Control of Nematodes and Related Diseases in Potatoes
K.A. Rykbost l , J. Maxwell', and R.E. Ingham 2
Introduction
Root-knot nematodes (Meloidogyne
chitwoodi) and corky ringspot disease (CRS),
caused by tobacco rattle virus and vectored by
the stubby-root nematode (Paratrichodorus
allius), remain among the most costly pests for
potato production in the local area and in the
Pacific Northwest. Crops seriously infected by
either of these problems are reduced to the
status of culls. In each of the past five years,
crops in fields with no history of CRS have
become infected, inflicting substantial
economic losses on growers. Fields in several
areas of the northwest have been abandoned for
potato production due to the presence of
stubby-root nematodes and the lack of
satisfactory control options.
The choice of control measures for rootknot nematodes depends on nematode
populations, climatic conditions, soil conditions
(organic or mineral soils), and crop rotation
alternatives. Chemical control options range
from an application of Mocap (ethoprop) at low
population levels, at a cost of about $150/acre,
to the extreme case of Mocap plus Vapam
(metham sodium) at about $300/acre.
Mocap, Vapam, and Telone II (1-3,
dichloropropene), which are all somewhat
effective in controlling root-knot nematodes,
have not provided acceptable control of stubbyroot nematode and CRS in previous research.
The most effective control for CRS, Temik
(aldicarb) was reinstated for use on potatoes in
the Pacific Northwest in 1996. However, a
restriction of 150 days-to-harvest was imposed
for its use. This effectively limits the use of
Temik to late maturing varieties in long season
production areas.
Research in the Columbia Basin
continues to evaluate combinations and
application rates of products in an attempt to
identify lower cost control options for both pest
problems. With the possibility that Temik may
become available with a less restrictive days-toharvest interval, a study was conducted at the
Klamath Experiment Station in 1996 to
evaluate the efficacy of Mocap, Telone II, and
Temik, alone or in combinations, in a field with
fairly high nematode populations.
Procedures
The experimental site was a Poe fine
sandy loam soil that has been in a barley/potato
rotation for over 10 years. The site has been
used for nematode research in alternate years
since 1990. Relatively high populations of
stubby-root and root-knot nematodes occur
throughout the field. Plot areas were
established in October 1995 to accommodate
six treatments and four replications in a
randomized complete block design. Individual
plots were 16 feet (6 rows) wide and 42 feet
long. Three plots in each replication received
15 gpa of Telone II shanked in on 18-inch
spacing at 16-inch depth with a V-ripper on
October 24. The field was plowed in midNovember.
1 /Superintendent/Professor and Biological Sciences Research Technician III, respectively,
Klamath Experiment Station, Klamath Falls, OR.
Associate Professor, Department of Botany and Plant Pathology, Oregon State
University, Corvallis, OR.
Acknowledgments: Funding provided by the Oregon Potato Commission, and Rhone-Poulenc is
gratefully recognized.
7 6
Klamath Experiment Station
Soil depths of 0 to 8 inches and 8 to 16
inches were sampled in each plot on April 4.
Samples were composites from about 15
probes/plot. A second set of samples was
obtained from all plots on October 7, 5 days
after potatoes were harvested. These samples
were from the top foot of soil. All soil samples
were sieved and a 250 g soil sample was
extracted with wet sieving-sucrose
centrifugation by personnel in the nematology
laboratory at the OSU Department of Botany
and Plant Pathology.
Mocap EC was applied at 12 lb aiJacre
with a conventional ground sprayer and
incorporated in the top 8 inches of soil with a
disc on May 3. Two plots in each replication
were treated with Mocap. Temik was applied
in the seed furrow to the four center rows of
designated plots at a rate of 3.0 lb ai/acre
during planting on May 23. Russet Burbank
potatoes were planted at 12-inch seed spacing
in 32-inch rows with a two-row, assisted-feed
planter. No granular insecticides were applied
to non-Temik treatments at planting.
Standard cultural practices were
followed except for nematode control
measures. Vines were desiccated with Diquat
applied at 1.0 pint/acre on September 12. All
tubers from 30 feet of the center two rows of
each plot were harvested with a one-row,
digger-bagger on October 2 and stored until
grading on October 30. All tubers with visible
root-knot nematode blemish were classified
blemished, regardless of size or other grade
considerations. Twenty tubers from each plot
were cut into quarters and cut surfaces were
inspected for CRS symptoms. Any evidence of
CRS, regardless of severity, constituted a
positive CRS determination. Yields were
recorded by size and grade for unblemished
tubers. CRS infection did not result in
downgrading tubers otherwise classified as
No.ls.
Results and Discussion
Fall applied Telone II greatly reduced
the population of both root-knot and stubbyroot nematodes (Table 1). Populations of rootknot nematodes in April were highly variable in
plots not treated with Telone II, ranging from 0
to over 700/250 g dry soil in individual plots.
The distribution between soil layers indicated
higher populations in the top 8 inches in plots
not treated with Telone H. Stubby-root
nematode populations ranged from 0 in 5
treated plots to about 100/250 g dry soil in one
untreated plot. Stubby-root nematodes
appeared to be uniformly distributed in the top
16 inches of soil.
Populations of root-knot nematodes
after harvest were much higher than have been
observed in previous research at this site. The
very warm weather in July and August may
have resulted in an additional nematode
generation compared with a cooler season. The
mean population for the control treatment was
five times greater than the highest population
observed in a 1994 study conducted in the same
field. All plots treated with Telone II had
significantly fewer root-knot nematodes than
the untreated control. Stubby-root nematode
populations were lower in October than in
April samples. In Temik treatments, no stubbyroot nematodes were found in five of the eight
plots. No CRS symptoms were observed in
samples from any of these plots. However, no
stubby-root nematodes were found in two
replications of the control treatment, while CRS
infection in these plots was over 60 percent.
Crop development appeared normal
through the season, with no apparent effect of
treatments on foliage. Total yields were very
uniform across treatments (Table 2).
Differences in grade between treatments were
mainly due to the level of root-knot nematode
blemish. In three out of four replications, all
tubers in untreated control plots were
blemished. Root-knot nematode populations in
post-harvest samples ranged from 1,500 to
Klamath Experiment Station
77
3,350/250 g dry soil in these plots. About 37
percent infection was observed in the fourth
replication where the population was 390/250 g
dry soil. The extent of blemish was also
variable in other treatments. For example,
blemish in Mocap treated plots was 4, 8, 38,
and 100 percent in the four replications. The
corresponding populations were 82, 54, 2,032,
and 1,602/250 g, respectively. While large
differences were found between treatments in
the percent of blemished tubers, the only
statistically significant difference was between
the control and all other treatments. The
average blemish in treatments was in good
agreement with post-harvest root-knot
nematode populations.
Nematode blemish data show trends
that are consistent with results observed in
previous research at the same location. Mocap
was not as effective as Telone II in reducing
blemish. The combination of Telone II and
Mocap did not improve control compared with
Telone II applied alone. Temik appeared to be
slightly more effective than Mocap, either alone
or in combination with Telone II. It should be
noted that the application rate for Telone II was
less than would normally be applied for the
root-knot nematode populations found at this
site. However, the lower rate was selected
specifically to determine whether it would be
adequate when used in combination with
Mocap.
CRS infection levels were also highly
variable in most treatments (Table 2). Mocap
did not provide any reduction in CRS. The fall
samples show higher stubby-root nematode
populations in plots treated with Mocap than in
7 8
the untreated plots. Telone II reduced the
incidence of CRS by about one-third, but not
significantly. Stubby-root populations in fall
samples were the same in Telone II treated
plots as in the control. Temik, alone or in
combination with Telone II, significantly
reduced CRS compared to control and Mocap
treatments.
Summary
The only treatment that provided
commercially acceptable control of both rootknot nematode blemish and CRS infection was
the combination of Telone II and Temik.
Mocap reduced blemish and root-knot
nematode population by about 50 percent, but
was ineffective in controlling stubby-root
nematodes or CRS infection. Based on
previous research, Telone II would probably
have achieved acceptable control of tuber
blemish at 20 or 25 gpa application rates. As in
previous trials, Telone II did not control CRS
even though it appeared to control stubby-root
nematodes based on populations observed in
spring samples.
The study once again demonstrated the
efficacy of Temik for CRS control. The 150
days-to-harvest restriction precludes use of
Temik for most crops in the Klamath Basin.
Crops planted in mid-May could not be
harvested prior to mid-October. If the preharvest interval is reduced to 120 days, Temik
will return as the best, if not the only, feasible
control measure for CRS caused by tobacco
rattle virus and vectored by stubby-root
nematodes.
Klamath Experiment Station
Table 1. Effect of fumigation and nematicide treatments on population of root-knot and stubbyroot nematodes at Klamath Falls, OR, 1996.
Treatment I
Root-knot
April 4
0-8"
8-16"
Oct. 7
0-12"
Stubby-root
April 4
8-16"
0-8"
Oct. 7
0-12"
nematodes/250 g dry soil
Control
209
112
1886
23
39
5
Mocap
206
167
943
28
25
14
Temik
80
38
870
26
19
2
Telone II
0
2
308
1
1
7
Telone II + Mocap
6
5
100
1
1
5
Telone 11 + Temik
0
0
58
0
2
1
84
251
NS
54
271
NS
694
103
1077
13
120
24
14
149
NS
6
143
12
Mean
CV (%)
LSD (0.05)
1 /
Mocap: applied at 12 lb. ai/acre May 3
Temik: applied at 3.0 lb. ai/acre May 23
Telone II: applied at 15 gallons/acre October 24, 1995
Klamath Experiment Station
79
00
Table 2. Effect of fumigation and nematicides on yield, grade, root-knot nematode blemish, and corky ringspot infection of
Russet Burbank at Klamath Falls, OR, 1996.
Treatment
Yield U.S. No. is
4-12 oz. >12 oz. Total
Bs
Yield
No. 2s Culls
Blemish
Total
cwt/A
g
lg.
tri
Po
P.
cA
aa.
Infection level
Blemish
CRS
%
Control
40
5
45
10
7
4
282
347
82
56
Mocap
145
10
155
30
19
14
149
366
38
60
Temik
182
18
200
44
23
19
84
368
22
1
Telone II
217
18
235
51
13
12
61
370
16
34
Telone II + Mocap
197
15
212
46
25
11
41
334
12
34
Telone II + Temik
236
23
259
61
24
18
11
373
3
6
Mean
CV (%)
LSD (0.05)
169
39
99
15
84
NS
184.
40
112
40
34
20
18
69
NS
13
82
NS
105
93
147
360
8
NS
29
88
38
32
80
38