Brodie, B. B.* Biology and Distribution of Potato Cyst Nematodes in North America and
their Economic Impact on Potato Production.
E-mail: bbb2@cornell.edu
[Vol. 78, 445]
Soil and climatic conditions in all major potato production areas of North
America are conducive to the development of potato cyst nematodes (Globodera
rostochiensis and G. pallida) making these nematodes a threat to the entire potato
industry. Their life cycle is completed in 38 to 58 days with longer time required at lower soil
temperatures. Population increase varies from 10 to 35 fold per growing
season with greater increases at lower population densities. There is only one generation
per year. Encysted eggs can remain dormant in the soil for many years in the absence of
a host. Only G. rostochiensis (golden nematode) is present in the United
States and only in the State of New York where two pathotypes, Ro1 and Ro2, exist. Both G.
rostochiensis and G. pallida are present in Newfoundland, Canada but only G.
rostochiensis is known to exist on Vancouver Island. Only G. rostochiensis is reported
from Mexico but other closely related Globodera species are also present. If left
uncontrolled, the golden nematode can cause up to 80% loss in yield. Since the initiation of
statutory control programs (quarantine and regulations) in North America, no yield
losses have been recorded. Indirect losses are manifest in the expenses of managing the
statutory programs and in restrictions of activities of other agricultural industries,
which lowers the total agricultural income of the infested region.
Brown, C. R.*,
H. Mojtahedi
,
G. S. Santo
,
P. Hamm
,
R. Novy
,
D. Corsini
,
S. Love
,
& S. James.
Defending the Profitability of Growing Potato in the Columbia Basin: Development of Corky Ringspot and Columbia Root-knot Nematode Resistant
Germplasm.
*E-mail: cbrown@pars.ars.usda.gov
[Vol. 78, 446]
Columbia root-knot nematode (Meloidogyne
chitwoodi) and corky ringspot
disease (CRS) together pose serious risks to potato quality and yield in the Columbia
Basin of Washington and Oregon. The cost of two fumigation treatments approximates $
350 per acre, and the types of fumigants pose environmental problems that will
continue to be controversial in the future. Resistance to Columbia root knot nematode has
been found in several wild Mexican species, and has been introgressed into advanced
backcross generations. Resistance to CRS is ubiquitous in advanced breeding materials.
The resistance to CRS has been found to be active against the tobacco rattle
virus, and not against the vector, stubby root nematode (Paratrichodorus
allius). Combined resistance to root-knot and CRS has been selected for the first time in materials that have
long tuber shape, high yield, and good frying qualities. The patchy occurrence of CRS,
even in fields with severe infection pressure, poses a problem in reliability of screening
results. It appears that supplementation of inoculum, by adding soil with pot-grown
viruliferous nematodes to field plots, may help to homogenize severity of disease
pressure, but timing of inoculation appears to be crucial. There appears to be considerable
variation in virulence of nematode-virus isolates from different fields and specific
interactions between certain clones and nematode-virus cultures has occurred, meaning that some
clones are resistant to one culture, but not another. Similarly, some clones are
resistant in one field but not another. Fortunately, strong non-specific CRS resistance has also
been found and used to advance the program.
Christ, B. J.*. Powdery Scab: An Emerging Disease on Potato.
E-mail:
ebf@psu.edu
[Vol. 78, 447]
Powdery scab, caused by Spongospora subterranea f. sp. subterranea has
become an increasing problem in North America. Many questions exist on the biology
and epidemiology of this disease. We have monitored soil moisture and temperature
over the past five years to correlate environmental conditions with levels of
powdery scab. High levels of the disease were associated with cool wet growing seasons. We
have focused on screening germplasm and cultivars for disease reaction to powdery
scab. All trials are conducted on grower’s fields that were known to have
naturally high occurrence of powdery scab. Randomized complete block designs with 4-6
replications were used. Kennebec was used as a susceptible check in the
trial. Resistant lines for tuber symptoms included Norkotah Russet, Russet Burbank,
Snowden and NY112. However, tuber severity does not necessarily correlate to severity of root symptoms. For example Snowden was moderately resistant for
tuber symptoms but highly susceptible for root symptoms. Other trials have focused
on chemical control using the susceptible cultivar Kennebec. Although no
significant difference were observed among treatments there were definite trends. For
seed treatment trials, Evolve had 22 percent more healthy tubers than the control.
In furrow applications of Omega also yielded 18-20 percent more healthy tubers than the
no treatment control. Trials to examine best time of applications of a suitable
chemical are needed. Other types of management practices need to be assessed in order
for developing adequate management practices for powdery scab.
Collins, Wanda*. The Global Initiative on Late Blight
(GILB). cip-ddg-research@cgiar.org.
[Vol. 78, 448]
The Global Initiative on Late Blight
(GILB) was launched in 1996 to bring
scientists and development experts from both developing and developed
countries together to increase the efforts and resources devoted to this problem.
Forty-one participants from developing and industrialized countries met at a project
design meeting at the International Potato Center (CIP) in Lima, Peru in 1996. A
three-phase, 10-year program was planned with specific priorities for each phase. GILB is
coordinated from the International Potato Center, Lima, Peru. GILB operates
as a facilitating, rather than a funding or research mechanism in itself, and
serves as a focal point for members to decide on common objectives, priorities and to share
results. In 1999, three years after its beginning, GILB organized a global conference
(GILB’99), entitled Late Blight: A Threat to Global Food Security, in Quito,
Ecuador. Some 165 participants from 40 countries attended to review the late blight situation
in general, and the progress of GILB and its participants towards control of the disease.
Because of the progress made in GILB during its first three years of existence
towards meeting the initial priorities, and because of changes in the external environment
affecting late blight research (such as the rapid advances in molecular technologies), new
priority areas were established to guide GILB activities for the next three years. The
five priority areas for GILB are Breeding for Host Resistance, Pathogen Studies,
Integrated pest management for late blight
(IPM-LB), Training and Information.
Cooke, Louise R.*. Potato Late Blight in Ireland and
Europe. [Vol. 78, 448]
Potato late blight in Ireland, as elsewhere in Europe, belongs exclusively to
the new population of Phytophthora infestans and is distinct from the more
recent new introductions into North America. Typically, European P. infestans populations show little variation
for
allozyme genotype (Gpi, Pep), contain both A1 and A2 mating types and metalaxyl-resistant
and -sensitive strains (the frequency varying between regions and years).
Metalaxyl-resistant strains tend to be A1, but there is no clear association between mating type,
allozyme genotype and metalaxyl resistance as in N. America. The A2 mating
type is found more often on tomato than potato, in allotment gardens than commercial
fields and in northern rather than southern and western Europe. In the Netherlands
and Scandinavia, where the A2 mating type is common, oospores occur in the field
and may act as primary inoculum. In Ireland, the A2 mating type is rare so
oospore formation is unlikely. Because the Irish climate favors blight epidemics in most years and
susceptible potato cultivars are widely grown, control depends on fungicides. In Northern
Ireland, metalaxyl-resistant strains occurred in under 50% of isolates in every year
since 1995 (except 2000, 57%). It is recommended that systemic fungicides are used early
in the program with not more than three applications to limit selection for
resistance. After this, formulations containing a translaminar (e.g. dimethomorph) or a
non-systemic protectant (fluazinam or mancozeb) may be used, followed by fentin to reduce
tuber infection. In practice, most growers use a range of products, although some
still rely entirely on protectants and generally apply c. 8 sprays per season.
Cubeta, Marc A.*, Paulo C.
Ceresini & H. David Shew. The Contribution of Population Biology and Genetics for Developing Seed
Certification Programs for Rhizoctonia.
*E-mail: marc_cubeta@ncsu.edu
[Vol. 78, 449]
Rhizoctonia disease of potato, caused primarily by the fungus Rhizoctonia
solani anastomosis group 3 (AG-3), occurs in most potato production areas of the
world. Several molecular techniques are available to identify R. solani AG-3
in pure culture and various DNA-based techniques are currently being developed for
identification of the fungus from plant tissue and soil. For the past four years our laboratory
has been using somatic incompatibility, amplified fragment length polymorphism (AFLP)
analysis and polymerase chain reaction (PCR)-based single locus genetic
markers to examine the genetic diversity and structure of populations of R. solani
AG-3
on seed tubers from Canada, Maine and Wisconsin and in North Carolina soils (NC).
Results suggest that populations of R. solani AG-3 exhibit a high level of
genetic diversity and have a mixed population structure with evidence for both clonality and
recombination. High levels of genetic diversity were detected in introduced seed tubers from
Canada, Maine, and Wisconsin (50 different genotypes among a total sample of 58
isolates). Seven of these genotypes were amongst the most frequent genotypes observed in
North Carolina commercial potato production fields. The potential utility of
molecular-based techniques to better understand aspects of population biology and genetics
for predicting ecologically important variation related to disease management
will be discussed in the context of developing seed certification strategies for
Rhizoctonia.
Dale, M.F.B., Barker, H. & Brown, D.J.F.* Tobacco Rattle (TRV) and Potato Mop-Top Viruses (PMTV) in Europe.
[Vol. 78, 451]
TRV and PMTV viruses are transmitted by (Para)trichodorid nematodes and the fungus
Spongospora subterranea respectively, causing "spraing"
disease. Both viruses have long survival periods, up to 10 years for PMTV and several decades for
TRV. Symptoms of "TRV-spraing" and "PMTV-spraing" are similar.
There are no acceptable chemical controls for the PMTV, whilst oximecarbamates are used
for TRV vector nematode control they have been withdrawn in some areas. The reactions
of potatoes to TRV are resistant, susceptible (infected symptomlessly), or
sensitive (hypersensitive -spraing symptoms), dependant on virus/potato
interactions. Potato cultivars are sensitive or susceptible to PMTV, none are resistant. Not all
tubers from PMTV-infected plants become infected, on replanting a few develop into
infected plants i.e. is "self-eliminating". TRV in a sensitive cultivar is
self-eliminating. TRV persists in susceptible cultivars, potentially spreading the virus to new
areas. TRV in susceptible potatoes can adversely affect yield and quality traits. A
pre-plant soil test using a PCR-MD diagnostic is being developed for (Para)trichodorus species
and TRV. A PCR-MD has been developed for S. subterranea and an ELISA diagnostic
is available for PMTV. Lab. tests with engineered viral coat protein mediated
resistance (CPMR) proved successful for PMTV. SCRI is identifying molecular markers for
TRV resistance.
Davidson,
Robert D.* The Challenge to Produce High Quality Certified Seed Potatoes.
E-mail: rddavid@coop.ext.colostate.edu
[Vol. 78, 451]
Producing high quality certified seed potatoes with low disease levels has
been the goal of North American certified seed programs and seed growers for
well over eighty years. Programs have developed during this time based upon sound science,
buyer demands, the ability of the seed growing region to meet these demands,
quality control and solid economics. There have been notable successes in the arena of seed
production. Controlling diseases, particularly bacterial diseases, through
the use of tissue-culture production schemes has been extremely effective. In addition,
use of limited generation, flush-out certification systems and improved disease
testing technology has reduced certain disease problems even further. Work with
cultivar development programs and line selections has improved the quality and yield
of the seed product. However, there are new threats to seed production on the horizon. Soil
borne diseases and insect problems are on the upswing and will play a role in
further development of certification systems. Community diseases such as late blight,
PVY and potato leafroll are again causing significant concern. Finally, overall
increased costs and the potential for reduced profits are causing certified seed
producers to rethink many of their strategies. While these challenges are not
insurmountable, they will stress the certification programs and seed growers as adjustments are
made.
Deahl, Kenneth L.* & Richard W. Jones. Late Blight in the United States and
Canada.
*E-mail: deahlk@ba.ars.usda.gov
[Vol. 78, 452]
Late blight caused by Phytophthora infestans continues to be a very
serious problem throughout the potato production areas of the United States and
Canada. In order to develop more effective control strategies for late blight disease
management, many scientists have attempted to characterize the genetic and phenotypic
variation in various populations of the fungus. Although mating type changes provided the
first suggestion of major modification in P. infestans populations,
variations in metalaxyl sensitivity also indicated dramatic changes in this pathogen. The results
based on morphology, pathogenicity, ribosomal internal transcribed region 2 (ITS2)
sequence, isoenzyme markers, DNA fingerprints obtained using multilocus probe RG-57,
and genetic markers based on analysis of mitochondrial DNA (mtDNA) has permitted
further resolution of genetic diversity in populations from different
locations in the U.S. and Canada. Knowledge of this pathogen diversity has helped identify
potential new strategies for disease management and has assisted in determining where
future monitoring of the pathogen is necessary. However, despite all that has been
discovered with contemporary analyses of the population variability of P. infestans during
the last few years, particularly with DNA-based markers, many unanswered questions
remain to be investigated in the future.
Forbes, Greg*. Late Blight in Latin America, Africa and
Asia. [Vol. 78, 453]
The dynamics of potato late blight depend on the agro-ecosystem. In many
developing countries in Latin America, Asia and Africa, potatoes are grown
under sub tropical (generally winter production) or tropical highland conditions. Late
blight is particularly problematic in the latter. In the tropical highlands, potatoes
are grown year-round and aerial inoculum of Phytophthora infestans is present
most of the time. For this reason, late blight attacks can occur very early in crop
development, frequently just after plant emergence. Therefore, sanitation procedures, the primary
components of late blight IPM in the temperate zone, probably have little effect in the
tropical highlands. Late blight management in the tropical highlands is based
primarily on methods that reduce the rate of disease spread, such as host resistance and
fungicides. Host resistance has been used more extensively in developing countries than
in the US and Europe. Fungicides are used extensively in most developing countries, but
application technologies vary. Average daily temperatures found in the
tropical highlands (10 – 14 C) are sub-optimal for late blight development. This
reduces the rate of disease spread and thereby enhances the effects of host resistance
and fungicides. Low temperatures, therefore, compensate somewhat for the problem
of having inoculum present throughout the growing season. Many farmers in the
tropical highlands are unaware of germ theory and attribute late blight symptoms to
abiotic or even mystical factors. Highly participatory farmer-training models have been
modified from other cropping systems to be used with potato farmers in the tropical
highlands.
Geary, Brad*, D. A. Johnson,
P. B. Hamm, S. James & K. A. Rykbost. Silver Scurf, an Emerging Potato Disease.
*E-mail: brad_geary@byu.edu
[Vol. 78,
454]
Silver scurf, caused by Helminthosporium solani, has been recently
recognized as an important potato disease because of the increased incidence of this
disease, likely due to the increased occurrence of resistance to commonly used seed
treatments. Helminthosporium solani, which is limited to the periderm of tubers, is
particularly important on potato tubers sold on fresh markets because of the unsightly
blemishes this pathogen produces. Increased water loss through infected areas can
result in up to 13% less yield of marketable potatoes, which affect grower returns regardless
of processed or fresh markets. Tubers can become infected with H. solani in
the field or in storage. The primary source of inoculum is the seed-tuber, which has shown
a positive correlation between the amount of silver scurf on the seed and the
amount on the progeny-tubers. As the severity of seed-tuber infections increase, the
amount of silver scurf on the progeny-tubers also increases, indicating the importance
of using clean seed. The severity of silver scurf can be lessened through chemical and
cultural means. In Washington and Oregon, the fungicide seed treatments of Maxim +
Blocker, Quadris, Maxim, and Tops MZ had significantly less silver scurf on
progeny-tubers than progeny-tubers from untreated seed. Some cultural methods of minimizing
silver scurf include planting seed with minimal amounts of disease, vine kill two to
three weeks prior to harvest, do not leave tubers in the ground beyond skin set,
disinfect potato storages and store potatoes at lowest possible temperatures under
appropriate humidity to reduce spore production and new infections.
Inglis, D. A.* & M. L.
Powelson. Transmission of Phytophthora infestans from Potato Seed Tubers to
Sprouts: Implications for Management and Certification. *E-mail:
dainglis@wsu.edu
[Vol. 78, 460]
Potato tubers are the principle means for survival of Phytophthora
infestans (Pi). Infected tubers in cull piles are generally regarded as a primary source of
inoculum for initiating late blight epidemics in many production regions. However, new
information on seedborne inoculum leading to transmission of Pi from seed tubers
to sprouts and early-season establishment of late blight within a field, calls for new
approaches in management. Our research, and the results of others, has shown that
transmission of Pi from seed pieces contaminated during seed cutting and handling operations is
driven by inoculum density, cultivar susceptibility, physiological age of seed, and
time interval between contamination and seed fungicide treatment. Inoculum which
comes into con-tact with recently cut seed pieces can be managed preventively by
seed treatment with broad-spectrum fungicides, thereby circumventing transmission.
Latent seed tuber infections, however, cannot be controlled with this approach, and
measures such as banded applications with cymoxanil at 95% emergence to eradicate new
infections and to protect healthy foliage may be useful. Many factors which
reduce seed piece decay and promote seed vigor and rapid emergence, also provide
Pi with the opportunity to survive the soil environment, infect emerging sprouts and
spread in the canopy before foliar fungicides can typically be applied. Therefore, adoption
of certification standards for late blight may be justified in the future to
identify seed lots at risk for latent tuber infections.
Ingham, Russell E.* & Philip B. Hamm.
Certification Issues Surrounding Columbia Root-knot Nematode (Meloidogyne
chitwoodi). E-mail:
inghamr@science.oregonstate.edu [Vol. 78,
459]
Columbia root-knot nematode, Meloidogyne
chitwoodi, infects potato
tubers causing galling on the surface and minute, brown spots in the tuber as a
response to the presence of adult females. Both symptoms are quality defects that can cause
commercial crops to be devalued or rejected if 5-15% of tubers are affected.
Large acreage is infested in Idaho, Oregon and Washington and M. chitwoodi is
also reported from California, Colorado, Nevada and Utah. The nematode survives where soils
freeze, infects roots at 6 C or above, and thrives in warm soils, so the
potential range of CRKN may be far larger than its current distribution. Northern root-knot
nematode, M. hapla, is a problem on potato in many areas east of the current range of M.
chitwoodi and M. chitwoodi should be able to exist in those areas if introduced.
Since M. chitwoodi can develop during seed storage, and infective juveniles are
released from infected tubers, the potential for spread in seed is high. All seed should be
certified for zero tolerance to M. chitwoodi to prevent spread to new areas. Growers
with seed fields in regions where M. chitwoodi occurs should sample on a small
area basis (<10 acres/sample) before planting to ensure detection if present. Tuber
inspections for external galling and females in tubers should be conducted at harvest and/or,
preferably, before seed leaves storage for shipment. While USDA may conduct
shipping point inspections in many instances, it behooves seed producers to
be proactive and inspect all seed lots before shipping to affirm that M.
chitwoodi will not be spread.
Ingham, Russell E., Philip B.
Hamm* & Kenneth A. Rykbost. Control of Corky Ringspot in Oregon.
*E-mail: philip.b.hamm@oregonstate.edu
[Vol. 78, 460]
Corky ringspot (CRS) produces corky, necrotic arcs, rings or spots, which are
quality defects in tubers that can lead to crop rejection. The disease is caused by
tobacco rattle virus (TRV), vectored by stubby-root nematodes (Paratrichodorus
allius) in the western U.S. In Oregon, this disease is found in isolated fields in the Columbia
Basin (CB), Klamath Basin (KB), Central Oregon and Treasure Valley growing areas.
Furthermore, P. allius is widespread in many
fields that do not express symptoms when potatoes are grown. Introduction of TRV into these fields through infected potato or weed
seed or movement of soil carrying nematodes with TRV in wind, water or on machinery,
presents the potential for CRS to become more widespread. Corky ringspot was
suppressed by aldicarb until its suspension in 1989, whereupon many fields expressed high
levels of CRS. Metam sodium (MS) applied through chemigation or surface incorporated
ethoprop alone, do not control CRS, but combined treatments are effective.
1,3-dicloropropene (1,3 D) has been effective in the CB but requires the addition of ethoprop in the
KB. Combinations of MS and 1,3-D have controlled CRS in the CB. Post emergence applications of oxamyl have been effective in the CB if applications begin
before tuber initiation. All major cultivars grown in the region are susceptible, but
Russet Norkotah expresses fewer symptoms than Russet Burbank, Ranger Russet, Shepody or
Century, which are highly susceptible. Resistance is available in some current
breeding lines and in several cultivars grown in other regions of the U.S.
Johnson, Dennis A.*, Brad Geary, & L. Tsror
(Lahkim). Black Dot: A Recently Recognized Economically Important Disease of
Potato. *E-mail: djohnson@wsu.edu
[Vol. 78,
461]
Yield reductions ranging from zero to greater than 40% have been documented
in replicated, controlled experiments with potato due to Colletotrichum coccodes. Tubers greater than 280g in size can be especially reduced. Potato
cultivar, fungal
isolate and environmental factors affect yield. Below- and above-ground phases of the
disease are evident from quantifying the fungus in plant tissues. C. coccodes
is
disseminated through tuber-, soil-, and air-borne inoculum. The fungus can be detected in
below-and above-ground plant stems relatively early in the growing season but
symptoms usually are not expressed until later in the season. Infected seed tubers
increase the incidence of early season plant infections and provide a means of pathogen
dissemination. Foliar infections may result from wind blown inoculum and
foliage abrasion from blowing sand. Yields were decreased more in growth chambers at
daily temperatures of 23C and 28C than at 18C on Russet Burbank and Norkotah Russet
plants grown in soil infested with C. coccodes when compared with
plants grown in non-infested soil. Azoxystrobin reduces C. coccodes levels in stems and
tubers.
Lambert D.H.* & A.I. Currier.
Dynamics of Ridomil Resistance in Populations of Phytophthora Erythroseptica, cause of Pink Rot of
Potato. *E-mail: lambert@maine.edu
[Vol. 78, 465]
Many P. erythroseptica isolates in Maine are moderately
(MR, EC 50 ca
EC50, 4 ug metalaxyl/ml) or highly resistant (HR) to Ridomil. Pink rot field trials with
‘Russet Norkotah’ were conducted from 1997 to 2000 to investigate the rate at which
Ridomil effectiveness might be lost in pathogen populations containing resistant
strains. While effectiveness of foliar Ridomil treatments was variable, standard in-furrow
applications achieved 100% control of sensitive strains in all three such trials. No
control of HR strains and little or no control of MR strains occurred in the two trials
with mixed populations. In untreated areas, the percentage of MR isolates remained
constant over 3 yr. (15, 18, and 19%), indicating MR fitness equivalent to the sensitive
population. Incidence of tuber infection following 1 yr of oat, barley or soybean ranged
from 0.4 to 0.8% vs 11.1% in those plots which followed potato. This low contribution of
older and rotation crop inoculum implies that most P. erythroseptica inoculum
is produced by the most recent potato crop in such rotations and, therefore, that the
very high selective pressure of in-furrow treatment would rapidly increase the
proportion of resistant individuals. Following a Ridomil treatment which reduced infection
by sensitive strains 50%, the incidence of MR tuber isolations increased from
18% in 1999 to 31% in 2000, close to the 30% expected if potato debris was the only
significant source of inoculum.
Littell, Ramon C. Statistical Sampling for Soil-Borne Pathogens in Seed Potatoes. [Vol.
78, 466]
Seed potato certification requires sampling and testing for pathogens. In
most cases it is impossible to prove absence of pathogens with certainty. Then the goal
is to establish strong statistical evidence of low levels of infection. Adequate
sampling is required to show acceptably low levels of pathogens. Sampling and testing for
disease detection always presents difficult challenges, but problems are even harder
in the case of soil-borne pathogens because of notoriously aggregated distributions in
space. Aggregation, or clustering, increases probability rates of non-detection over
rates for randomly distributed pathogens. Knowledge of the degree of aggregation
obtained from preliminary sampling can assist in computing non-detection rates. If the
sampling units are soil cores and analytical methods are highly sensitive, then
bulking of samples can improve detection rates.
Lizárraga,
Charlotte*. Global Initiative on Late Blight (GILB) Linkage Groups.
E-mail:
cip-gilb@cgiar.org.
[Vol. 78, 467]
The Global Initiative on Late Blight (GILB) organized the global conference Late
Blight: A Threat to Global Food Security (GILB’99),
which was held 16 – 19 March 1999 in Quito, Ecuador. The purpose of this conference was to bring late
blight experts from all over the world to learn about the global status of the
disease, relevant technology development and implementation, and to decide on future concerted
action. During workshops and discussions in the later part of the conference,
participants expressed the need to be more closely linked to each other in order not to
duplicate efforts and to learn what others are doing. GILB’99 was appreciated as a
mechanism to do this, but a conference every three years was not judged sufficient to
provide the ongoing and close associations necessary for efficiently working together. To
meet this need Thematic (Breeding for Host Resistance, Integrated Pest Management,
Molecular Studies of the Pathogen, Variation and Evolution of Phytophthora
infestans) and Regional (Africa, Europe, Latin America, Southwest Asia, and East and
Southeast Asia) were formed. These groups are active and have web pages with contact
lists, laboratory protocols, databases and other information maintained by GILB
(http://www.cipotato.org/GILB. Many of them have held initial planning meetings
with GILB support.
Love, Stephen L.*,
Thomas A. Salaiz, Joseph J.
Pavek, & Charles R. Brown. Development of Russet-Type Germplasm with Resistance to Corky
Ringspot. *E-mail: slove@uidaho.edu
[Vol. 78, 467]
Corky ringspot (CRS), caused by tobacco rattle virus and vectored by stubby
root nematodes (Trichodorus sp.),
is rapidly becoming a widespread problem in the northwestern U.S. Genes for resistance are present in many European and a few
older North American cultivars. No consistent effort has been made to develop
resistance in the russet-type cultivars that are predominant in Northwest potato
production. This report summarizes eleven years of research to develop russet type germplasm
with resistance to CRS. Initial screening of existing USDA/ARS (Aberdeen)
germplasm identified two corky ringspot resistant clones, A72630-9 with resistance
derived from Bintje and A77567-7 with resistance derived from Multa. Both resulted from
crosses between CRS resistant and russet skinned parents. A cross between the
A72630-9 and A77567-7 produced A8259-5, a clone with russet skin, oblong shape, yellow
flesh, and a high level of CRS resistance. Screening of additional USDA/ARS germplasm
identified other sources of resistance, including AC Brador and progeny of
Fianna (NZA8904-2), PI407415 (A77715-6), Ukama (A89875-5), S.
andigena (A8793-6), and complex Polish germplasm (A90586-11). These resistant clones were hybridized
with russet parents and 105 progeny clones screened at Egin, ID and Pasco, WA from
1992- 2000. Twelve clones with russet parentage have been identified as having high
levels of resistance to CRS, combined with good adaptation. Six of these clones have
oblong to long shape and russet skin and will be important for developing new
cultivars with CRS resistance in combination with russet-type appearance and quality
characteristics.
Lulai, Edward C.*. Skin-Set: Measurement and Physiology of Resistance to Tuber Skinning
Wounds. E-mail: lulaie@fargo.ars.usda.gov
[Vol. 78,
468]
Tuber skinning wounds result in costly disease, blemish defects and shrinkage
in processing, seed and fresh market potatoes. We have conducted research and developed a body of information on the measurement of skin-set and the
physiology of periderm maturation and resistance to skinning wounding. Measurement of
the rate of skin-set development among diverse genotypes showed that resistance
to skinning developed slowly and varied from year to year. Cultural conditions appeared to be as important as vine killing in affecting the rate of skin-set
development. Contrary to anecdotal information, periderm maturation and the development of resistance to skinning wounds were not accompanied by skin
thickening or large changes in skin tensile strength. Periderm maturation is characterized by a reduction in vapor conductance which is more pronounced in
russeted than smooth skinned varieties. This water vapor loss may be
important in skin-set development because our results show that the cellular damage
incurred upon skinning is restricted to the turgor sensitive cells of the native
periderm, i.e. the phellogen. The radial walls of active phellogen cells are thin, fragile,
easily fractured and are the source of susceptibility to skinning. After growth
ceases, the phellogen becomes inactive, the cell walls strengthen and thicken and are no
longer easily broken; thus the skin is firmly held in place. Our current research is
directed towards identifying the biochemical processes and regulatory mechanisms
responsible for skin-set and related wound-healing.
Miller J. S.*.
Significance of Sexual Reproduction in Phytophthora
infestans Epidemiology. E-mail:
jsmiller@uidaho.edu
[Vol. 78, 468]
Phytophthora infestans is heterothallic, unlike the majority of Phytophthora
species. When antheridia and oogonia of two isolates of alternate
compatibility type come in contact, sexual reproduction leads to the production of thick walled
oospores. Oospores can also form in vitro in the absence of both mating types when
pathogen cultures become old or stressed, and when isolates are exposed to fungicides
or other fungal species. The presence of both A1 and A2 compatibility types in
portions of Europe has led to sexual recombination, resulting in a diverse population
structure. In North America, evidence for sexual recombination has been found in only a few
localities. Two results of most consequence to potato production from sexual recombination are 1) the formation of oospores and 2) the generation of new,
aggressive strains of the pathogen. The first consequence is important
because oospores can survive in soil in the absence of the host plant. Oospores can
also remain viable after exposure to low temperatures. Even though oospore germination
levels are typically low, oospores can serve as an early source of inoculum when potato
crops are first established in a field. Data supporting the second consequence of
sexual recombination have been harder to obtain in North America. However, recent
work suggests that oospore progeny can be aggressive, therefore making control of
late blight more difficult. Laboratory studies have shown that the US-11 clonal
lineage may have been generated from a cross between isolates of the US-6 (A1) and
US-7 (A2) lineages. The US-11 is highly aggressive and has caused significant
potato crop losses in portions of North America.
Mills, Dallice*. Emerging Technologies for Detection of Plant Pathogens: Advantages and
Limitations. E-mail: millsd@science.oregonstate.edu
[Vol. 78, 469]
The traditional biochemical and immunological techniques for the detection of
plant pathogenic organisms and viruses have been greatly augmented by the
development of a number of DNA-based methods during the past decade. Some DNA-based
detection methods have the potential for greater specificity and sensitivity than
techniques such as the enzyme-linked immunosorbent assay (ELISA), the indirect fluorescent
antibody staining assay (IFAS), or nucleic acid probe hybridization. Key to recent
major advances in DNA-based technology was the development of the polymerase chain
reaction (PCR), an enzymatic process whereby a specific sequence of DNA from
the genome of any organism and the RNA from a virus may be amplified several
million-fold from a nucleic acid primer set. The design of the primer pair and, therefore,
the region of the genome that is amplified may impose certain limitations with
respect to the method by which the PCR product may be analyzed, the level of sensitivity
and specificity achieved, whether the results can be verified by DNA-based
methods, whether different pathogens that can be simultaneously analyzed, the number
of samples that can be analyzed, the time required to obtain results, and the
cost of assaying each sample. To be discussed are detection techniques as diverse as
rep-PCR-mediated genomic fingerprinting, nucleic acid sequence-based amplification
(NASBA), reverse transcriptase polymerase chain reactions (RT-PCR), telomere
RFLP analysis, and a comparison of the strategies, performances, limitations and
advantages of using either TaqMan or Molecular Beacon assays. Factors that interfere
with PCR assays will also be discussed.
Mojtahedi, Hassan*, G. S. Santo
, P. E. Thomas
, J. M.
Crosslin
, & C. R. Brown.
Distribution, Biology and Efficiency of the Vector of Tobacco Rattle
Virus Causing Corky Ringspot Disease of Potato in the Pacific Northwest.
E-mail Chuck Brown: cbrown@pars.ars.usda.gov
[Vol. 78, 470]
Paratrichodorus allius was identified in 30% of soil samples collected
from potato fields in Washington, Oregon, Idaho. Tobacco rattle virus (TRV) was
transmitted to Samsun NN tobacco from only 10% of P. allius-infested soil samples.
TRV caused typical symptoms on tobacco. Only 0-6% individual P. allius from
potato fields expressing either severe or mild corky ringspot (CRS) symptoms transmitted
TRV to tobacco. All viruliferous populations of P. allius, regardless of
origin, transmitted TRV to Russet Burbank and Russet Norkotah potato tubers provided the soil
moisture level in pot cultures was maintained at or above field capacity. The severity
of CRS, however, depended on the virus isolate and vector density. In pot cultures,
three P. allius/250 cm 3 of soil was
enough to initiate CRS on potato. Under field conditions, the tubers of Russet Burbank and Russet Norkotah developed 2 mo after planting,
and quickly become vulnerable to P. allius feeding, and TRV infection. The
severity of CRS increased for 35 and 50 days after the first detection of symptoms in
Russet Norkotah and Russet Burbank tubers, respectively. P. allius populations
remained low and unchanged during the growing season. The field observations confirmed our
conclusions from greenhouse tests that potato tubers become vulnerable to TRV transmission soon after they are developed and should be protected from P.
allius invasion for at least 8 weeks.
Pavlista, Alexander, D.*. Relating Potato Skinning to Field Measurements of
Skin-Set. E-mail: apavlista@unl.edu
[Vol. 78,
475]
Desiccating potato vines, chemically or mechanically, hastens tuber skin set
which is commonly estimated by skinning tubers with an abrasive apparatus. A
hand-held torque meter was developed for measuring tuber skin resistance to sheering as
a measure of skin set. To demonstrate practical use of the torque meter to
growers, sheer resistance must be related to the actual skinning of tubers. In conjunction
with desiccation trials, ’Atlantic’ and ’Snowden’ potato tubers were
individually measured for skin sheer resistance followed by skinning. There is a linear relationship
between skin sheer resistance and tuber skinning. The regression equations were sheer
force (oz*in) = 49-0.19* skinning (%) for Atlantic and sheer force (oz*in) = 54-0.24*
skinning (%) for Snowden. As vines desiccated over the three weeks after chemical or
mechanical treatments, skinning decreased and sheer force resistance increased. The
objective measuring of sheer resistance was directly related to the more subjective
measuring of tuber skinning with an R-square of 0.8. The torque meter can be used in the
field or office. The major difficulty was maintaining a constant pressure on the tuber
skin while applying the torque force.
Proudfoot, Kenneth G.* Potato Wart Disease - Reality and Myth.
E-mail:
harphill@roadrunner.nf.net [Vol. 78, 477]
Potato wart disease results from the infection of the potato plant by the
non-mycelial, obligate parasitic soil-borne fungus Synchytrium endobioticum. The
fungus is native to the high Andes region of Peru and Bolivia where it attacks both
cultivated and wild Solanum species. Wart disease and its causal agent were first described by Schilbersky in
1896. The disease was spread from the British Isles to other European countries, and
worldwide by the trade in potatoes. The disease is confined to areas where soil
temperatures are below 20°C and soil moisture is adequate
for the movement of infective zoospores. The fungus is characterized by the formation of a thick walled resting spore
which may remain viable for up to 40 years. Spread between countries, or between farms,
is primarily through the planting of tubers which had been grown in infested
soil. Very limited spread from a focus of infection may occur through the movement of
contaminated soil. Infective zoospores remain viable for less than two (2)
hours, and their mobility is limited to less than 25 mm. Using various differential
varieties, several pathotypes of the fungus have been identified in Europe, North and
South America. Typically the disease occurs more frequently under conditions where crop
rotation is not practiced and growers retain their own seed for planting, e.g.
subsistence or part-time farming and gardening. Although a pest of quarantine importance, this fungus
pales into insignificance compared to the potential threat of such pathogens
as the cool tolerant form of the brown rot bacterium, or the virus Y strain causing
potato tuber necrosis.
Rowe, Randall C.*. Verticillium dahliae in North American Certified Seed Potatoes -
How Important is it?
E-mail: rowe.4@osu.edu
[Vol. 78,
479]
In an extensive survey of commercial potato seed lots produced across North America,
Verticillium dahliae was detected in 65 of 224 seed lots
tested, a successful isolation rate of nearly 30%. Isolates of the fungus recovered in this survey
were assessed by vegetative compatibility analysis. All 162 isolates of V.
dahliae tested belonged to vegetative compatibility group (VCG) 4. Among these, 64% belonged
to VCG 4A, 33% to VCG 4B, and 3% to VCG 4AB. Of 39 isolates tested in the greenhouse, all were pathogenic to potato
cv. Superior. In plants inoculated
with VCG 4A isolates, disease symptoms developed earlier, were more severe, and plants
died earlier as compared to VCG 4B isolates. Results of this survey suggest that:
1) commercial certified seed tubers from diverse locations in N. America are
commonly infected with V. dahliae and thus may serve as primary sources of the
pathogen, 2) potato isolates of V. dahliae in North America belong to VCG 4A and 4B
and these strains are widely distributed via seed tubers, and 3) VCG 4A isolates are a
distinct pathotype of V. dahliae that is highly aggressive to potato. The
potential importance of these findings to the N. American potato industry will be discussed.
Sabba, Robert P.* & Edward C. Lulai. Biochemical Changes in Cell Wall Structure Associated with Periderm
Maturation. E-mail: rpsabba@wisc.edu
Skinning of potato tubers results in hundreds of millions of dollars of
losses to producers every year. Potatoes are susceptible to skinning when they are
harvested before periderm maturation (skin-set) is complete. Research in our laboratory
has shown that skin-set is dependent on changes in the cell wall structure of the
meristematic cell layer of the periderm (phellogen). In particular, radial
walls of the phellogen cells thicken during skin-set. The biochemistry of these changes in
wall structure are poorly understood. Our research indicates that an increase in
un-esterified (acidic) pectins in phellogen cell walls accompanies maturation of the
periderm. Staining with ruthenium red indicates that the phellogen cell walls of
immature periderm include esterified pectins, but are lacking in un-esterified
pectins. Periderm maturation is accompanied by an increase in un-esterified pectins in
phellogen walls. Although peroxidases are involved in the cross-linking of many polymers in
the cell wall, we could not detect any changes in peroxidase activity (in situ) in
phellogen walls that accompanied maturation of the periderm. Development of wound periderm
differs from native periderm, in that wound periderm maturation is not accompanied by
an increase in un-esterified pectins in the phellogen cell walls. We are
continuing to investigate the biochemical changes in phellogen cell walls associated with
skin-set development.
Secor, Gary, A.* & Neil C. Gudmestad. The Impact of Soil (and Seed) Borne Diseases without Vectors on
Seed Potato Certification. *E-mail:
gary.secor@ndsu.nodak.edu
[Vol. 78, 481]
Seed potato certification was instigated primarily to insure varietal purity.
Disease tolerances became part of certification because disease symptoms often masked
agronomic features used for variety identification. Certification of seed
potatoes requires meeting the requirements of both field and post harvest inspections
and tests, including a shipping point inspection conducted by a combination of state and
federal certification officials. Meeting tolerances for diseases that are only
seed-borne, such as viruses and bacterial ring rot, are required to pass field inspection.
Tolerances for disease that are both seed- and soil-borne are left to the judgment of the
inspector but may be recorded in inspection documents. However, disease tolerances exist
for many diseases that are both seed- and soil-borne in order to issue a shipping
point inspection required to complete the certification procedure. This presentation will
review seed potato certification rules and regulations for seed-and soil-borne diseases,
and the biology and epidemiology of the many diseases in this category. Some of these
diseases are Rhizoctonia, Verticillium wilt, Fusarium dry rot, black dot,
silver scurf, late blight, pink rot, early blight, scab, bacterial wilt, powdery scab,
nematodes and wart.
Thomas, P. E.*,
H. Mojtahedi ,
J. M. Crosslin, & G. S. Santo. Reduction and Elimination of Tobacco Rattle Virus from Soils in Alfalfa
Cultures. E-mail: pthomas@pars.ars.usda.gov
[Vol. 78, 487]
Historically, it has been difficult or impossible to eliminate tobacco rattle
virus (TRV) from soils once the virus became established with its nematode vector.
Numerous experiments conducted in greenhouses or fields of the Columbia Basin indicate that TRV may not survive in pure alfalfa (Medicago
sativa) cultures. Initially, we observed that TRV could no longer be isolated from parts of a heavily
contaminated field 3 years after it was seeded to alfalfa and maintained
essentially weed free. In contrast, TRV was routinely isolated from weedy parts of the
same field that were not seeded to alfalfa. The nematode vector remained prevalent in
the alfalfa field. TRV was markedly reduced but not completely eliminated after 3 years
in alfalfa plots that were not maintained completely free of weeds. TRV was occasionally
detected at extremely low concentrations, only by PCR procedures, in the
roots of eight alfalfa cultivars grown in contaminated soil in greenhouse pots (8 or 16
weeks) or in field plots. However, TRV could not be isolated from such roots by mechanical
or by nematode transmission after roots were washed and transplanted to virus free
soil. In contrast, virus-free nematodes routinely acquired TRV from the roots of
susceptible tobacco plants handled in the same manner. Nematodes that initially
transmitted TRV to tobacco lost ability to transmit after maintenance on alfalfa for 3 months
in a greenhouse.
Thomas, P. E.*,
K. Thompson, R. E. Ingham, & J. M. Crosslin. An Apparent New Strain of Tobacco Rattle Virus that causes Systemic
Symptoms in Potatoes. *E-mail:
pthomas@pars.ars.usda.gov
[Vol. 78, 486]
Systemic foliage infection of potato with tobacco rattle virus (TRV) is rare
and has never been reported to occur routinely. Circular areas in which plant
emergence was delayed for 4 to 6 weeks after seeding were observed in two potato fields of
Colorado, one in 1998 (Cultivar Norkotah Russet) and another in 2000 (Cultivar Russet
Nugget). Emerging stems of plants in affected areas were malformed and contained
necrotic streaks. Foliage had irregular ringspot and mottle symptoms, and flowering
was delayed. Tubers from affected areas had typical TRV symptoms and produced
positive PCR assays for TRV while tubers from unaffected areas were free of symptoms
and produced negative PCR assays. The foliage, stems, and roots of affected
plants routinely assayed negative for TRV by ELISA but positive by PCR analysis. TRV
was isolated from some roots by mechanical transmission to tobacco. A single
nematode vector species of TRV (Paratrichodorus
allius) was found in soils in
the affected areas. These results implicate TRV as the cause of the disease and indicate that the
strain of virus involved routinely moves systemically in plants in its naked RNA form.
Studies are underway to confirm this conclusion experimentally. The symptoms
putatively caused by the virus provide a striking visual assessment of TRV distribution
in contaminated fields that previously was never observed.