As a general rule, use only certified seed. Certified seed is inspected by professionals and considered to meet certain strict standards. For example, certification almost invariably prohibits the known presence of bacterial ring rot, rootknot nematodes and certain other diseases which can lead to devastating crop losses. Strictly enforced tolerances have been established for other lesser pests and diseases. Certification is not a guarantee of quality.
Certification reports typically emphasize what is seen during visual field inspections.
Diseases present at low levels during inspection may go unobserved, increase dramatically before harvest, and lead to seed tuber infection levels far beyond tolerances. To guard against this problem, most states require some type of winter grow-out test in which a small sample of each seed lot is planted and grown out for further visual inspection. Winter testing requirements vary considerably among states and among Generations within states using Limited-Generation programs, as most do. Factors other than field diseases can also affect the performance of seed. Seed handling during harvest, storage, and shipping may have more influence on crop performance than disease readings reported by certification inspectors.