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UC Pest Management Guidelines

Small tubers produced on plantlets grown under greenhouse conditions may be used to grow the first generation of seed potatoes in the field.


Seed Certification and Seed Tuber Handling

(Reviewed 1/08, updated 5/08)

In this Guideline:

Seed certification. Many pests can be transmitted in infected seed tubers, including bacterial ring rot, blackleg, common scab, late blight, potato viruses, powdery scab, Rhizoctonia, root knot nematodes, silver scurf, and wilt diseases. Stem cutting and micropropagation techniques have been developed to obtain pest‑free potato plants for propagation and production of certified seed tubers. Several generations of plants are grown in the field to produce certified seed tubers that will be sold to commercial growers.

Certified seed tubers are not guaranteed to be disease free. They are certified to have shown no more than certain low percentages of pest and disorder symptoms during the inspections required by a state's seed certification program. The allowable level of symptom expression for each pest or disorder is called a tolerance level, and these levels vary from state to state. A zero tolerance exists for certain pests, such as bacterial ring rot and root knot nematode. To pass these tolerances, seed lots must be inspected at least twice in the field during the growing season and be inspected in storage or at the time of shipment. Samples of each seed lot are grown and inspected in winter field trials in warm locations such as southern California and Hawaii, or in winter greenhouse plantings. Pests for which tolerances are enforced and the range of tolerances among the western states are listed in the table below.

Range of Tolerance Levels for Latest Generation of Seed in Western States Seed Certification Program
Disease Symptoms Range of tolerance level (%)
leafroll 0.2–0.4
PVX 1.0–4.0
mosaic virus 0.5–2.0
calico (AMV) 0.5–1.0
Total all viruses 1.0–3.01
haywire 0.5–2.0
witches' broom 0.5–1.0
spindle tuber 0.0–0.1
blackleg 0.25–0.4
bacterial ring rot 0.0
late blight 1.02
giant hill 0.53
root knot nematode 0.0
varietal mix 0.1–1.5
1  Does not include PVX. Total proportion of tested plants showing symptoms of viruses other than PVX cannot exceed this level.
2 Shipping point tolerance
3 Colorado and Montana

Seed tuber handling. Store seed tubers at 35° to 38°F. About 2 weeks before cutting, warm seed tubers gradually to 50° to 55°F and hold them at that temperature with a relative humidity greater than 90% and good ventilation. This reduces the amount of tissue tearing during cutting and encourages wound healing (suberization) after potatoes are cut, greatly reducing the incidence of seed piece decay after planting.

If tubers are cut when they are just beginning to sprout, a stage sometimes called "peep" or "peek" emergence is more rapid, and you can more easily choose a seed piece size that gives the number of sprouts you want. Cut seed tubers before sprouts exceed about 1/8 inch (3 mm) in length to avoid breaking them and reduce the chance of spreading disease during cutting. If sprouts are broken, spread of mechanically transmitted viruses is more likely and seed pieces may develop multiple sprouts, which are weaker and may form too many stems per hill. A seed piece size of 1.5 to 2.5 ounces is recommended for optimum performance in most areas. The larger size is recommended for cultivars that have few eyes, such as CalWhite and Russet Nugget. Cut seed in an area out of drafts. Seed treatments may be applied at this time to protect against certain diseases (see table below). Follow good sanitation practices during cutting; clean and disinfect cutting equipment thoroughly between seed lots (link to table of disinfectants, if we add one). Protect the cut seed from sun and wind when hauling.

Plant cut seed pieces immediately in moist soil (60 to 80% of field capacity) that is at a minimum of 45°F to accelerate emergence and wound healing after planting. If you cannot plant cut seed immediately, hold it at 50° to 55°F with good aeration and high humidity to speed wound healing. Store cut seed only where adequate airflow can be maintained throughout the pile. Do not store cut seed in bulk trucks. Do not plant seed that is cooler than the soil, particularly early in the season.


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463
General Information
B. J. Aegerter, UC Cooperative Extension, San Joaquin Co.
H. Carlson, UC Cooperative Extension, Siskiyou Co.
R. M. Davis, Plant Pathology, UC Davis
L. D. Godfrey, Entomology, UC Davis
D. R. Haviland, UC Cooperative Extension, Kern Co.
J. Nuñez, UC Cooperative Extension, Kern Co.
A. Shrestha, UC IPM Program/Kearney Agricultural Center, Parlier

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