Many pests can be transmitted in infected seed tubers, including bacterial ring rot, blackleg, bacterial soft rot, 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. Check specific Potato Seed Certification Standards for pest tolerances enforced in California. Depending on your own situation and tolerance for particular diseases, your seed purchasing decision might involve a stricter standard.
Seed tuber handling
Store seed tubers at 38°F. To increase seed age, higher temperatures can be used. About two 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, the spread of mechanically transmitted viruses increases 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. Seed treatments may be applied at this time to protect against certain diseases and insects (see table below). Follow good sanitation practices during cutting; clean and disinfect cutting equipment thoroughly between seed lots. 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.