Guidelines for Seed Potato Selection, Handling and Planting
PP-877 (Revised), July 1997
Gary A. Secor, Professor, ND Agricultural Experiment Station
Neil C. Gudmestad, Professor, ND Agricultural Experiment Station
Duane A. Preston, NDSU Extension Service, Minnesota Extension Service
H. Arthur Lamey, Professor, NDSU Extension Service
Selecting Seed
Examine Seed for Disease
Prepare to Accept Seed
Disinfect
Seed Handling
Cutting
Planting
Disinfectants Commonly Recommended for Potato Handling Equipment and
Storage Facilities
A major limiting factor in profitable potato production is disease, which can be
seed-borne, soil-borne, or both. Vegetative propagation of potatoes and cutting prior to
planting increases the chances for transmission of many yield-limiting diseases caused by
fungi, bacteria, viruses and nematodes. Total control of many of these diseases is often
impossible, and a combination of control strategies is necessary.
The seed potato industry and associated certification system with visible disease
tolerances was formed as one strategy to minimize this disease. Even though certification
schemes are primarily designed to verify varietal purity, the process also minimizes
disease in seed potatoes by establishing disease tolerances. Planting certified seed does
not guarantee freedom from disease or disease-causing agents. For example, the bacteria
that cause ring rot and blackleg can often be present but in a latent condition (that is,
not causing outward signs or symptoms of disease). Fungal organisms such as Fusarium
and Helminthosporium solani may also be on or in the seed tuber. Viruses are
invisible to us and may be present but undetected. The handling and sanitation procedures
of the seed potato buyer have a strong impact on the performance of the seed lot,
including the spread and expression of disease in the crop and the resultant quality and
yield. This circular recommends general guidelines for disease prevention in selection,
handling and planting of seed lots for the buyer of seed potatoes to help maximize
production of a healthy and high quality potato crop.
Purchase certified seed. You can be assured that such seed conforms to guidelines of
varietal purity, field inspections, and disease tolerances according to certification
agency rules and regulations. Buying year-out-seed, culls, oversize or seconds may lead to
problems. Ask about the pedigree of the seed; where did it originate, how many generations
old is it, who increased the lot, etc. Examine field inspection reports, storage
inspections and results of winter grow-out tests in Florida, California, or other southern
grow-out areas. Pay particular attention to disease observations and remarks made by field
inspectors on field inspection reports.
If possible, make a personal visit to seed growers you are considering purchasing seed
potatoes from. Note the general appearance of their fields and storage facilities. The
potatoes should be in dark, well-ventilated storage with high humidity but no free
moisture, free from visual disease, and not excessively sprouted. Ask if the seed has been
treated with Mertect or Dithane ST going into storage to protect from Fusarium dry rot.
Ask if seed has been treated with Ridomil in the field to protect it from pink rot and
leak (water rots). Ask if bacterial ring rot has been on the farm in the past three years.
If so, be aware this disease could be present. Call or write the state certification
agency for seed lot records from the past season; they are available to the public. Some
facilities, such as the Seed Health Testing Lab at NDSU, offer testing services of seed
lots for diseases and pathogens for a fee.
Examine Seed for Disease
There are several diseases to watch for and some simple tests to help evaluate seed
lots for disease.
Virus Diseases
Certification readings are the best gauges of virus content. Virus symptoms are
generally not visible in tubers, except for leaf roll, which causes net necrosis in
certain cultivars, notably russets. Some varieties, notably Shepody and Russet Norkotah,
do not readily express visible mosaic symptoms, and virus can be present without
detection. Serological tests (ELISA) are often conducted for PVY mosaic in these
varieties, either in the production or winter grow-out fields. Such tests are routinely
used for PVX.
Rhizoctonia
The black scurf stage on the tubers should not exceed 5% of the tuber surface (Figure 1 – 47KB color photo). Coverage greater than
this can result in yield and quality losses in the field. This disease is most active in
cool soil and causes damage by pinching off the developing sprouts, which must regrow.
This process delays emergence, lowers yield and increases off-grade tubers.
Fusarium Dry Rot
The seed should not have many tubers with symptoms of Fusarium dry rot (Figure 2 – 86KB color photo). A 1 to 2% level is reasonable
and should not be exceeded in most years; federal regulations allow 1% dry rot at
shipping. Fusarium inoculum on tubers without dry rot can be determined by cutting
20-30 tubers in half, placing them in a large paper bag or box, and shaking. Read dry rot
after 10-14 days incubation at 50-60�F in high humidity. This will indicate the potential
for Fusarium seed piece decay and the possible necessity for a seed treatment. In recent
years, many isolates of Fusarium sambucinum (synonym = F. sulphureum), the
agent responsible for dry rot, from throughout the US and Canada have become resistant to
thiabendazole, the active ingredient in Mertect, resulting in reduced disease control. A
modification of this test to determine resistance is to treat cut tubers with TBZ prior to
incubating the seed in paper bags or boxes.
Bacterial Ring Rot
Look for external symptoms of cracks in the skin (Figure 3
– 59KB color photo); cut the stem end of suspect tubers for internal symptoms of ring
rot. If no external symptoms are seen, cut the stem end of 100 or more tubers selected at
random and look for characteristic ring rot discoloration in the vascular ring (Figure 3 – 91KB color photo). Because other diseases (Fusarium)
or disorders (freeze injury) may resemble ring rot, squeeze suspect tubers and look for
cloudy bacterial ooze from the vascular ring. Suspect tubers should be sent to a competent
official for confirmation of ring rot. Do not use seed lots known to be infected with ring
rot; seed lots with ring rot are not eligible for certification. The bacteria causing ring
rot may be latent in a seed lot without causing symptoms for up to two years, and may be
at such a low incidence as to avoid detection during visual inspections. Tests are
available to index seed lots for the presence of latent ring rot bacteria. This testing
can be done at the Seed Health Testing Lab at NDSU for a fee.
Soft Rot/Blackleg
If more than 1% of the tubers show symptoms of soft rot or tuber blackleg (Figure 4 – 61KB color photo), seed may have the potential for
excessive seedpiece decay. Erwinia bacteria cause soft rot and wet seedpiece decay
and can be latent. Because Erwinia can reside in the lenticels of the tuber, the
number of tubers infested with (carrying) the bacterium gives an estimate of the potential
for seed decay. This can be determined by wrapping 40-50 tubers in wet paper towels, and
plastic wrap over that for 5-7 days at 50-60�F. A more severe test is to puncture 10
lenticels per tuber with a toothpick prior to wrapping. Soft rot should be odor free,
mushy and wet but not sticky and stringy. A sticky, stringy decay with a bad odor
indicates Clostridium decay and should not be scored. These tests will indicate the
potential for soft rot seed piece decay if conditions are favorable for decay. Research
results indicate that extra care is needed for seed lots with more than 50% tuber soft
rot. If handled properly (see seed handling and planting recommendations), acceptable
stands and yields can be obtained. Reducing bruising of seed during handling is the most
important consideration for reducing soft rot in the seed.
Verticillium and Fusarium
These two fungal pathogens can cause wilt and early dying. They are easily visible as
vascular discoloration in the stem end of the tuber (Figure 5
– 94KB color photo). Internally borne inoculum is not as important as inoculum on the
surface of the tuber or in the soil. For the Red River Valley, the amount of Verticillium
or Fusarium in the seed does not contribute greatly to the amount of
wilt that results in the field. However, tuber borne inoculum does act as a source of Verticillium
that will contaminate the soil.
Scab
This disease, caused by soil borne Streptomyces, is soil borne. Infected seed
serves to introduce the scab organism into the field but does not provide inoculum for
infected progeny tubers. Scab infection comes from scab in the soil, not the seed tubers.
Excessively scabby seed is unattractive (Figure 6 – 54KB
color photo).
Silver Scurf
This is primarily a seed-borne disease, although low levels of inoculum may survive in
the soil from one season to the next. It is best to purchase seed with minimum amounts of
visible scurf, but in reality, most seed lots have some silver scurf, and the disease may
not be observed because many silver scurf lesions are too small to be seen. Silver scurf
occurs as scattered golden/silver patches on the skin of the potato, primarily at the stem
end (Figure 7 – 48KB color photo). The fungus causing
silver scurf, Helminthosporium solani, sporulates on seedpieces shortly after
planting and moves to progeny tubers during the growing season. Silver scurf-affected
tubers are sources of inoculum in storage, and the disease spreads in storage, even at
seed holding temperatures. Selection of disease-free seed is desirable, but impractical.
The use of seed treatment fungicides is a better option for managing seed-borne inoculum.
Late Blight
Presence of late blight in seed can be serious and lead to epidemics later in the
season. Late blight can overwinter in infected seed and appears as irregular purplish
lesions externally (Figure 8 – 33KB color photo), and, if
cut about 1/4 inch deep, as granular rust colored areas internally (Figure
9 – 36KB color photo). Late blight tubers do not break down when stored cold
(38F) and can act as one source of blight in the field when planted. Greater than 90% of
late blight affected seed tubers decay due to secondary soft rot in the field at planting,
but the few infected seed pieces that grow can start an epidemic under favorable
conditions. Preliminary research has shown that late blight can be spread during cutting
which can lead to infected plants early in the season. Seed treatment fungicides have been
shown to reduce this infection. Federal regulations allow 1% late blight at shipping. If
late blight is suspected, send a sample to a competent pathologist for confirmation. Late
blight free seed is the best option.
Early Blight
This is a soil-borne disease caused by Alternaria solani and is sometimes found
in seed lots. It is not considered a seed-borne disease, but a severely infected seed lot
may suffer stand and emergence problems due to secondary soft rot and dead eyes.
Nematodes
Generally not a serious problem in the Red River Valley. However, certain areas may
contain nematodes and more surveys are necessary to determine economic importance. Seed
infected with nematodes should not be planted. Planting of nematode infested seed can be
the initial source of field infestation which will increase in future years and be
difficult to control.
Pink Rot and Leak
Most seed affected by these diseases, collectively called water rots, usually decays
during the storage season and does not persist until spring for planting. However,
affected seed should be removed at grading and not planted. Affected seed is watery and
has a pink to charcoal black discoloration. Application of metalaxyl in the field when
tubers are nickel to quarter in size provides excellent control of these diseases.
Prepare to Accept Seed
After you have selected the seed, agreed to a price and scheduled a delivery time, your
storage should be prepared to accept the seed. What you do from here on determines to a
large degree how well your seed will perform. Do not store seed in a storage where sprout
inhibitors such as CIPC have been used unless the entire ventilation system,
including plenums, flumes, fans, ducts, etc. have been thoroughly cleaned. The seed house
must be thoroughly cleaned and disinfected to eliminate carryover of disease causing
organisms. Ring rot bacteria can persist up to three years on dried surfaces and are
capable of surviving freezing temperatures. Clean out all major trash (tubers, vines,
dirt, broken boxes, old bags, etc.) and discard or burn. If potatoes are discarded, don't
leave them near the storage or in a pile (cull pile). A cull pile is a potential source of
many diseases (soft rot, ring rot, late blight, viruses). Instead, burn, chop, compost,
freeze or bury discarded potatoes.
After a rough cleanup, thoroughly wash the storage bins, walls and floor with hot soapy
water using a high pressure washer and rinse. Follow this with liberal application of
disinfectant. In general, disinfectants must be in contact with the surface to be
disinfested for a minimum of 10 minutes to kill bacteria. A foaming agent can be added to
some disinfectants to help the chemical stay in place, such as on a wall, for 10 minutes.
Table 1 lists disinfectant groups commonly recommended and some of their characteristics.
There are literally hundreds of brand name disinfectants; most registered disinfectants
are effective when used properly at the high label rate.
In addition to the disinfectants in the table, live steam can be used to sanitize. The
temperature of the steam contacting the surface to be disinfected must exceed 150�F.
Caution: Do not confuse condensed water vapor with colorless steam. Condensed water vapor
(clouds) may be at less than the required temperature. Exposure time should be five
seconds for fresh, wet bacterial material and 20 seconds for dried bacterial material.
Steam must be used properly to be effective. Do not rush steam cleaning. Steam may be most
useful for equipment rather than the entire storage because of the need for high
temperatures and small surface area covered with the steam appliance.
Following any of the disinfection procedures, rinse well with cold or hot water, remove
excess water, replace equipment and make necessary repairs. Wood surfaces can be treated
with a wood preservative such as copper-8-quinolinolate, which is fungicidal and somewhat
bactericidal. Do not use creosotes or coal tar since they have been canceled for this use
in potato storages.
A separate storage or bin(s) should be available for incoming seed. Keep lots separate
if possible. When the seed arrives, inspect the load(s) to be sure it's what you ordered,
including correct cultivar, lot and tags. Be sure it has not been damaged or frozen in
transit. Store the seed in cool (40-42�F), well-ventilated bins at 85-90% humidity. Avoid
or minimize bruising during the handling operation, as any breaks in the tuber skin may
act as entry sites for disease. Bruising of the seed during handling is the main factor
causing poor seed performance. Warm the seed 7 to 10 days prior to cutting and
planting to lower the reducing sugars and give the sprouts a head start on growing. The
recommended temperature is 50-55�F; this temperature, along with fresh air and humidity,
hastens the wound healing process. The warming time varies depending on cultivar, but
avoid excessive sprouts, which can spread disease during cutting. Alternatively, seed can
be cut, then warmed seven to 10 days prior to planting, but in practice this may result in
excessive decay from bacterial soft rot and Fusarium dry rot. Precut seed should be piled
no more than 6 feet high at temperatures less than 60F with plenty of air for drying and
wound healing. This practice is beneficial for cultivars such as Nooksack and Atlantic,
which have erratic emergence due to long tuber dormancy.
Use a clean disinfected cutter. Seed cutters with "open cell" sponge rollers
that can absorb water can also absorb ring rot bacteria, which can persist in the sponge
from one year to the next and cannot be killed by disinfection procedures. Therefore, use
of cutters with water impermeable (closed-cell) sponge rollers is recommended. Keep the
blades sharp and adjusted to deliver an average seedpiece weight of about 2 ounces. Clean
and disinfect cutting equipment, preferably each day and definitely between seed lots.
This will reduce the spread of bacterial disease such as ring rot and blackleg. Watch cut
seed (at least spot check) for disease - especially ring rot. If ring rot is discovered,
the seed lot should not be planted. Remove and destroy that seed lot, and thoroughly
disinfect all cutting equipment and facilities. Provide workers with disinfectants and
wash facilities to prevent bacteria from entering the seed cutting area. Dip pans are only
effective for a 10 minute dip – a quick dip may not work. It may be better to keep a
pair of rubber boots soaking in disinfectant and change boots when entering the warehouse.
Provide workers with plastic disposable booties and new gloves daily.
The three main points of planting are: 1) get good seed, 2) handle it carefully, and 3)
use cultural practices that encourage quick emergence. The following checklist of cultural
practices to follow at planting seed potatoes will minimize disease and maximize emergence
and stand:
- seed and soil should be the same temperature; 50�F is optimum
- avoid wet, soggy soils
- handle seed gently
- do everything possible to encourage quick emergence
- plant shallow and hill plants as they emerge
Seedpiece decay (SPD) can be a major problem in some years in all production areas. The
two major causes of SPD are bacterial and fungal. The fungus Fusarium and the
bacterium Erwinia carotovora are capable of causing SPD. Fusarium SPD tends
to be dry, slow-moving decay whereas Erwinia SPD tends to be a wet, fast-moving
decay. Both forms are capable of moving from the decaying seed piece into the vascular
system of the plant causing wilt and blackleg
Fusarium decay in the field is favored by a wide range of temperature and
moisture conditions. Because this pathogen needs a wound to enter the tuber, it is
important to speed the healing of cut seed. Seed containing Fusarium dry rot in
storage will continue to decay when planted.
Most seedpiece decay is caused by Erwinia bacteria. Erwinia seedpiece
decay (SPD) is favored by cool, wet weather. The cool weather slows down seedpiece growth
and the moisture favors the bacterium, which is very sensitive to drying. Excess water
also inhibits resistance of the potato to disease. Erwinia can enter not only
through cut surfaces and wounds but, more importantly, resides in the lenticels (breathing
pores) of the tuber. When conditions of low oxygen occur in the field (wet soil), the
bacteria become active and can rapidly decay the seedpieces. Most potatoes, except those
recently derived from stem cutting programs, contain Erwinia bacteria in the
lenticels. In a sense, bacterial seedpiece decay is a disease waiting for the right
conditions to trigger it.
There are two important factors that affect SPD: 1) the quality of seed - how clean
(disease-free) the seed is, and 2) the environment it is planted in - moisture,
temperature, soil type, etc. Based on these principles, the following planting
recommendations have been formulated for control (or at least partial control) of SPD.
Treat the seed with a recommended seedpiece treatment chemical. There are many
chemicals available - experience and the results of professional evaluations in your area
should guide you to which one to use. Do not use thiabendazole compounds (TBZ, thiophanate
methyl) as a seed treatment if Mertect has been used on the seed in and/or out of storage.
Use of thiabendazole as a seed treatment in addition to prior treatment may cause abnormal
sprouting disorders, such as tuber formation on the seed pieces instead of sprouting, and
may result in poor stands. Some of the registered seed treatments contain captan,
thiophanate methyl, mancozeb, bleach and fludioxonil, or mixtures of these compounds.
Except for bleach, all seed treatments are fungicides. They are primarily aimed at the
seed borne pathogens Fusarium coeruleum and sambucinum, Helminthsporium
solani and Rhizoctonia solani. In recent years, many isolates of H. solani
and F. sambucinum have become resistant to benzimidazole fungicides commonly used
as seed treatments (thiabendazole and thiophanate methyl). Seed treatments containing
these compounds as active ingredients are not as effective as they once were at reducing
disease caused by resistant strains of these fungi.
The inert ingredients, or carriers, are part of the seed treatments and may play a role
in seed treatment performance. Bark carriers, ground up bark of fir, alder or other trees,
tend to absorb more moisture than mineral carriers such talc, gypsum or clay. The mineral
ingredients may trap water, becoming gummy and cutting off oxygen, leading to more seed
piece decay under wet soil conditions. Almost all seedpiece treatments are fungicides.
None of the treatments will affect lenticel-borne Erwinia. However, because Fusarium
infected seed is more susceptible to blackleg, controlling Fusarium indirectly
helps control blackleg. None of the treatments affect fungi (especially Fusarium)
already in the seedpiece. They affect only pathogens on the cut surface and in the soil
close to the seedpiece.
Seed treatments can be used going into storage, at the time seed is removed from
storage, or at cutting time. Application time will depend on the individual operation and
the particular need for disease control. The usual application time is immediately after
seed cutting; consult the product label to determine timing of the seed treatment. Various
application methods are available for dust and liquid application of seed treatment
fungicides, but more work is needed in the area of fungicide application for dust control
and complete coverage of uncut and/or cut seed.
Blackleg can be minimized in a seed lot containing up to 50% of the tubers infested
with Erwinia by bruise avoidance, warming the seed, planting in warm soil, and planting
shallow to encourage rapid emergence and establishment on their own roots.
If all these recommendations are followed, the seed you bought has a better chance of
performing as you think it should. Remember that no single control guarantees freedom from
disease. All these recommendations should be combined in an integrated approach with the
goal of minimizing losses to disease and maximizing seed performance.
Table 1. Disinfectants commonly recommended for potato handling
equipment and storage facilities.
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Inacti- Inacti- Recom-
vated by vated Recommended mended
Disin- organic by hard Corrosive concentration exposure Shelf
fectant matter water to metal Safety for use time life Comments
---------------------------------------------------------------------------------------------------------
Quaternary Some No Slight Use Label 10 min 1-2 yr Diluted disinfectant
ammonium caution directions relatively safe,
compounds (see concentrated form is
comments) poisonous. Stainless
Hypochlor- Yes No Yes Irritant, 1:50 (0.1%) 10 min 5.25% Quick acting,
ites (except caustic or 1:200 bleach inexpensive; caustic
(5.25% iron) (see stable to skin and clothing.
bleach) comments) 6 mo. Use at 1:50 when
at room mixing with water only.
temp. Is more effective at
pH 7-8 than at normal
pH of 10-11. For
maximum effectiveness,
use 1 part 5.25%
bleach: 200 parts water,
0.6 parts white vinegar.
Gives concentration of
256 ppm.
Iodine Some No Yes Relatively Label 10 min 1-2 yr. Do not take internally.
compounds (except safe. Use directions No longer effective if
iron) caution. it loses Yellow-brown
color.Tamed iodophor
compounds work best.
Phenolic Some; not No No Oral Label 10 min 1-2 yr Provides residual action.
Compounds greatly poison. directions These have name "phenol"
Use on label of ingredients.
caution.
Chlorine No No No Non-toxic Label 10 min 2 wks Use potentiated form by
Dioxide directions when mixing base + activator.
mixed. Broad spectrum activity
1-2 yrs against viruses, fungi,
in and bacteria; does not
separate produce THM.
containers.
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This circular was produced with the cooperation and financial support of Norarits crop
protection seed treatment business unit.
PP-877 (Revised), July 1997
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