able to build up resistance.

CONTACT VS SYSTEMIC

How Contact Fungicides Work

• Multi-site activity (therefore effective against a wider range of fungal diseases).
• These fungicides act to kill fungus by overwhelming it with substances which are poisonous to it (e.g. copper).
• These fungicides primarily based on inorganic chemicals such as copper, aluminium, sulphur. Also upon the dithiocarbamates combined with zinc and manganese ions.
• Low risk of resistance build-up.
• Must be present before infection begins

FUNGAL INFECTION PROCESS (from Fungicides_McManus.pdf)

• Inoculation - fungal spore lands on plant surface
• Adhesion - fungal spore exudes a glue so that it sticks to plant surface even in rain
• Germination - fungal spore takes up water and germ tube emerges
• Penetration - germ tube enters plant, either by poking through epidermis or by entering stomata or wounds
• Plant-pathogen recognition - chemical or molecular signals are exchanged between fungus and plant so fungus knows it has infected suitable host
• Infection - fungus invades plant by growing in or between cells, and releases spores from plant surface

TIMING OF APPLICATION

Fungicides must be applied at suitable times to be effective. Contact protectives must be applied before infection. Systemics must be applied at right stage of plant development (eg at bud break).

How Systhane (example of systemic fungicide) Works From IntroToFungicides.pdf

Systhane is a Triazole, which is a sterol inhibitor (DMI). Demethylation inhibitor fungicides (DMI) inhibit the biosynthesis of sterols (enzymes used by fungus) in fungal membranes.

So Systhane works by inhibiting the biosynthesis of sterols (enzymes used by fungus) in fungal membranes.