A MILD STRAIN OF POTATO SPINDLE TUBER VIROID FROM CHINA IS SIMILAR TONORTH-AMERICAN ISOLATES

At the potato breeding farm of Keshan Potato Research Institute (KPRI) , Heilongjiang, China, potato cultivar Kexin #1 often did not develop ''spindly'' tubers when infected with the potato spindle tuber viroid (PSTVd). Using return-polyacrylamide gel electrophoresis, isolates of the mild strain of PSTVd were detected from the potato tubers as well as from the true potato seeds. Reaction of KPRI-PSTVd was studied on t wenty-two potato cultivars of North American origin under field and gr eenhouse conditions. Symptoms in these potato cultivars consisted of a n upright growth of plants, stunted plants, and elongated, spindly tub ers with longitudinal growth cracks, as typically produced by the Nort h American PSTVd strains. KPRI-PSTVd was also shown to be transmitted in true seed of nine potato cultivars tested. The nucleotide sequence of KPRI-PSTVd is identical to the published sequence for the type isol ate of the PSTVd mild strain and differs in only one position from tha t of a newly sequenced mild strain from Wisconsin (WB). Possible sprea d of mild strains of PSTVd through the distribution of Irish Cobbler p otatoes from North America to Southeast Asia and China is discussed.