A necrosis at the tip of cones was observed on hop (Humulus lupulus), cultivar "Nugget", grown in Oregon in the early 1990's. Fusarium sambucinum and F. avenaceum were recovered from symptomatic cones in 1998 and preliminary inoculation experiments suggested both Fusarium species could cause hop cone necrosis. Studies were carried out to (1) examine pathogenicity and demonstrate Koch's postulates for hop cone tip blight using isolates of F. avenaceum and F. sambucinum obtained from hop cones; (2) examine isolates of F. avenaceum and F. sambucinum derived from other diseased plant hosts, and other Fusarium species derived from hop cones, for ability to cause cone necrosis; and (3) survey commercial fields to determine Fusarium populations on 'Nugget' cone parts. Isolates ofF. avenaceum and F. sambucinum recovered from diseased hop cones were used for pathogenicity experiments. In addition, cone inoculations were done with single isolates of F. avenaceum and F. sambucinum from diseased sweet corn roots, one isolate of F. sambucinum recovered from a diseased potato tuber, individual isolates of F. equiseti and F. oxysporum from hop cones. Cones of two hop cultivars, 'Nugget' and 'Willamette', were collected from three different farms on three sampling dates and inoculated with spore suspensions of hop-derived F. avenaceum and F. sambucinum at concentrations of 10��, 1O���, and 10��� conidia/ml to examine dosage effects. Necrosis was evaluated 2, 4, 6, and 8 days after inoculation. Percent cone necrosis decreased as inoculum concentration of either F. avenaceum or F. sambucinum decreased, and was lowest on water-treated cones, for all three sampling dates. The respective Fusarium species were recovered from symptomatic cones. Cone necrosis developed following cone inoculation with F. avenaceum or F. sambucinum from potato or corn. Hop cones inoculated with F. equiseti or F. oxysporum also developed necrosis, but at relatively lower levels compared to the other Fusarium species used for inoculations. For the surveys in commercial hop fields, burr and cone material were collected on five different dates. Fusarium sambucinum was recovered most frequently, but F. avenaceum was also found. Both Fusarium species were recovered from asymptomatic burr and cone materials throughout the growing season. In general, Fusarium species, F. equiseti, F. oxysporum, F. culmorum, F. solani, plus F. avenaceum and F. sambucinum were found more frequently early in the season on stigmatic tissue, and Fusarium recovery decreased as the season progressed. Fusarium prolferatum and F. monilforme were recovered rarely. / Graduation date: 2004