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244 Factors were studied that limited take-all development on
wheat following sorghum in doublecropping systems. Two
CORRELATION OF ARMILLARIA ROOT ROT WITH DAMAGE TO TREES field studies with plots exhibiting decreased take-all
DEFOLIATED BY THE JACK PINE BUDWORM. K.I. Mallett and W.J.A. severity following sorghum had lower soil pH than plots
Volney. Forestry Canada, Northern Forestry Centre, 5320 - 122 where wheat followed soybean. Bioassays for microbial
St. Edmonton, Alberta, Canada T6H 3S5 suppressiveness indicated wheat:sorghum doublecropped soils
were suppressive to take-all at levels similar to soils with
Pinus banksiana trees severely defoliated by the jack pine long histories of wheat monoculture. Growth chamber studies
budworm (Choristoneura pinus) in 1985 and '86 were dissected to indicated no differences in disease development on wheat in
determine the extent of root infections by Armillaria sp. The artificially infested soils after amending with various
severity of damage to the tree crowns was correlated with the summer crop residues, including sorghum, or planting with
incidence of root infection: 100 % of the dead trees were these summer crops, indicating no differences in pathogen
infected with Armillaria sp., 60% of the trees whose upper survival.
crowns had been killed were infected, but only 20% of the
healthy trees were infected. The value of these observations
in hazard-rating jack pine stands is discussed. 249
RHIZOSPHERE COMPETENCY OF FIVE Fusarium SPECIES ON MAIZE PLANTED
245 IN THE FIELD. C. M. Ocamb, T. Kommedahl, and P. M. Burnes,
Department of Plant Pathology, University of Minnesota, St.
VARABILITY IN VM CE OF OOSPORE INOCUIUM OF 1HYflJORA Paul, MN 55108.
CAPSICI AND THE RELATIONSHIP OF ME DENSITY OF OOSPORES IN
SOIL TO PI I4JTAMFLI. J.H. Bowers and D.J. Mitchell. Dept. In 1988, maize seeds were coated with one of five species of
of Plant Pathology, Univ. of Florida, Gainesville, FL 32611. Fusarium and planted in the field. Virtually the entire root
systems of the seedlings were colonized by each of three
Oospore progeny from 20 crosses of pathogenic isolates of species. Of these, F. moniliforme constituted 90%; E.
Phytophthora capsici were found to vary in pathogenicity to proliferatum, 48%; and F. oxysporum, 39% of the colonies
pepper. Only oospore inoculum from specific crosses caused isolated from roots. Seedling root systems were not extensively
disease in pepper seedlings with an initial inoculum density colonized by F. solani (9%) or F. equiseti (11%). The seedling
of 25 oospores per gram of soil. The highest mortality (30 to root systems of the dry control, water control, and the captan
75%) resulted fra oospore inoculum produced from crosses in- treatment yielded primarily F. oxysporum and F. solani.
volving certain isolates of A2 compatibility type. Oospore Rhizosphere competence differed in laboratory, greenhouse, and
inoculum from other crosses caused little or no disease (0 to field tests. Field studies offer evidence that Fusarium species
5%). Oospores produced from one cross resulted in 40 and 60% present in the seed differ as potential inoculum sources for
plant mortality with inoculum densities of 10 and 15 oospores root infections.
per gram of soil, respectively. The ID50 was calculated to be
41 oospores per gram of soil. Inoculum efficiency was calculated
using the multiple infection transformation as the
estimated number of infections per propagule and was found to 250
be 0.011. EFFECT OF INOCULUM DENSITY OF CEPHALOSPORIUM GRAMINEUM AND SOIL
pH ON CEPHALOSPORIUM STRIPE OF WINTER WHEAT. L. P. Specht and
246
T. D. Murray. Department of Plant Pathology, Washington State
University, Pullman, WA 99164-6430.
SURVIVAL OF FUSARIUM OXYSPORUM F. SP. VASINFECTUM IN
SOIL AS AFFECTED BY FOUR CROPPING SYSTEMS. J. P, McEntee,
R. D. Martyn, and J. L. Starr. Department of Plant Pathology and
Microbiology, Texas A&M University, College Station, 77843.
Winter wheats (cvs. Stephens and Nugaines) were grown in soil
adjusted to pH values of 4.5-7.6 and maintained at -0.01 MPa.
Soil in pots was drenched with conidial suspensions of Cephalosporium
gramineum when seedlings were in the four- to five-l
Microplots infested with a sand:cornmeal inoculum of Fusarium
oxysporum f. sp. vasinfectum (FOV) were planted with a FOVsusceptible
cotton cultivar (Rowden), a resistant cultivar (Auburn 634),
a nonhost (Sorghum), or left fallow. Populations of FOV were
determined initially and again at mid- and end-of-season. At the end
of the season, plants were examined for vascular browning (VB) and
isolation frequency of FOV from root and stem tissue. Final FOV
populations were highest in microplots planted to sorghum, while no
difference was observed between FOV populations from Rowden and
Auburn 634. The fallow treatment had the lowest FOV populations.
Differences in plant numbers with VB and in the nature of the VB
were observed between cotton cultivars. VB incidence and severity was
greatest in Rowden; however, no significant difference in isolation
frequency of FOV was observed between cultivars. Examination of the
sand:cornmeal inoculum revealed that it was composed largely of
microconidia, chlamydospores and mycelia. When dried under sterile
conditions, the microconidial and mycelial populations decreased
rapidly, leaving primarily chlamydospores.
stage. Inoculum density of C. gramineum, soil pH, and cultivar
h "
'
all had significant (P