Avocado sunblotch is a disease caused by the Avocado sunblotch viroid (ASBVd) – a circular single-stranded RNA molecule in the family Avsunviroidae which is only about 250 nucleotides in length. Despite its tiny size, and the fact that this viroid is not classified as a living organism, the presence of ASBVd in an avocado host can lead to the appearance of severe symptoms such as the formation of coloured, sunken lesions on avocado fruit, discoloured streaks on young stems, and discolouration and malformation of leaves. Although the appearance of these symptoms will not usually lead to death of a healthy avocado tree, fruit symptoms will impact market value of the fruit, and asymptomatic trees may have significantly reduced yield when compared to their uninfected counterparts.

While previous research on ASBVd has been largely focused on mapping its distribution, refining detection techniques and monitoring the physiological effect of infection on avocado trees and fruit, there is as yet no definitive evidence to explain how the viroid causes disease at the molecular level. Current research within the ARP aims to elucidate molecular mechanisms of avocado sunblotch disease by investigating host responses to viroid infection, as well as how viroid populations change within avocado hosts over time. By expanding our understanding of how ASBVd causes host symptoms at the molecular level we will pave the way for future research which will aid South African avocado growers in combatting this disease.

 

*Read more about Avocado Sunblotch Viroid on our Fact sheet here.

 

ARP Team Members

Melissa Joubert: Investigating molecular mechanisms for disease caused by Avocado sunblotch viroid (ASBVd).

 

                        

 

Rosellinia necatrix Berl. ex Prill. is an ascomycete pathogen that targets a multitude of different plant hosts in various tropical and temperate regions. As the causal agent of white root rot (WRR) it has caused significant economic losses within the agricultural and forestry industries. Symptoms and the presence of R. necatrix were confirmed in a commercial avocado orchard in the Limpopo Province (South Africa) in 2016. Since then, it has been detected in Mpumalanga, Western Cape and Kwa-Zulu Natal. Control options for WRR are limited due to the pathogen’s hardy resting structures, deep soil penetration, and resistance to common fungicides. Additionally, common avocado rootstocks in South Africa appear to be susceptible to white root rot. Currently, research in the ARP is aimed at understanding R. necatrix and it's affect on avocado, as well as potential control strategies.

 

 * Read more about Rosellinia necatrix on our Fact sheet here.

 

ARP Team Members

Phinda Magagula: The detection and management of Rosellinia necatrix in avocado orchards.

Khanyisile Botshelo: In planta antagonistic potential of Trichoderma species against Rosellinia necatrix.

Tsakane Miyambo: Investigating the genetic diversity, population structure and virulence of Rosellinia necatrix in South Africa.

Jesse Hartley: Monitoring the spread of Rosellinia necatrix on avocado in South Africa.

 

       

 

An ambrosia beetle, commonly known as Polyphagous Shot Hole Borer (PSHB) Euwallacea fornicatus (Coleoptera: Curculionidae: Scolytinae), is considered to be a pest, due to its ability to damage trees by acting as a vector for a pathogenic fungi. Fusarium euwallaceae, the fungal symbiont of the PSHB, is inoculated into the tree by the beetle. Eventually, the fungal symbiont prevents the transport of water and nutrients by invading the xylem, that leads to Fusarium dieback and the eventual death of the host tree. This pest-pathogen complex has emerged as an invasive pest in Israel and the United States of America (California), causing severe damage and significant economic losses to agricultural, ornamental and urban trees, especially to their avocado industries.

This pest-pathogen complex was detected in South Africa damaging Platanus x acerifolia (London Plane) trees in the National Botanical Gardens, KwaZulu-Natal. Recently, it was detected on a backyard tree and in a commercial avocado orchard. Control management strategies for this pest complex are limited due to inefficient trapping mechanisms, lack of biocontrol measures and inefficient fungicides. The use of resistant/tolerant trees could serve as a potential control strategy. Current research within the Avocado Research Programme (ARP) is therefore aimed at identifying Fusarium spp. isolates sampled from avocado trees to determine the extent of the threat to industry, after which the taxonomy of these isolates will be defined.  We are also in the process of determining the threat on various, commonly growth avocado cultivars through the use of multiple pathogenicity trials.

 

*Read more about Ambrosia beetles and Fusarium dieback on our Fact sheet here.

 

ARP Team Members

Casey Gill: PacBio sequencing of the fungal mutualists associated with Xylosandrus crassiusculus.

 

                  

 

The advent of next-generation sequencing (NGS) technologies has revolutionised research capacity and its broad applicability over the past decade. These technologies have directly impacted researchers’ ability to understand host-pathogen interactions, holistically, and on a molecular scale. Recently, the avocado industry has begun to unravel some of the underlying mysteries that pertain to prevalent avocado pests and diseases; the Avocado Research Programme (ARP) has been highly influential in this space.

As a founding member of the Avocado Genome Consortium - an international collaborative effort which was established in 2016 - the ARP has directly contributed to the development of a high quality, chromosome-level reference genome assembly for Persea americana (avocado). The Avocado Genome Consortium has used this new reference genome to re-sequence more than 10 additional cultivars and rootstocks as part of an ongoing project, with plans to increase this number soon. Provided with this data, researchers will be able to accelerate the arduous process of selecting rootstocks and cultivars with desirable traits.

As part of the broader objectives of the Avocado Genome Consortium, transcriptomic data has also been generated. This data will be utilized to answer fundamental questions pertaining to the evolutionary biology, gene expression, physiological processes, and molecular pathways in avocado. Transcriptomic data from a dual RNA-sequencing experiment - involving both susceptible and partially resistant avocado rootstocks challenged with Phytophthora cinnamomi - was also used to identify avocado defence targets and discovery of pathogen effectors involved in disease development. The data from this work has been published across several research articles and will be used to further our understanding of the avocado-P. cinnamomi interaction.

 

The researchers involved are:

  • David Kuhn, USDA, Florida
  • Patricia Manosalva, UCR, California
  • Noëlani van den Berg, UP, South Africa
  • Antonio Javier Matas Arroyo, Departamento de Biología Vegetal, University of Malaga, Spain
  • Aureliano Bombarley Gomez, Virginia Tech Horticulture, USA
  • Randy Ploetz, University of Florida, USA
  • Alan Chambers, University of Florida, USA

 

ARP Team Members

 

Phytophthora root rot (PRR) is the most severe and damaging disease in avocado plantations in South Africa. The causal agent, Phytophthora cinnamomi, is an oomycete that also causes disease on thousands of other plant species. The pathogen is a heterothallic species that can reproduce by sexual or asexual means. It can persist in soil or infected plant material for extended periods of time, and there is no means to eradicate this oomycete from infected areas once the pathogen has successfully established in soils. Currently, phosphite injections, along with the use of good agricultural practices, are commonly used as methods of control for PRR.

Given the severity of the disease, however, little is known regarding the biology, population diversity, and the molecular basis for the P. cinnamomi - avocado interaction. Current research of ARP therefore focuses on characterizing the genetic diversity and monitoring pathogen populations using microsatellite markers, and using genomics and genetics tools to characterize effectors genes (RxLR and Crinkler) as well as other genes that are known to contribute to the pathogenicity of the oomycete. The sensitivity and effectiveness of phosphite control and its mode of action towards P. cinnamomi is also being investigated.

 

 * Read more about Phytophthora cinnamomi on our Fact sheet.

 

ARP Team Members

Aaron Maringa: Investigating the adhesion of the oomycete Phytophthora cinnamomi during the early infection of avocado.

Katelyn Baird: Characterising the role of Phytophthora cinnamomi NLP proteins in plant cell infection.

Ncobile Kunene: Development of a transformation system for Phytophthora cinnamomi.

Dr Sylvans Ochola: Establishment of Nicotiana benthamiana pathosystem for functional characterization of Phytophthora cinnamomi effectors and avocado R genes.

Kayla Midgley: Characterising the role of Phytophthora cinnamomi CRN effector proteins in host plant cell death.

Susanna Anbu: -Expression profiling of Nep-like protein (NLP) effector genes during infection in avocado.

Shalya Moodley: Assessing the virulence of dominant Phytophthora cinnamomi genotypes from South African avocado orchards.