Viroids and their potential danger to potatoes in hot climates

Viroids are the smallest agents known to be pathogenic to plants. Unlike viruses, they are devoid of a protective protein coat and are composed entirely of circular ribonucleic acid of low molecular weight (ca. 85,000-130,000 d. In spite of their small size, viroids cause serious diseases of avocado, chrysanthemum, citrus, coconut, cucumber, hop, potato and tomato. Although viroid diseases have been reported in both tropical and temperate regions of the world, they induce more severe symptoms at high temperatures. They have wide host ranges and at least six different viroids can induce symptoms in potato similar to those caused by the potato spindle tuber viroid. This creates difficulties in making positive identifications. Viroids spread readily by contaminated knives, sickles, and hilling and cultivating equipment. Viroids are detected by bioassays, by gel electrophoresis on polyacrylamide gels or by nucleic acid hybridization tests. Viroids are not a potential danger to potato crop in hot climates.