Download Modes of Action for Plant Disease Management Chemistries

Modes of Action for Plant Disease
Management Chemistries
Michael Matheron
University of Arizona
Yuma Agricultural Center
Biological mode of action
Fungicidal action can be expressed in
one of two physically visible ways.
• Inhibition of spore germination.
• Inhibition of fungus growth.
Physiological mode of action
What happens at the cellular level to
cause the visible effects on spore
germination and fungal growth?
Why is it important to be familiar
with the physiological mode of
action of a fungicide ?
• For resistance management and
preservation of fungicide effectiveness.
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.
A typical cell and cell components
–Electron transport chain
–Enzymes
–Nucleic acid metabolism
and protein synthesis
–Sterol synthesis
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.
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.
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.
Inhibition of sterol synthesis
(Inhibit demethylation of ergosterol)
• Ergosterol is the major sterol in most fungi.
• It is essential for membrane structure and function.
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.
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.
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.
Plant activators
• Acibenzolar (Actigard)
• Harpin (Messenger)
• Biological control organisms
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.
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
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.
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
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
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
Phytochemicals
• Different from phytoalexins in that phytochemicals are
induced by wounding.
• Phenolics
– Furanocoumarins, Coumarins, Tannins, Lignin,
other phenolics
• Terpenoids
• Alkaloids
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
Mode of action - Actigard
Induction of Systemic Acquired Resistance
Mode of action - Messenger
Salicylic Acid Pathway
Classical SAR
SAR gene expression
harpin
Jasmonic Acid Pathway
Pest resistance
Ion-exchange
receptor
Signal
amplification
PDF1.2 and other
gene expression
Increased
nutrient uptake
Gene expression
involved
in plant growth
Increased
photosynthesis