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86 when root numbers were lowest. This indicates that citrus
roots may be able to compensate for root damage or reduced
THE `FECT OF CXMPOSTED MUNICIPAL SEWAGE SLUDGE (CIVISS) ON numbers with increased efficiency of root uptake.
PHYTOPHTHORA ROOT ROT AND SCLEROTINIA CROWN AND STEM ROT OF
ALFALFA. R. P. Woodward and R. B. Carroll, Department of Plant
Science, University of Delaware, Newark, DE 19717-1303. 90
Saranac alfalfa was planted into 15.2 cm pots containing autoclaved
silt loam soil amended with C(SS obtained from three
municipalities. Experimental design was a randomized complete
block with five replications. Treatments were loading rates of
11.2, 22.4, 44.8, 112.0, and 224.0 Mg/ha of each CMSS.
Phytophthora megasperma f. sp. medicaginis (Pmm) and
Sclerotinia trifoliorum via ixig (St) ito inocula oil achamed rio were toplatin. applied separately Aterresistance
via mixing into each amended soil prior to planting. After
nine weeks, disease development was rated. No significant
differences (P=0.05) were noted with respect to 41455 origin,
Significant Pmm disease suppression occurred at 44.8 and
112.0 Mg/ha. Suppression of disease caused by St occurred at
all loading rates. Isolations from infected root and crown
tissues indicated less Pmm was recovered as the CMSS rates
increased but St remained constant.
87
SOILS SUPPRESSIVE TO BLACK ROOT ROT OF BURLEY TOBACCO IN
WESTERN NORTH Carolina. Julie R. Meyer and H. D. Shew. Dept.
of Plant Pathology, North Carolina State University, Raleigh,
NC 27695-7616.
SoiLs suppressive to black root rot were identified by th
abs supprese in blac rotrter identified bythe
absence of disease to black in root fields rot planted and containing in cultivars 2-500 with cfu/g low soil of
Thielaviopsis basicola. Suppressiveness was confirmed under
controlled environmental conditions with several pathogen
isolates and host cultivars.
ies
Suppressiveness
and sultivaro. Suppressive e
was
s
not
of T.
the
result of reduced survival of the chlamydospores of T.
basicola. The suppressive factor appears to be abiotic and
associated with soil acidity. Soil amendments were used to
separate the effects of different components of acidity (soil
pH, base saturation, exchangeable Al) on disease. The results
suggest that soil Al may be the mechanism of suppressiveness in
these soils.
EFFECT OF INSECT DEFOLIATION ON SEVERITY OF FUSARIUM CROWN-
ROT OF ALFALFA. P. D. Colyer,I J. W. Lee, 2 and S. S.
Quisenberry. 91
2 Louisiana State University Agricultural Center,
Louisiana Agricultural Experiment Station, Red River Research
Station,I Bossier City, LA 71113 and Department of Entomology,
COLONIZATION OF SOLANUM TUBEROSUM L. BY COLLETOTRICHUM COC
2
Baton Rouge, LA 70803.
(WALLR.) HUGHES A.W. Barkdoll and J.R. Davis, Univ. of Idaho
R&E Center, Aberdeen, ID 83210
Alfalfa, Medicago sativa L. variety 'Florida 77', was inoculated
with three different isolates of Fusarium and defoliated
to varying levels with yellowstriped armyworms, Spodoptera
ornithogalli (Guenee), to determine the effect of insect
defoliation on the development of crown-rot under greenhouse
conditions. There were no significant interactions between
short-term insect defoliation and Fusarium crown-rot on forage
quality, yield, or root carbohydrate reserves. Although
insect defoliation alone did reduce plant height, yield,
and maturity (18,33, and 30%, respectively) at the first
harvest, no significant effects were observed at two subsequent
harvests. Fusarium oxysporum Schlecht was the most virulent
of the three isolates tested.
Colletotrichum coccodes can be found in roots, tubers, stem
bases and apices of potato in the field. Tubers from foliar
inoculated potatoes contained significant differences (P=O.Ol)
in colony forming units (cfu's) of C. coccodes of 2000 and 300
in stem and bud ends respectively. Surface disinfestation of
tubers with 10% clorox did not reduce cfu's. In view of the
high cfu's of C. coccodes in tubers and stems an experiment was
conducted to evaluate fungal colonization of potato originating
from infested seed tubers. Russet Burbank mini-tubers wgre
inoculated with a conidial suspension (50 ul of 9.2 x 10
conidia/ml) of C. coccodes and were planted in the greenhouse.
The resulting pl-ants were destructively sampled three times
during the season. Stem apices and bases, roots and soil were
sampled for C. coccodes at each period. At no sampling period
88
was C. coccodes detected in stem bases or apices. In contrast,
C. coccodes in soil increased from zero to above 0.2 cfu/g soil.
Root infection at the last sampling increased from 0 to 15%.
CORRELATION BETWEEN SAMPLES FOR ESTIMATION OF INOCULUM DENSITY
OF CYLINDROCLADIUM CROTALARIAE IN SOIL. A. K. Culbreath, r1.
K. Beute, and B. B. Shew, Depts. of Plant Pathology, Coastal 92
Plain Univ., Expt. Raleigh, Stn., NC Tifton, 27695. GA 31793 and North Carolina St.
COLONIZATION AND PATHOGENICITY OF FUSARIUM OXYSPORUM AND F.
Four quadrants, each consisting of 63 (13.7 mi
SOLANI ON ESSEX SOYBEAN. G. M. Farias and G. J. Griffin.
2 ) contiguous
plots were established in a field naturally infested with
Cylindrocladium crotalariae in Martin County, NC, in 1988.
Half adjacent of the area field was was planted planted to peanuts. to corn the Two previous quadrants year and an
(designated peanut or corn) were situated in each area. Two
independen samplest e corn)weresisuting
of 12 acm diam. To
independent samples, each consisting of 12 (2.5 cm diam. x
16.5 cm) soil
density
cores, were taken from all plots. Inoculum
elutriation-selective
(ID) of C. crotalariae
medium technique.
was estimated
Both
using
samples
an
gave
similar estimates of mean ID in each quadrant; ID was
in
greatest
peanut quadrants. Estimates of ID from the two samples
were highly correlated (p < 0.01) in peanut quadrants but were
not significantly correlated in corn quadrants,
Department of Plant Pathology, Physiology and Weed Science.
Virginia Polytechnic Institute and State University. BlaCksburg,
VA 24061.
Colonization Clnzto of fEsxsyenctldn Essex soybean cotyledons by yFsru Fusarium oxysp xsou
occurred 1 day after planting in naturally infested soil and the
fungus was present at high frequency (25%) after 4 days. F.
oxysporum and F. solani colonized the lower hypocotyl and -
emerging 4 days both roots fungi 2 and were 3 days found after colonizing planting, t elongatin respectively. Afer
portion of the hypocotyl. The hypocotyl-root transition 9 , upper zone had
the highest frequency (20-28% of 2-mm tissue segments)
nization
of colo-
by each species at 4 days. F. solani had higher
colonization rates per unit of inoculum than F. oxysporum (0.042
and 0.023 colonizations/m root/propagule/g soil after 4 days,
89
respectively). In soil-temperature tank tests at 20 C and -0.01
MPa water potential, all of the four
solani
F. oxysporum
isolates
and four
tested
F.
delayed seedlilig emergence and caused
significant reductions in stem length and plant fresh weight.
ROOT UPTAKE OF RUBIDIUM-86 BY CITRUS ROOTS AS AFFECTED BY ROOT
PATHOGENS, SEASON AND IRRIGATION. Q. A. Menge, E. L. V.
Johnson, E. Pond and H. Liu. Dept. of Plant Pathology, 93
University of California, Riverside, CA 92521.
THE EFFECT OF PLANTING DATE ON FUSARIUM WILT OF M4USKMELON IN
Uptake of Rb" by 1-cm pieces of excised citrus root tips was CALIFORNIA. 0. J. Jacobson and I. R. Gordon, Dept. of Plant Pathmeasured.
In a greenhouse experiment, roots from Troyer ology, University of California, Berkeley 94720.
citrange inoculated with Phytophthora parasitica absorbed 36%
less Rb" than roots not inoculated with P. parasitica. In the A randomized complete block design was employed with planting
field, trees treated with nematicides and fungicides (oxamyl dates as blocks and susceptible and resistant cultivars as
and metalaxyl) consistently exhibited a 27-57% greater uptake treatments; the experiment was conducted for two years. Cortical
of Rb" than did non-treated trees. Most of this increase in root colonization by Fusarium oxysporum f. sp. melonis was
uptake was observed July-October when P. parasitica and nematode measured on seedlings and at full fruit load. Rate of disease
populations were high. Uptake of Rb" was greater in roots of progress and incidence at harvest were both higher during hotter
citrus receiving irrigation which was 80% and 120% of the portions of the growing season. Root colonization by the
evapotranspiration demand (ETD) than by those receiving 100% of pathogen was not affected by planting date; cultivar differences
the ETD. Root uptake of Rb" fluctuated considerably on a were apparent only at full fruit load and may represent xylem
seasonal basis and was greatest during the summer months and colonization of the susceptible host. Inoculumn varied sig-
1146 PHYTOPATHOLOGY