COTTON PRODUCTION ACROSS THE UNITED STATES IS UNDER SERIOUS THREAT FROM A SOIL-INHABITING, DENSITY DEPENDENT FUNGUS, CALLED FUSARIUM OXYSPORUM F. SP. VASINFECTUM RACE 4 (FOV4), WHICH CAUSES FUSARIUM WILT. PREVIOUSLY, IT WAS FOUND ONLY IN CALIFORNIA, BUT DURING THE PAST DECADE THE DEVASTATING EFFECT OF FOV4 HAS MOVED INTO THE WIDER COTTON BELT AND IS A SYSTEMIC PROBLEM. LEFT UNCHECKED, FOV4 COULD DECIMATE THE ENTIRE COTTON GROWING INDUSTRY IN THE UNITED STATES. STABLE AND DURABLE GENETIC RESISTANCE TO FOV4 IN UPLAND COTTON IS NOT AVAILABLE, DESPITE FOCUSED EFFORTS. FORTUNATELY, IN GOSSYPIUM BARBADENSE (PIMA COTTON), STABLE AND DURABLE RESISTANCE TO THE SAME FOV4 PATHOGEN WAS DISCOVERED BY OUR TEAM AND IS CURRENTLY DEPLOYED IN COMMERCIAL VARIETIES. THIS APPROACH INCLUDES ROBUST PROTOCOLS FOR BUILDING INOCULUM DENSITY AND INCLUDING PROPRIETARY CHECKS TO MAP THE SEVERITY OF DISEASE ACROSS THE FIELD OVER THE PAST TWO DECADES, WHICH ARE RELIABLE AND REPEATABLE ACROSS YEARS. FIELD SCREENINGOF 1,000 UPLAND COTTON LINES BEGAN IN 2018, AND WE HAVE IDENTIFIED PROMISING FOV4 RESISTANT UPLAND GERMPLASM. IN THIS PROJECT, WE WILL CONTINUE SCREENING UPLAND GERMPLASM AND TO IMPROVE FOV4 RESISTANCE BY STACKING FAVORABLE ALLELES FROM MULTIPLE RESISTANCE SOURCES, AND TO ELUCIDATE THE GENETICS OF RESISTANCE. THIS PROJECT WILL DELIVER NEW SOURCES OF FOV4 RESISTANT GERMPLASM, HIGHLY RESISTANT LINES, QUANTITATIVE TRAIT LOCI (QTL)S AND BIOMARKERS TO TRACK AND PREDICT RESISTANCE.