1Department of Plant Pathology, and 2Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695, USA

Background and objectives
Sweet potato, Ipomoea batatas, is the most valuable vegetable crop produced in North Carolina, with farm-gate returns of $60 million. It is grown on approximately 35,000 acres [1]. Because sweet potatoes are prone to deleterious mutations in skin colour, flesh colour, shape, low yields and other characters, the Certified Sweetpotato Seed Program in North Carolina was established in 1962 to provide high-quality roots of varieties that are true-to-type and free from pests. Historically, carefully selected 'seed' roots are increased in numbers in the Certification Program in four stages: Breeder, Foundation, Registered and Certified seed. Although the North Carolina Certified Sweetpotato Seed Program is effective in delivering high quality 'seed' roots, it does not provide 'seed' that is free of viruses and possible other pathogens. Clonally propagated sweetpotatoes may accumulate viruses which affect plant vigour, productivity and other horticultural characteristics. Sweet potato feathery mottle virus (SPFMV) is widespread in sweet potato seed, and causes foliar discoloration and 10-20% reduction in yield. However, strains of SPFMV cause russet crack (a very damaging root-surface necrosis), and internal cork lesions on roots of popular sweet potato varieties [2]. The objectives of the project are to improve the existing North Carolina Certified Sweetpotato Seed Program by: (i) incorporating virus indexing into the Certification Program; (ii) eliminating russet crack; (iii) improving 'seed' quality; and (iv) increasing yields.

Materials and methods
Development of virus-indexed micropropagated planting material in a sweet potato variety involves selecting superior hills, establishing 0.5-1.0 mm in vitro cultures of the apical meristem, micropropagating, growing plants under insect-free conditions in the greenhouse, assaying plants for SPFMV by grafting on the Brazilian morning glory Ipomoea setosa, evaluating mericlones in the field for production characteristics, and then increasing seed stocks in the foundation field under strict isolation.

Results and conclusions
Tests have been conducted since 1989 to evaluate the effects on yield and root quality of virus-indexed, micropropagated planting material. In all cases, yields from virus-indexed mericlones were comparable to or higher than yields from virus-infected plants [3]. Problems were experienced with SPFMV infection in the foundation increase field, and with variability in vine characteristics and colour of skin and flesh of roots from plants micropropagated from the same mother plant. A significant difference was observed in the reduction of russet crack, from 23% in grower 'seed' to less than 0.2% in 'seed' produced from virus-indexed micropropagated plants. Commercial sweet potato growers benefit from the use of virus-indexed micropropagated root stocks from the North Carolina Certification Program by having reduced levels of russet crack, reduced incidence of disease, improved uniformity, shape and colour, and high yields of harvested roots.

1. Wilson LG, Averre CW, Baird JV et al., 1989. North Carolina Cooperative Extension Service Bulletin AG-09.
2. Cali BB, Moyer JW, 1981. Phytopathology 71, 302-305.
3. Schultheis JR, 1994. HortScience 28, 727.