Producing a clean sweetpotato plant begins in a lab. Working in a sterile laminar airflow hood and using a dissecting microscope, a small growing tip is cut away, or excised, from an infected plant and placed into a tube of medium that contains all the nutrients required to grow. The tips, referred to as microshoot tips or meristem tips, are 0.1 to <0.5 mm in diameter. The tubes are incubated under lights and in approximately 5 to 9 months, during which the plantlet is transferred to fresh medium every 3 to 4 weeks, the tip develops into a small plantlet about 6 cm high with shoots and roots.
The new plant that has been regenerated from the microshoot tip is tested to confirm that the process successfully eliminated viruses. It is then used to produce propagation material that goes on to further cycles of propagation in a protected greenhouse.
At this stage, depending on the NCPN center, the green plants are supplied to a grower or, in North Carolina, a certified seed grower. Alternately, the green plants may be planted in a field to produce certified or clean roots to sell.
Excising sweetpotato microshoot tips for virus elimination therapy
Ipomoea setosa (Brazilian morning glory) plants ready to be graft inoculated with a section of a sweetpotato vine.
The I.setosa leaf on the right shows vein clearing and stunting which are typical symptoms of virus infection.
Visible symptoms of virus infections in sweetpotatoes can be easy to miss; they vary with the cultivar and environmental conditions. Even if there are no visible symptoms the disease may still affect potato quality, size and yield negatively. For this reason, to be sure that you are not propagating a virus along with your sweetpotato it is necessary to test the plants for viruses.
The NCPN-SP centers and programs test for viruses by various methods including: indexing on Ipomoea setosa, qPCR, and HTS. I. setosa or Brazilian morning glory is a sensitive indicator of many sweetpotato viruses. The test involves grafting a section of a sweetpotato stem onto several I. setosa plants, then observing them for 3 to 6 weeks to see if they develop symptoms. For more details see Minimum Virus Testing Protocols for Sweetpotato Plants in the National Clean Plant Network – Sweetpotato (PDF)