| Summary: | 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
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