TOWARDS GUAVA WILT DISEASE RESISTANCE IN SOUTH AFRICA

Abstract

Soil-borne vascular wilt pathogens cause among the most devastating plant diseases worldwide. Guava wilt disease (GWD) caused by Nalanthamala psidii, resulted in the loss of more than half the guava production area in the Limpopo and Mpumalanga provinces of South Africa during the 1980s. Resistant guava selections were developed but renewed outbreaks of guava wilt disease in 2009, now also affecting the tolerant ARC-ITSC developed cultivar 'TS-G2', is placing the guava industry under threat once again. Alternative control measures are currently not available and host resistance remains the most logical choice for control. However, the lack of known sources of plant resistance and the emergence of additional pathogen races pose as major obstacles. Accurate identification and knowledge about the genetic variation within the pathogen population are therefore key aspects in the development of resistant guava cultivars. The aim of this study was firstly to determine the identity of 18 new fungal isolates obtained from infected 'TS-G2' trees. The internal transcribed spacer of the ribosomal RNA gene cluster (ITS), nuclear large ribosomal sub-unit (LSU), as well as the partial second largest subunit of the RNA polymerase II (rpb2) and the larger subunit of ATP citrate lyase (acl1) gene exons and introns of the new fungal isolates and reference isolates were sequenced and compared. Sequence analysis established the identity of the new isolates to be identical to N. psidii reference strain (CBS 439.89) previously isolated in South Africa, but differed from N. psidii stains from Malaysia and Taiwan. Although species diagnosis was possible, it was not possible to show geographic distribution- or determine pathogenicity relationships between isolates. The development of microsatellites or Simple Sequence Repeat (SSR) markers using high throughput sequencing was therefore investigated. A total of 15 645 SSRs were identified consisting mostly of tri- and tertra- nucleotide motifs. Selected SSRs will be used to facilitate further fungal population studies.