Characterization of species associated with Pythium soft rot of ginger and evaluation of Pythium Oligandrum as a biocontrol
Amongst the 1,400 species within the Zingiberaceae, ginger (Zingiber officinale Roscoe) is believed to be the most commonly used spice for cooking, medicine and confectionery purposes. Following more than a thousand years of domestication and development in the East, ginger is now grown worldwide throughout the tropics and subtropics. Ginger has drawn great attention from scientists over the years for its antibacterial and antifungal chemicals; nonetheless, ginger is a host for many bacteria,
... or many bacteria, fungi, oomycetes and viruses. Among these phytopathogens, Pythium, an oomycete that causes Pythium soft rot (PSR) disease is of the most concern and is widely distributed throughout ginger growing regions. Losses due to PSR in pre-and post-harvested ginger are generally from 5-30%, but losses up to 100% have been recorded in India and Australia. Pythium myriotylum was believed to be the causal pathogen for the PRS disease outbreaks recorded on the two oldest ginger farms in Australia in 2007. However, P. myriotylum isolated from ginger exhibited differences in pathogenicity on ginger compared with P. myriotylum isolated from other crops in Australia. In addition, from 2007 onwards, PSR was observed on many other farms. A three-year-project (2012 to 2015) was initiated to assess the diversity within P. myriotylum occurring on Australian ginger farms as well as to study the presence of other Pythium spp. occurring on these ginger farms. The species status of P. myriotylum recovered from ginger with PSR symptoms in Queensland had been previously called into question since numerous PSR isolates exhibited morphological features which had been believed to be unique to P. zingiberis. Although the species status of P. myriotylum and P. zingiberis has been discussed, no comprehensive and comparative assessment has been made to date. International reference isolates of these two species, P. myriotylum CBS254.70 and P. zingiberis NBRC30817 were imported for exclusive re-examination along with samples of P. myriotylum from the collection of the University of Queensland. Since the reference isolate of P. zingiberis NBRC30817 showed minor divergence (less than 1%) in gene sequences (20 gene regions analysed) and very similar morphology, it was proposed to be reunited with the taxon P. myriotylum. The identification of P. myriotylum as a pathogen of PSR disease of ginger in Australia was confirmed. A collection of Pythium spp. (173 isolates) was isolated from diseased ginger rhizomes and soils collected around diseased ginger. Eleven distinct Pythium spp. were identified based on morphology and molecular sequences. Pathogenicity assays confirmed that P. myriotytlum isolates recovered from PSR ginger were pathogenic on ginger rhizomes and plants at a wide temperature range from 20 -35 °C. Pythium aphanidermatum was pathogenic on ginger at 30 -35 °C. Nine iii other species assessed were either non-pathogenic or pathogenic on the ginger rhizome only. Subsequent sequence assessments showed that P. myriotylum isolates from different farms were genetically uniform and had a host preference to ginger. Isolates of P. myriotylum derived from different hosts and origins showed differences in pathogenicity to ginger. Sequence diversity based on single nucleotide polymorphism (SNP) was investigated for these isolates of P. myriotylum. Overall, within 22 nuclear and mitochondrial genes studied, a higher number of true and unique SNPs were discovered in P. myriotylum isolates obtained from PSR ginger in Australia compared with other P. myriotylum isolates. Additionally, the use of the enzyme HinP1I in a PCR-RFLP of the CoxII gene successfully discriminated P. myriotylum isolates from PSR ginger from other P. myriotylum and Pythium spp. Using the same PCR-RFLP of the CoxII gene, P. myriotylum was also detected directly from PSR infected ginger; thus providing a potential diagnostic tool. The recovery of Pythium oligandrum, a well-known mycoparasitic oomycete, from soils around PSR ginger led to an investigation of antagonism and mycoparasitism of P. oligandrum against an isolate of the PSR pathogen, P. myriotylum Gin1. It was apparent that P. oligandrum did not produce substances toxic to P. myriotylum Gin1, but it did parasitize and compete against P. myriotylum Gin1 in vitro assays. Disease indices recorded in pathogenicity assays in Petri plates containing dual cultures of P. oligandrum and the target pathogen were reduced significantly compared with those recorded with single cultures of P. myriotylum Gin1. However, subsequent pot trials with co-inoculation of these two Pythium species showed no significant difference from control inoculations with P. myriotylum Gin1 only. The symptoms of soft rot on ginger were not only induced by Pythium, but were also found to be induced by Pythiogeton (Py.) ramosum, a less studied oomycete. This reports Py. ramosum as a pathogen of ginger occurred at high temperatures (30 -35 °C), and is the first record of its pathogenicity on ginger. It also was confirmed to be pathogenic on excised ginger, carrot and potato roots/tubers and other tested crop species in Petri plate assays. Py. ramosum also exhibited intraspecific variation within the ITS and CoxI genes. iv Declaration by author This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis.