Agriculture Canada, Research Branch, Research Station Frederiction, New Bruoswik, Canada E3B4Z7.
The spindle tuber disease of potato, which was previously believed to be of viral etiology, has been shown to be caused by an infectious RNA of low-molecular weight (127,000 daltons), which is characterized by the absence of a dormant phase (viron) and by genome which is much smaller than those of known viruses. The RNA is single stranded, covalently closed ring of 359 nucleotides, whose complete primary sequence is known. The infectious RNA is termed “viroid” and six additional viroid-diseases of plants are known. The spindle tuber viroid is highly infectious, readily transmitted by mechanical means, contaminated tools and also via true seeds, pollens and ovules of potato and other plants. It has a wide host-range. Although symptomlessly carried in most plants it causes diagnostic symptoms in many cultivars of tomato and Scopolia sinensis plants. Mild and severe forms of spindle tuber viroid are known, which cause 20% and 64% yield loss, respectively.
