Klamath Experiment Station
Effect of Seedpiece Treatment with Fungicides on Russet Norkotah, 1997
K.A. Rykbosti, J. Maxwell', and K. Locke'
A
bstract
Russet Norkotah seed was hand cut,
treated with several standard and two
experimental fungicides, suberized for 11
days, and planted in a randomized complete
block design experiment with four replications. Data gathered included emergence over
time, yield, grade, tuber size distribution, and
post-harvest silver scurf infection. Two experimental fungicides delayed emergence by
several days. All treatments and the untreated
control achieved similar final stands of about
95 percent. No differences in canopy vigor or
maturity were observed among treatments.
Very high yields were found in all treatments.
The experimental compound. LS 132, significantly altered tuber size distribution, resulting
in lower yield of small tubers and higher yield
of large tubers. In all other respects, no yield
or grade differences between any treatments or
the untreated control were observed. Slight
suppression of Rhizoctonia sclerotia five
weeks after harvest was observed on tubers
from Tops MZ, LS 132, Maxim, and
Mancozeb treatments. Following incubation at
ideal conditions for silver scurf development,
the incidence and severity of silver scurf was
significantly affected by seedpiece treatments.
Mancozeb 8 and Maxim had significantly less
silver scurf infection than all other treatments;
Tops 2.5 and Tops 5.0 were not significantly
different than the untreated control; and Tops
MZ, LS 130, and LS 132 treatments were
intermediate in level of silver scurf infection
in tubers.
Introduction
Control of seed-borne diseases through
use of fungicides is a very common practice in
the potato industry. Seedpiece decay caused by
fusarium (Fusarium sambucinum) infection is
the most common target for fungicide use on
seed. Rhizoctonia, caused by Rhizoctonia
solani, and silver scurf, caused by
Helminthosporium solani, are becoming a
greater concern in recent years. Most recently,
interest in protecting seed from new, more
virulent strains of late blight, caused by
Phytophthora infestans, has been added as a
goal. The development of resistance to fungicides in Fusarium and silver scurf is widely
recognized. Russet Norkotah, which has
become an important fresh-market variety,
seems to be more susceptible to silver scurf
than Russet Burbank. Advances in the development of fungicides to control these diseases
through seed treatment include Maxim, which
has been registered for Fusarium, Rhizoctonia,
and silver scurf control; Tops MZ, reported to
have improved efficacy for Fusarium, Rhizoctonia, and silver scurf; and new products from
'I Superintendent/Professor and Biological Sciences Research Technician III, respectively, Klamath
Experiment Station, Klamath Falls, OR.
2 / Klamath County Cooperative Extension Agent, Klamath Falls, OR.
Acknowledgments: Appreciation is expressed to Gustafson, Incorporated and the Oregon Potato
Commission for financial support, and to Dr. Phil Hamm and Ms. Joy Jaeger at HAREC for laboratory evaluation of silver scurf infection.
64 Klamath Experiment Station 1997
Klamath Experiment Station
Effect of Seedpiece Treatment with Fungicides on Russet Norkotah, 1997
Gustafson (LS130 and LS132) which may
provide some control of seed-borne late
blight. An experiment was established at
Klamath Experiment Station (KES) to
evaluate these and standard seed treatment
fungicides on Russet Norkotah in 1997.
Procedures
Generation III Russet Norkotah
seed with a virus-free greenhouse reading
was obtained from a Klamath County seed
grower. Tubers were hand cut to 1.5- to
2.0-oz seedpieces on May 9. Cut seed
batches of 50 lb were treated with carefully
measured quantities of seven products and
mixed several times between containers to
get uniform product distribution on
seedpieces. One batch was left untreated
after cutting. All lots were isolated from
each other and allowed to suberize for 11
days at approximately 55 °F and 95 percent relative humidity. Seedpieces were
counted and bagged in paper for individual
plot rows immediately before planting on
May 20.
The experiment was a randomized
complete block design with four replications of two-row, 33-foot plots. Seedpieces
were planted with a two-row, assisted-feed
planter at 8.7-inch spacing in 32-inch
rows, with 42 seedpieces/plot row. Fertilizer was banded on both sides of rows at
1000 lb/acre of 16-8-8-14 at planting. All
other cultural practices were as described
on page 13. Vines were desiccated with
Diquat applied at 1.0 pint/acre on September 2 and 7. Potatoes were harvested with a
one-row, digger-bagger on September 26.
All tubers from both rows of each plot were
weighed in the field. Approximately. 120 lb/plot
were stored at 55 °F and 95 percent relative
humidity until they were graded on October 30.
Tubersmere graded as Bs (< 4 oz); U.S.
No. is 4-8 oz, 8-12 oz, or > 12 oz; U.S. No. 2s; or
culls. One 10 lb random sample of 8- to 12-oz No.
Is from each plot was evaluated for specific
gravity by the weight-in-air, weight-in-water
method. Immediately after removal from water,
tubers were inspected visually for Rhizoctonia
sclerotia. The percentage of tubers in this sample
with Rhizoctonia black scurf lesions was recorded. There was no evidence of silver scurf
lesions on any tubers at this time. A second set of
25 tubers from the 8- to 12-oz fraction of U.S. No.
ls was washed with tap water to remove adhering
soil, air dried, and packaged in individual boxes
for shipment to the Hermiston Agricultural Research and Extension Center (HAREC) for silver
scurf evaluation.
Samples were incubated in the dark for 26
days at 70 °F in wet/dry cycles for silver scurf
evaluation. Tuber samples were sprayed with
nonchlorinated water to wet all surfaces, placed in
perforated poly-film bags, and boxed individually.
Tubers were sprayed to wet their surfaces at fiveday intervals. On December 1, all samples were
evaluated for silver scurf infection. A silver scurf
rating (R) was established as follows: infected
tubers with < 1 percent surface blemish were
scored as 1; 1-10 percent surface blemish scored
as 2; > 10 to 25 percent surface blemish scored as
3; and > 25 percent surface blemish scored as 4.
The score times number of tubers with that score
were added for each 25 tuber sample and divided
by 25. The resulting R is 0.00 if no blemish
occurs on any tubers, 1.00 if all tubers had silver
Klamath Experiment Station 1997 65
Klamath Experiment Station
Effect of Seedpiece Treatment with Fungicides on Russet Norkotah, 1997
scurf blemish on less than 1 percent of the
surface, or 4.00 if all tubers had more than 25
percent surface blemish.
Field weights were adjusted for soil or
other losses to calculate yields. All data were
statistically analyzed using MSUSTAT software.
Results and Discussion
Plant emergence was monitored at 23,
28, and 41 days after planting. The LS fungicides significantly delayed emergence at 23
and 28 days after planting (Table 1). At 41
days after planting. stands were not different
for any treatments. Canopy vigor was uniform
among all treatments throughout the season.
Vines were beginning to senesce by late
August.
Yields were very high for Russet
Norkotah, as observed in other 1997 KES
trials (Table 2). No significant differences
were observed in total No. ls or total yield
even though variability as measured by coefficients of variability (CV) was very low. The
only notable effect of treatments on yield
parameters was larger tuber size for the LS
132 treatment. The data suggest this product
may have reduced tuber set.
Effects of fungicide treatments on
Rhizoctonia black scurf were minor and not
statistically significant (Table 1). Most tubers
with any Rhizoctonia sclerotia had < 1 percent
surface infection. The trial site is not free of
Rhizoctonia. Serious Rhizoctonia stem lesions
are frequently observed on volunteer potato
plants commonly found in the field.
Although the silver scurf infection
levels following incubation under conducive
66 Klamath Experiment Station 1997
conditions were mild, treatments significantly
affected the incidence as measured by the
rating system used (Table 1). Mancozeb and
Maxim reduced infection compared to other treatments. The control and Tops 2.5 rated
significantly higher than Tops MZ, LS 130,
and LS 132.
The lack of major response to fungicide treatments in this trial is not surprising.
The seed used was very high quality with no
evidence of fungal disease infections. The
growing season was very favorable with
relatively mild temperatures and little weatherrelated stress on crops. In hindsight, it would
have been desirable to retain the silver scurf
samples in storage for several more months.
A number of studies in the Red River
Valley and Pacific Northwest have shown
improved Fusarium, Rhizoctonia, and silver
scurf control using Tops MZ and Maxim
compared with older, standard products or
untreated seed. In many of these studies,
diseases are inoculated to insure their presence. The LS fungicides are showing promise
in protecting against late blight. Delayed
emergence and effects on tuber size distribution, and possibly tuber set, observed in this
study require further evaluation. More fungicide seed-treatment studies are planned for
1998.
Conclusions
Rhizoctonia and silver scurf have
become issues of serious concern in the
Klamath Basin. Rhizoctonia causes stand
losses and deformed tubers when favorable
conditions occur early in the season. Silver
scurf has been responsible for rejection of lots
Klamath Experiment Station
Effect of Seedpiece Treatment with Fungicides on Russet Norkotah, 1997
for fresh-market use. The silver scurf problem
has become much greater since the conversion
from Russet Burbank to Russet Norkotah.
Preliminary results from this and other studies
in the region suggest the new fungicide
Maxim and the older products Tops MZ and
Mancozeb may offer improved control of
these diseases. Additional studies in this area
are planned.
Klamath Experiment Station 1997 67
Klamath Experiment Station
Table 1. Effect of seedpiece fungicides on plant emergence and on rhizoctonia and silver scurf
infection of tubers of Russet Norkotah grown at Klamath Falls, OR, 1997.
% Emergence
6/12
6/17
6/30
Treatment
9e
Rhizoctonia
Silver scurf
rating (R)
Untreated control
Tops 2.5 @ 16 oz/cwt
Tops 5.0 @ 8 oz/cwt
Tops MZ @ 16 oz/cwt
25
26
34
24
79
83
85
79
95
97
95
96
39
38
40
28
0.80
0.77
0.63
0.42
LS 130 @ 8 oz/cwt
LS 132 @ 8 oz/cwt
Maxim @ 8 oz/cwt
Mancozeb 89% @ 16 oz/cwt
10
6
32
15
69
49
85
76
95
96
97
90
45
23
16
20
0.32
0.37
0.08
0.00
Mean
CV (c7c)
LSD (P= 0.05)
21
41
13
76
10
11
95
4
NS
31
74
NS
0.42
38
0.24
Table 2. Effect of seedpiece fungicides on yield, grade, and tuber size distribution of Russet
Norkotah grown at Klamath Falls, OR, 1997.
Treatment
4-8 oz
Yield U.S. No. ls
> 12 oz
8-12 oz
total
Bs
Yield
No. 2s culls
total
cwt/A
Control
Tops 2.5
Tops 5
Tops MZ
157
163
170
157
194
202
207
204
146
137
142
145
497
502
518
506
39
44
42
35
28
31
9
14
19
7
2
5
583
583
571
560
LS 130
LS 132
Maxim
Mancozeb 8%
132
101
169
137
206
174
219
200
167
236
122
151
505
511
509
488
32
26
41
29
19
25
12
16
16
15
7
15
572
576
569
547
Mean
CV (c/c)
LSD (P= 0.05)
148
15
34
201
13
NS
156
24
55
505
6
NS
36
21
11
19
56
NS
10
82
NS
570
4
NS
68 Klamath Experiment Station 1997