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Type: Thesis
Title: Investigating the host range and origins of Phoma koolunga (Ascochyta blight of field pea)
Author: Keirnan, Elizabeth Carole
Issue Date: 2020
School/Discipline: School of Agriculture, Food and Wine
Abstract: Ascochyta blight (synonym: blackspot) is a serious, globally distributed, primarily foliar disease of Pisum sativum L. (field pea). It is typically caused by a combination of three or four fungal species that can exist independently of each other, called a complex. Phoma koolunga, identified in 2009 in South Australia, is the most recent addition in the Ascochyta complex. Despite multiple international studies on Ascochyta blight of field pea, P. koolunga has not been reported anywhere else in the world and the origins of the pathogen, and if it occurs on other legume species remain unknown. This study provides new information on the host range of P. koolunga on leguminous plants in controlled growth room conditions. To establish a host range, disease incidence and severity were assessed on 41 legume species comprising Australian native, weed, crop, pasture legumes and wild type Pisum, Lathyrus and Vicia species, following inoculation using two isolates of P. koolunga. All legumes tested, except Cicer arietinum (chickpea), developed leaf lesions and some also had stem and tendril lesions. Incidence and severity differed significantly among species but not consistently between isolates. The ability of the P. koolunga isolates to cause lesions on a wide range of legumes, including natives, in controlled environment conditions, suggests that it has a broad host range in humid and mild temperature conditions conducive for disease. Although all 17 native species developed some degree of leaf spotting, seven were considered susceptible because disease incidence was greater than 55 percent. This research also details the isolation, identification and classification of Didymellaceae fungi causing leaf spots, collected from legumes during field studies undertaken to investigate a possible native origin of P. koolunga. Samples from plants with leaf spots were collected from field pea growing regions throughout New South Wales, South Australia and Victoria taken back to the laboratory and cultured. The resultant fungal isolates were identified based on both morphology and phylogenetic analyses of the internal transcribed spacer region and part of the RNA polymerase II subunit B gene region. P. koolunga was not detected on native, weed or pasture legumes that had leaf spot symptoms in any of the regions visited, and only one isolate was recovered from field pea in the entire 2-year collection period. However, six novel species from the family Didymellaceae were isolated from Australian native legumes, five were from South Australia and one from New South Wales. The locations are represented by four different Australian Indigenous Peoples native language groups. Representatives of those groups were approached to request permission to use a suitable Aboriginal word for species epithet and permissions granted. These fungi are described here as Didymella djirangnandiri from Swainsona galegifolia, Didymella kaurna from Gastrolobium celsianum, Neodidymelliopsis tinkyukuku from Hardenbergia violaceae, Nothophoma garlbiwalawarda from Senna artemisioides, Nothophoma naiawu, and Nothophoma ngayawang also from S. artemisioides. Additional findings from the field collections were the identification of three new host-pathogen associations for Australia. Didymella pinodes, the primary pathogen responsible for Ascochyta blight of field pea, was isolated from leaf spots on naturalised species Vicia cracca (tufted vetch) in New South Wales and on Senna artemisioides from five different locations across South Australia. The discovery that these legumes may serve as an inoculum reservoir hosts for D. pinodes has implications for epidemiology and management of Ascochyta blight of field pea because both commonly occur in field pea growing regions throughout South Australia. Didymella lethalis was isolated from naturalised species, Lathyrus tingitanus (tangier pea), growing in a creek bed located in a well-used recreation area in Adelaide, South Australia. Phylogenetic analyses indicated that P. koolunga has a close relationship with the recently named species Ascochyta boeremae and supports the re-naming of P. koolunga as Ascochyta. Confirmation of the correction in nomenclature to Ascochyta koolunga comb. nov. was achieved with PCR followed by sequencing at two additional loci, the partial gene regions of ß-tubulin and the partial large subunit nrDNA (LSU). In summary, the controlled growth room results revealing a wide legume host range, and field collection results yielding no isolations from legumes other than field pea, suggest that P. koolunga is unlikely to have originated as a pathogen of Australian native legumes and provides no evidence regarding possible origins.
Advisor: Scott, Eileen
Davidson, Jenny
Dissertation Note: Thesis (MPhil) -- University of Adelaide, School of Agriculture, Food and Wine, 2020
Keywords: Pisum savitum
Australian native legumes
disease incidence
disease severity
alternative host
pathogen reservoir
multigene phylogeny
Provenance: This thesis is currently under Embargo and not available.
Appears in Collections:Research Theses

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