1. Modes of Action for Plant Disease Management Chemistries
Michael Matheron
University of Arizona
Yuma Agricultural Center

2. Biological mode of action Fungicidal action can be expressed in one of two physically visible ways.
Inhibition of spore germination.
Inhibition of fungus growth.

3. Physiological mode of action What happens at the cellular level to cause the visible effects on spore germination and fungal growth?

4. Why is it important to be familiar with the physiological mode of action of a fungicide ?
For resistance management and preservation of fungicide effectiveness.

5. The physiological mode of action
Fungicides are metabolic inhibitors and their modes of action can be classified into four broad groups.
Inhibitors of electron transport chain.
Inhibitors of enzymes.
Inhibitors of nucleic acid metabolism and protein synthesis.
Inhibitors of sterol synthesis.

6. A typical cell and cell components
Electron transport chain
Enzymes
Nucleic acid metabolism and protein synthesis
Sterol synthesis

7. Inhibition of electron transport chain (Respiration in mitochondria)
Sulfur
Disrupts electron transport along the cytochromes
Strobilurins (azoxystrobin, kresoxim-methyl, pyraclostrobin, trifloxystrobin)
Inhibit mitochondrial respiration, blocking the cytochrome bc1 complex.

8. Inhibition of enzymes Copper
Nonspecific denaturation of proteins and enzymes.
Dithiocarbamates (maneb, manzate, dithane, etc)
Inactivate –SH groups in amino acids, proteins and enzymes.
Substituted aromatics (chlorothalonil, PCNB)
Inactivate amino acids, proteins and enzymes by combining with amino and thiol groups.
Organophosphonate (fosetyl-Al)
Disrupts amino acid metabolism.

9. Inhibition of nucleic acid metabolism and protein synthesis
Benzimidazoles (thiophanate-methyl)
Inhibit DNA synthesis (nuclear division).
Phenylamides (mefenoxam)
Inhibits RNA synthesis.
Dicarboximides (iprodione, vinclozolin)
Inhibits DNA and RNA synthesis, cell division and cellular metabolism.

10. Inhibition of sterol synthesis (Inhibit demethylation of ergosterol)
Ergosterol is the major sterol in most fungi.
It is essential for membrane structure and function.

11. Sterol inhibiting fungicides
Imidazoles (imazalil)
Triazoles (propiconazole, myclobutanil, tebuconazole, triflumazole)
Morpholines (dimethomorph)
Inhibits sterol production at different site than imidazoles and triazoles. Affects cell wall production.

12. Why is it important to know the physiological mode of action of fungicides ?
For resistance management and preservation of fungicide effectiveness.
Incorporate fungicides with different modes of action into a disease management program.
In alternation or as a mixture.

13. Plant activators
In contrast to conventional fungicides, plant activators have no direct effect on pathogens.
Plant activators induce plants to produce natural disease-fighting compounds.

14. Plant activators Acibenzolar (Actigard) Harpin (Messenger)
Biological control organisms

15. Natural Plant Defense Mechanisms
Salicylic acid pathway – Induces SAR (systemic acquired resistance), a natural biological defense response to pathogen attack.
Jasmonic Acid Pathway - Induces the production of disease and insect defense compounds.

16. Salicylic Acid Pathway
Production of active oxygen (hydrogen peroxide, peroxidase)
Peroxidases have been associated with fungal cell wall degradation and pathogen defense signaling
Thickening plant cell wall
Increasing lignification
Production of phenolic esters that strengthen cross linking

17. Salicylic Acid Pathway
Systemic and local accumulation of Pathogenesis Related Proteins (PR-Proteins)
chitinases
ß-1,3 Glucanase
Systemic accumulation of anti-microbial compounds called phytoalexins.

18. Chitinases
Chitin is the major component of all fungal cell walls except for the Oomycetes
Chitinases break down fungal cell walls
Chitinases can break down insect exo-skeletons
Activity is greatly enhanced by Glucanase

19. ß-1,3 Glucanases
Glucans and cellulose are the major components of Oomycete cell walls
Antifungal activity is most often in combination with Chitinase
Direct defense: Degrade fungal cell walls
Indirect defense: Promoting the release of oligosaccharides that act as elicitors of defense reactions

20. Jasmonic Acid Pathway
Farmer and Ryan (1990) discovered that jasmonic acid volatilized from sagebrush could trigger defense gene expression in adjacent tomatoes
Jasmonic acid volatiles act as attractants for beneficial insects
Jasmonic acid induces the production of disease and insect defense compounds.
Defense Proteins
Phytochemicals

21. Phytochemicals
Different from phytoalexins in that phytochemicals are induced by wounding.
Phenolics
Furanocoumarins, Coumarins, Tannins, Lignin, other phenolics
Terpenoids
Alkaloids

22. Examples of plant activators
Acibenzolar (Actigard)
Harpin (Messenger)
Harpin is a natural protein found in many common pathogenic microorganisms;
Erwinia amylovora, E. chrysanthemi, Pseudomonas syringae, Pseudomonas solanecarum, Xanthomonas campestris.
Biological control organisms

23. Induction of Systemic Acquired Resistance
Mode of action - Actigard
Induction of Systemic Acquired Resistance

24. Mode of action - Messenger
Salicylic Acid Pathway
Classical SAR
SAR gene expression
harpin
Jasmonic Acid Pathway
Pest resistance
receptor
Ion-exchange
Signal amplification
PDF1.2 and other
gene expression
Increased nutrient uptake
Increased photosynthesis
Gene expression
involved
in plant growth