Fungi, bacteria, and viruses are three major types of pathogens causing plant diseases and majority of growers know about them or at the very least, have heard about them. But how many know about the world’s smallest infectious pathogens e.g., viroids? Perhaps few! So, what are viroids? Viroids are infectious agents, smaller than viruses, containing single-stranded, circular RNAs without a protein coat. We know viruses as light travelers; they take the necessities along and by high jacking the biological machinery and translation system of plant host, produce proteins and make copies of themselves. Viroids, however, have taken the minimalism lifestyle to the extreme. They are very small, contain a naked RNA with 220-450 nucleotides, without a protein coat or genetic materials to code proteins (Flores et al. 2009). Due to the lack of any proteins in their system, they cannot recognize or bind to the healthy cells for entry and use passive mechanisms of transmission which is considered a drawback.

For replication, viroids have somehow managed to take advantage of the rolling-circle replication process which is originally used to duplicate DNAs. The details of the mechanism are beyond the scope of the current topic or the need of audience. But in brief, they use host’s RNA Polymerase II enzyme to make long, attached chains of their genomes that needs to be cleaved at right places before they become entirely functional. The cleavage methods that viroid use at this point, classify them into two main families: the Pospiviroidae which uses host’s RNA Polymerase III to cleave and RNA Ligase to fuse the fragments; and the Avsunviroidae which are self-sufficient and carry their own tool for cleavage, hammerhead ribozyme (Figure 1) (Flores et al. 2009).   

Unlike other plants, avocado (Persea americana Mill.), is not subject to a variety of viral diseases. The occurrence of viral and viroid diseases in Avocado is limited to two viroids: Avocado Sunblotch Viroid (ASBV), the smallest known viroid (247 nt) belonging to the Avsunviroidae and Potato Spindle Tuber Viroid (PSTV) belonging to the Pospiviroidae. The first only infects avocado and is economically very important and damaging to the avocado industry. While the latter has a wider range of plant hosts and is much more important for vegetables and ornamentals. ASBV symptoms have been documented in California since the early 1900’s and the name sunblotch was given to this disease because symptoms resemble those of sunburn. Despite the first description of sunblotch in early twentieth century in California (Coit, 1928), distribution and economic impact of ASBV on California avocado industry is not well-known yet. Currently, the disease is well-distributed in most of the avocado growing regions, worldwide and impacts yield and fruit quality. Up to 75% and 30% yield reduction was observed in symptomatic and asymptomatic ‘Hass’, respectively (Saucedo-Carabez et al., 2014). ASBV symptoms may vary under the influence of different environment, avocado cultivar, and variant of viroid. The distribution of the viroid within a tree is uneven and irregular and sometimes, the viroid exists in the tree but does not show symptoms, such trees are called symptomless carriers which play a significant role in the epidemiology and spread of the disease. Viroids are stable molecules which could survive outside the cell environment for almost 7 weeks (Mehle et al. 2014), but the resilience of ASBV is to the degree that it was detected in pollen samples in beehives after three months in South Africa (Roberts et al. 2023). Symptoms. ASBV causes symptoms on all tree parts including fruit, leaves, branches, and twigs. While symptoms on leaves e.g., discoloration and physical distortion may rarely be seen, fruits typically show depressed/sunken scars in white, yellow, and red color which are dominantly seen at the pedicle end (Kuhn et al. 2017). Sometimes severely infected fruit show necrotic areas at the center of the crevices (Figure 2, a-c). Contradictory reports exist regarding the impact of ASBV on yield, some reports state that infected trees produce abundant but small fruit in size while in some, tree yield was reported to be severely reduced. Small and misshaped fruit resulting from the viroid impact fruit marketing value. As seen in Figure 2 (d-e), alligator skin/bark is another typical symptom of ASBV which is described as rectangular cracking appearance on bark of large branches and trunk of old trees. Infected trees may look stunted and old trees grow into a low and flattened shape with branches bowing toward the ground.

Transmission. Viroid transmits through budwoods and seeds from symptomatic or asymptomatic trees (Eskalen and Faber, 2016). Natural root grafting is another way of ASBV transmission in groves (Whitsell, 1952). Pollen from an infected tree could only infect the seed but not the pollen recipient tree unless it is infected through other ways (Desjardins et al., 1979, 1980). Seeds from asymptomatic trees spread the disease up to 100% but only less than 5% transmission occurred with seeds from infected trees (Desjardins, 1987). We do not have any evidence showing that insects are involved in disease spread. There were, however, assumptions that honeybees might be involved in spreading disease through pollen (Desjardins et al., 1979; Eskalen and Faber, 2016). In Queensland, Australia researchers found the viroid in pollen samples in pollination hives but not in the honeybees, while in South Africa, the viroid were detected in both pollen and the insect (Roberts et al. 2023). As mentioned above, viroids are reliant on passive mechanical transmission such as wounds caused by contaminated grafting or pruning tools and injection equipment (Desjardins et al., 1980).   

Control. So far, there is no treatment for ASBV. Preventive and sanitation measures such as disinfection of tools used for pruning, grafting, and injecting, producing, and planting disease-free grafting and seed materials are the primary ways to contain the disease. Uprooting the infected symptomatic trees despite being expensive could slow down the spread. Very fundamental and critical steps must be taken, though, at the nursery stage where disease-free budwoods and rootstocks grown from seeds.

Future Explorations. The pace of the global avocado production is remarkedly high compared to other tropical fruits, and it is projected to reach 12 Mt by 2020 (FAO 2021). Such increase in production is driven by high consumer demand and requires trading of large number of fruit and even grafting materials for expanding the plantation. Therefore, to facilitate the trading process, and avoid spread of the disease and quarantine on shipments, it is crucial to ensure the ASBV-free materials are produced. To meet this goal, we first need to optimize reliable, sensitive, fast, and somewhat affordable techniques to detect the diseased avocado trees in California. However, detection is challenging due to the uneven and irregular distribution of viroid in avocado trees and diagnosis based on symptoms is not the case for ASBV due to the existence of symptomless carriers.

Another interesting and challenging point that will require the attention of researchers about ASBV is that some symptomatic trees suddenly become asymptomatic and vice versa. It is assumed that stress play a part in infected trees shifting from asymptomatic to symptomatic, but more research is needed for clarification.

Observations show that all avocado cultivars are susceptible to ASBV but more information about the susceptibility of rootstocks and their roles in root-to-root transmission or combination of rootstock-scions is needed to be explored. Our next steps on the status of this disease will be focused on understanding the distribution and economic impacts, developing fast and reliable detection techniques with ability to detect the pathogen in any infected part of the tree, possible variants associated with the different symptoms and developing advanced control methods to contain the pathogen.

References

Coit J.E. Sunblotch of the avocado. Calif. Avocado Soc. Yearb. 1928; 20:27–32.

Desjardins P.R., Saski P.J., Drake R.J. Chemical inactivation of avocado sunblotch viroid on pruning and propagation tools. Calif. Avocado Soc. Yearb. 1987; 71:259–262.

Desjardins P.R., Drake R.J., Atkins E.L., Bergh B.O. Pollen transmission of avocado sunblotch virus experimentally demonstrated. Calif. Agri. 1979; 33:14–15.

Desjardins P.R., Drake R.J., Swiecki S.A. Infectivity studies of avocado sunblotch disease causal agent, possibly a viroid rather than a virus. Plant Dis. 1980; 64:313–315.

Eskalen, A., and Faber, B. A. 2016. Avocado Sunblotch Viroid (ASBVD). UC IPM Pest Management Guidelines: Avocado. UC ANR Publication 3436.

Flores R, Gas ME, Molina-Serrano D, Nohales MÁ, Carbonell A, Gago S, De la Peña M, Daròs JA. Viroid replication: rolling-circles, enzymes and ribozymes. Viruses. 2009 Sep;1(2):317-34.

Kuhn D.N., Geering D.W., Dixon J. Avocado sunblotch viroid. In: Hadidi A., Flores R., Randles J., Palukaitis P., editors. Viroids and Satellites. Academic Press; Cambridge, MA, USA: 2017. pp. 299–300. 

Mehle, N., Gutiérrez-Aguirre, I., Prezelj, N., Delić, D., Vidic, U., and Ravnikar, M.  Survival and transmission of potato virus Y, pepino mosaic virus, and potato spindle tuber viroid in water. Appl. Environ. Microbiol. 2014, 80:1455-1462.

Roberts, J.M.K., Jooste, A.E.C., Pretorius, L.S., and Geering, A.D.W. Surveillance for Avocado Sunblotch Viroid Utilizing the European Honeybee (Apis mellifera). Phytopathology. 2023, 113:559-566.

Saucedo-Carabez J.R., Téliz-Ortiz D., Ochoa-Ascensio S., Ochoa-Martinez D., Vallejo-Pérez M.R., Beltrán-Pena H. Effect of Avocado sunblotch viroid (ASBVd) on avocado yield in Michoacan, México. Eur. J. Plant Pathol. 2014; 138:799–805.

Whitsell, R. 1952. Sunblotch disease of avocados. Calif. Avocado Soc. Yearb. 1952, 37, 215–240.