The Avocado Sunblotch Viroid: An Invisible Foe of Avocado

Abstract

This review collects information about the history of avocado and the economically important disease, avocado sunblotch, caused by the avocado sunblotch viroid (ASBVd). Sunblotch symptoms are variable, but the most common in fruits are irregular sunken areas of white, yellow, or reddish color. On severely affected fruits, the sunken areas may become necrotic. ASBVd (type species Avocado sunblotch viroid, family Avsunviroidae) replicates and accumulates in the chloroplast, and it is the smallest plant pathogen. This pathogen is a circular single-stranded RNA of 246–251 nucleotides. ASBVd has a restricted host range and only few plant species of the family Lauraceae have been confirmed experimentally as additional hosts. The most reliable method to detect ASBVd in the field is to identify symptomatic fruits, complemented in the laboratory with reliable and sensitive molecular techniques to identify infected but asymptomatic trees. This pathogen is widely distributed in most avocado-producing areas and causes significant reductions in yield and fruit quality. Infected asymptomatic trees play an important role in the epidemiology of this disease, and avocado nurseries need to be certified to ensure they provide pathogen-free avocado material. Although there is no cure for infected trees, sanitation practices may have a significant impact on avoiding the spread of this pathogen.

Mexico: A Landscape of Viroid Origin and Epidemiological Relevance of Endemic Species

“…Like the characteristic endemism of plant species belonging to the Solanaceae family, the origin of the avocado plant has been determined to belong to a particular region of Mexico. In this context, it has recently been proposed that the origin of ASBVd resembles the origin of its natural host [ 20 ]. This proposal relies on the observation that the southern region of California imported seeds from Mexico at the beginning of the establishment of the avocado industry.…”

“…Likewise, symptomatic trees may become asymptomatic for underexplored reasons. More research is needed to understand the distribution and possible variants associated with the different symptoms that have been identified [ 20 ].…”

“…Recently, it has been proposed that the origin of avocado groves is linked to the origin of ASBVd in the Mexican highlands [ 20 ]. In biological observations, it was noted that in symptomatic avocado fruits with ASBVd, the embryo generally atrophies, while in asymptomatic fruits with a lower viroid load, viable embryos are present.…”

Pest categorisation of the avocado sunblotch viroid

“…ASBVd has no known insect vector and the major means for ASBVd dispersion is via infected propagative material (Vallejo P erez et al, 2017). Transmission most often occurs by grafting infected budwood (Wallace, 1958) or growing rootstock seedlings from infected seeds, often asymptomatic (Saucedo-Carabez et al, 2019). This is a major characteristic that led to the evolution and perpetuation of ASBVd (Duran-Vila et al, 2008) and resulted in a frequent infection of genetic material held in national germplasm collections (Ronning et al, 1996;Olano et al, 2002;Tondo et al, 2010;Schnell et al, 1997Schnell et al, , 2011.…”

Rational design of a survey protocol for avocado sunblotch viroid in commercial orchards to demonstrate pest freedom

“…Avocado sunblotch viroid has many biological characteristics that are conducive to the creation of pest free production sites, as recently reviewed by Kuhn et al [ 5 ] and Saucedo Carabez et al [ 6 ]. The viroid has no arthropod vector and in nature its host range is restricted to avocado.…”