**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** THE SMALL GRAIN CEREAL CROPS OF WHEAT, OATS, AND BARLEY ARE IMPORTANT AGRICULTURAL COMMODITIES IN THE UNITED STATES AND MANY OTHER COUNTRIES OF THE WORLD. UNFORTUNATELY, THEY ARE ATTACKED BY RUST DISEASES CAUSED BY THE FUNGAL PATHOGENS PUCCINIA GRAMINIS F. SP. TRITICI (PGT) (WHEAT STEM RUST), P. CORONATA F. SP. AVENAE (PCA) (OAT CROWN RUST), AND PUCCINIA HORDEI (PH) (BARLEY LEAF RUST). YIELD LOSSES DUE TO THESE RUST DISEASES CAN EXCEED 40% DURING SEVERE EPIDEMICS. THE DEPLOYMENT OF RESISTANT VARIETIES IS THE MOST EFFECTIVE MEANS FOR CONTROL OF THE CEREAL RUST DISEASES. THIS STRATEGY IS BASED ON EXPLOITING GENETIC VARIATION FOR NOVEL RESISTANCE (R) GENES. MOST R GENES ENCODE IMMUNE RECEPTOR PROTEINS THAT RECOGNIZE PATHOGEN 'EFFECTOR' PROTEINS WHICH ARE DELIVERED INTO HOST CELLS DURING INFECTION. EFFECTORS RECOGNIZED BY IMMUNE RECEPTOR PROTEINS ARE REFERRED TO AS AVIRULENCE (AVR) FACTORS AS THEIR RECOGNITION LEADS TO THE PATHOGEN'S INABILITY TO INFECT LINES CARRYING THE APPROPRIATE R GENE IMMUNE RECEPTOR. THE MAJOR CHALLENGE IN BREEDING CEREAL CROPS FOR DURABLE RUST RESISTANCE IS THE EVOLUTION OF PATHOGEN POPULATIONS TO OVERCOME RESISTANCE CONFERRED BY R GENES THROUGH MUTATIONS IN THE CORRESPONDING AVR GENES. THIS HAS RESULTED IN MANY BOOM AND BUST CYCLES, WHEREBY NEWLY DEPLOYED RESISTANCE GENES PROVIDE PROTECTION FOR ONLY A LIMITED TIME AND ARE THEN OVERCOME BY NEWLY EVOLVED VIRULENT PATHOGEN ISOLATES. THERE IS A DEARTH OF KNOWLEDGE CONCERNING AVR GENES AS ONLY THREE HAVE BEEN IDENTIFIED IN THE CEREAL RUST FUNGI, ALL FROM PGT. CHARACTERIZATION OF THE BROAD REPERTOIRE OF AVR GENES IN CEREAL RUSTS IS CRITICAL FOR UNDERSTANDING HOW INTERACTIONS WITH THE PLANT'S HOST IMMUNE SYSTEM INFLUENCE THE EVOLUTION OF VIRULENCE IN THESE IMPORTANT AGRICULTURAL PATHOGENS. THIS KNOWLEDGE, IN TURN, IS CRITICAL FOR DEVELOPING NOVEL RESISTANCE BREEDING STRATEGIES FOR CEREAL CROPS, THEREBY REDUCING THE IMPACT OF THESE DEVASTATING DISEASES FOR PRODUCERS AND END-USERS ALIKE.VERY LITTLE IS KNOWN ABOUT THE ROLE OF RUST EFFECTORS IN PROMOTING DISEASE IN THE ABSENCE OF IMMUNE RECOGNITION BY HOST PLANT RECEPTORS. THE OVERALL AIM OF THIS RESEARCH IS TO DETERMINE HOW HOST IMMUNITY AFFECTS VIRULENCE EVOLUTION IN CEREAL RUST FUNGI. THIS RESEARCH PROJECT AIMS TO ADDRESS KNOWLEDGE GAPS BY 1) GENERATING COMPLETE HAPLOTYPE-PHASED GENOME ASSEMBLIES FOR DIVERSE SETS OF PGT, PCA, AND PH ISOLATES FROM ACROSS THE WORLD; 2) IDENTIFYING GENES UNDERLYING AVIRULENCE PHENOTYPES AND THEIR GENETIC DIVERSITY; AND 3) UNDERSTANDING THE FUNCTIONS OF AVR EFFECTORS IN DISEASE. BY STUDYING THREE DIFFERENT RUST PATHOGENS ON DIFFERENT CEREAL HOSTS, WE WILL BE ABLE TO CONDUCT CROSS-SPECIES COMPARISONS TO IDENTIFY COMMON MECHANISMS OF VIRULENCE IN THESE SYSTEMS. THE KNOWLEDGE GAINED FROM THIS RESEARCH WILL BE CRITICAL FOR IDENTIFYING COMBINATIONS OF EFFECTIVE R GENES THAT CAN BE BRED INTO SMALL GRAIN CEREAL CULTIVARS, LEADING TO THE DURABLE CONTROL OF THE DEVASTATING RUST PATHOGENS. MOREOVE,R, THE DATA GENERATED FROM THIS WORK WILL INCREASE GLOBAL RUST SURVEILLANCE CAPACITY BY REDUCING THE NEED FOR COSTLY, LABOR-INTENSIVE INFECTION ASSAYS AND WILL FACILITATE RISK ASSESSMENTS FOR POSSIBLE RESISTANCE BREAKDOWN. THE PROPOSED RESEARCH WILL PROVIDE THIS CRITICAL MISSING ELEMENT IN DISEASE MANAGEMENT AND CROP PROTECTION PROGRAMS TO REDUCE BOOM AND BUST CYCLES OF RUST DISEASE EPIDEMICS IN SMALL GRAIN CEREAL CROPS, WHICH WILL ULTIMATELY CONTRIBUTE TO GLOBAL FOOD SECURITY.