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

Graduation date: 1995

Verticillium wilt of 'Willamette' hops (Humulus lupulus) was investigated to identifY the causal organism, to determine the incidence of the disease, and to explore the possibility of interactions with soil fertility and/or nematodes. In the first year of a three year study, sampling of yards followed a "searching for extremes approach". Selection of yards was based on a preliminary survey of all (35) hop growers in the Willamette Valley. Participating growers (10) were asked to identify one "good" and one "not-so-good" yard. Each of the 20 specified yards was subdivided into 4 plots; two representing a "good" and two representing a "not-so-good" area. In all 80 plots, data were collected to determine incidence of vascular colonization by Verticillium and stem necrosis in vines; soil and root parasitic nematode populations; concentrations of nitrate-N, ammonium-N, P, K, Ca, Mg, and pH in the soil surface, and nitrate-N, ammonium- N , and K in the subsoil; concentrations of total-P, K, and Zn in the leaves; and concentrations of nitrate-N, phosphate-P, and K in the petioles. The causal agents of the wilt were Verticillium dahliae in 13 yards and V. albo-atrum in one yard. Recovery of the pathogen within a yard ranged from 0 to 50% of sampled vines, while stem necrosis ranged from 0 to 68%. The frequency of infection was not significantly different among plots or yards, which suggests that the disease is present in all hop growing districts in Oregon. Soil nematode populations ranged from 0 to 3000 juveniles/100 g of dry soil. Heterodera humili (hop-cyst nematode) was the predominant parasitic nematode, while Pratylenchus (root-lesion nematode) and Paratylenchus (pin nematode) were recovered only occasionally. Densities of nematodes extracted from roots ranged from 0 to 2000 juveniles/g of moist root material and were primarily H. humili. A significant association between nematode populations and Verticillium incidence was not detected. Soil nutrient concentrations exhibited a high degree of variability among yards. The nitrate-N content, measured to a depth of 36" (90 cm) for individual hop yards, ranged between 65 (73) and 417 lb/A (468 kg/ha) with a mean value of 270 lb/A (302 kg/ha). Concentrations of ammonium-N were determined to be approximately one-fourth of the nitrate concentrations. Phosphorus and potassium concentrations ranged from 55 to 155 ppm and 118 to 799 ppm, respectively, in the surface soil. For the same depth, soil pH ranged from 5.15 to 6.78. Petiole concentrations of nitrate-N and potassium ranged from 0.16 to 1.3% and from 1.26 to 6.84%, respectively. While it is believed that the duration of the sampling period may have been responsible for the wide range in nitrate-N values, petiole potassium concentrations are thought to reflect the potassium content in the soil. The concentrations of K in petioles increased steadily with increasing soil test values up to 350 ppm K. Soil and tissue nutrient concentrations found within and among hop yards did not correlate significantly with the incidence of Verticillium wilt. However, petiole nitrate-N concentrations were significantly higher (p < 0.05) in plots infected with Verticillium (0.73%) as compared to noninfected plots (0.56%). / Graduation date: 1991