Avocado sunblotch viroid (ASBV) is a circular single-stranded RNA with 246-251 nucleotides. This viroid causes an important disease affecting avocado trees. Infections result in lower yields and poorer quality fruit. ASBV is the smallest known viroid that infects plants and is transmitted by pollen and infected seeds. Trees infected with the viroid often show no symptoms other than a reduction in yield. However, they are still carriers and can pass the disease onto other plants. Symptoms in more serious infections include depressed longitudinal streaks of yellow in the fruit. The fruit may also become red or white in colour. Symptoms in the leaf are uncommon but include bleached veins and petioles. Rectangular cracking patterns also occur in the bark of older branches. Detection of ASBVd is therefore of the utmost importance in controlling spread of this disease, as symptom-less trees represent a great danger in terms of spread of the viroid.