How are viroids primarily spread?

Viroids are primarily spread through mechanical means, such as pruning tools or other mechanical devices. This mode of transmission occurs because viroids are very small, infectious pieces of RNA that do not encode proteins and lack a protective protein coat. They can be easily transferred from one plant to another when tools that have come into contact with infected plants are used on healthy ones. This transfer can happen during practices like pruning, grafting, or handling plants without proper sanitization of tools.

Other potential methods of transmission, such as airborne spores or animal carriers, do not apply to viroids, as they do not have the same spreading mechanisms as pathogens like viruses or fungi. Soil contamination can contribute to the spread of some pathogens, but viroid transmission is primarily associated with direct physical contact facilitated by tools and equipment. Therefore, the correct understanding of viroid transmission emphasizes the importance of hygiene and sanitation in gardening and agricultural practices to prevent their spread.