Sour rot of tomatoes, caused by Geotrichum candidum, occurs in the field and postharvest settings regularly, although postharvest losses are severe only in some years on the Eastern Shore of Virginia (ESV) and other tomato production regions. Fungicide products and cultural control methods are tested for efficacy utilizing a traditional wounding technique that does not properly reflect natural sour rot infections. A new inoculation technique was optimized for G. candidum using negative pressure to infiltrate the tomato stem scar with pathogenic spores. This new method creates consistently high rates of infection and more successfully creates infections in mature green and breaker fruit. The population of G. candidum on the Eastern Shore of VA (ESV) was characterized using multilocus sequencing technique. The resulting phylogenetic tree defines four distinct groups, including two with uncommon loci that distinguish them from the majority of the population. Thirty-seven G. candidum isolates were inoculated to media amended with ten fungicides and antimicrobial compounds commonly used in tomato production and postharvest treatments. Propiconazole and tebuconazole completely inhibited growth of all colonies. Cultivar trials were conducted to determine if resistance or tolerance to G. candidum occurs. Ten commonly grown round and Roma cultivars on the ESV were similarly susceptible to G. candidum, even at low inoculum levels. Field and postharvest surveys of sour rot on tomato fruit attempted to correlate disease incidence with weather conditions in order to better understand the cause of sporadic infection. Few patterns were seen consistently throughout harvest periods and years. Rainfall was positively correlated with disease 2-3 days before surveys and temperature was negatively correlated with disease 5-7 days before surveys. No in-field weather conditions were correlated with postharvest disease incidence. Greenhouse trials were conducted to assess the influence of water congested tomato fruit on susceptibility to sour rot. Tomato plants were exposed to water inundation to mimic rainfall and varying levels of irrigation, both in order to congest tomato fruit. Though water congestion was achieved, tomato fruit were equally susceptible to sour rot infections. / Ph. D.