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

Induction of Systemic Acquired Resistance 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 receptor Ion-exchange Signal amplification PDF1.2 and other gene expression Increased nutrient uptake Increased photosynthesis Gene expression involved in plant growth