1	Fungicide Resistance: Past, Present and Future Outlook Frank P. Wong Department of Plant Pathology University of California, Riverside
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5	Fungicides and IPM Fungal diseases are the number one cause of plant loss caused by microorganisms Fungicides are a valuable part of IPM programs Tolerance for disease is low No effective cultural controls Most energy efficient way to control fungi Use fungicides properly and maintain their long-term utility
6	Fungicide Resistance Occurs when fungi adapt to fungicides which leads to immunity reduced efficacy Inheritable trait, passed on from one generation to the next Impact Direct crop loss Wasted time and money Loss of disease control options
7	Resistance in Turf and Production Nurseries Both are high risk situations Monoculture Constant cropping/perennial crop system High value/low disease tolerance Often limited in cultural control options/choices for resistant varieties
8	History of Fungicide Resistance Not of major concern until the introduction of site-specific fungicides (1970s) Multi-site (captan, chlorothalanil, EBDCs, sulfur) Single-site (benzimidazoles, DMIs, QoIs) Site-specific fungicide benefits: Systemic/ curative properties Reduced non-target effects/environmental impact Resistance development to these is much easier Small genetic changes in fungi overcome fungicides targeted at one enzyme or biochemical pathway
9	History of Fungicide Resistance
10	Fungicide Resistance Nursery Botrytis grey mold dicarboximides, benzimidazoles Phytophthora root rots phenylamides Turf Anthracnose benzimidazoles, QoIs, sterol biosynthesis inhib. Dollar spot dicarboximides, benzimidazoles Grey leaf spot benzimidazoles, QoIs Pythium blight phenylamides
11	How Does Resistance Occur? Resistant individuals occur naturally at a very low frequency (< 0.0001%?) Natural mutants Sexual and asexual recombination Resistant individuals are selected by fungicide applications Repeated fungicide use increases the frequency of resistant individuals When this frequency is high enough, control is impacted – “practical resistance” Fungicides do not “change” the fungus - only the population structure
12	Selection for Resistance
13	Resistance Risk Many factors contribute to the risk of resistance for any given fungcide
14	Biochemical Mode of Action BMOA is the primary determinant of the fungicidal activity of a chemical Most fungicides function by binding to fungal cell components and disrupting key metabolic functions Target specificity Multi-site Inhibitors Single-site Inhibitors
15	Multi-site Inhibitors Affect multiple metabolic processes
16	Multi-site Inhibitors
17	Multi-site Inhibitors
18	Single-site Inhibitors Affect a specific metabolic process Bind to proteins via “lock and key” process
19	Single-site Inhibitors Amino Acid and Protein Synthesis Anilinopyrimidines Sterol Biosynthesis DMIs Respiration (MET complex III) QoIs Respiration (unknown) Phenylpyrroles Lipid Biosynthesis Sub. Aromatic Hydrocarbons Dicarboxymides mRNA Synthesis Phenylamides Nuclear Division Benzimidazoles Respiration (MET complex II) Carboxins
20	Cross-Resistance Cross-resistance affects all fungicides with a similar biochemical mode of action Once resistance develops to a fungicide - all members of the same cross resistance group are affected Examples: Benzimidazole resistance affects benlate, topsin-M QoI resistance affects Quadris, Abound, Flint, Headline, Cabrio DMI resistance affects Rally, Elite, Rubigan, Orbit
21	Pathogen Biology Resistance occurs more frequently in pathogens that: Have a short generation time Have a high frequency of reproduction Are prone to genetic changes Each application is a selection event Higher chance of selection for resistance against large, fast reproducing pathogens
22	Relative Resistance Risk
23	Use Patterns Every application is a selection event Curative and eradicant applications are worse: Incomplete control Selection against a larger population Reduced rate applications are worse: Incomplete control Selection against a larger population
24	Physical Mode of Action
25	PLASVI Seedling Bioassay: Protectant and Postinfection Activity
26	Activity of Azoxystrobin Against Grapevine Powdery Mildew
27	Patterns of Resistance Development Resistance develops differently dependent on how many genes are involved in resistance Polygenic (many genes) Quantitative or “shifting” resistance DMIs, SIs, dicarboximides Monogenic (one gene) Qualitative or “disruptive” resistance Benzimidazoles, phenylamides, QoIs
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29	Patterns of Resistance Development Shifting Reduced control, especially at low label rates Reduced application intervals Slower to develop over time Disruptive No control at field rates Failures can be sudden and dramatic
30	Distribution of NY Grapevine Powdery Mildew Sensitivities to Myclobutanil 1995 vs. 1999
31	Grapevine Powdery Mildew Control
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33	Anthracnose Control
34	Resistance Management Strategies Reduce the overall number of selection events Alternate fungicides with different modes of action Partner must also work! Mixtures? site-specific + multi site = 1 selection event site-specific + site specific = 1 selection event each (selection for double resistance) Worse when mixture partner is used at ½ rate Partner must provide 80% control to prevent resistance selection (Delp 1986)
35	Selection for Resistance
36	Fungicide Alternation
37	Who Mandates Resistance Management Enforcement? FRAC – intra-industry committee that determines use limits for new and current chemistries Membership is primarily from large producers and focuses on larger fungicide groups Primarily based in EU Enforces resistance management through label restrictions
38	Fungicide Resistance Management - An Industry Effort
39	Future Outlook for Fungicide Resistance: Impact of FQPA and EPA standards Older multi-site chemistries will have increased restrictions B2 carcinogens new efforts to examine SIs All new fungicides must have reduced non-target effects All new fungicides will be single-site New fungicides will take longer to develop and register
40	Summary Resistance is the Achilles’ heel of all site-specific fungicides Resistance occurs naturally Avoid the development of practical resistance Many mechanisms of resistance Know the chemical class of fungicides you use Know the cross-resistance classes Two resistance patterns: Disruptive resistance = “all or nothing” Shifting resistance = “partial loss”
41	Summary Reduce the total number of fungicide applications and resistance selection events Alternate between cross-resistance groups Know the biochemical mode of action and cross resistance groups Reduce the population size being selected against Practice good disease management Don’t wait until the epidemic is raging Use fungicides effectively and efficiently Time applications based upon knowledge of disease epidemiology and development Don’t cheat with rates or application intervals Don’t rely on curative/eradicant activity It may cost you more in the end
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