title: Comparison of the biology and host range of downy mildew pathogens : Peronospora somniferi and Peronospora meconopsidis of opium poppy
Creators: name: Krishnamoorthy, K
Publisher: name: University of Tasmania
Year: 2021

description: Opium poppy is an economically important crop belonging to the family Papaveraceae. It is primarily grown for its opiate alkaloid compounds, including codeine, morphine, thebaine and noscapine, which are used in the manufacture of pharmaceutical products. The Australian state of Tasmania contributes more than 50% of licit global opiate production. Downy mildew, caused by infections with one of two distinct Peronospora spp., is a major threat to Australian opium poppy cultivation. Peronospora meconopsidis has been associated with poppy downy mildew in Tasmania since the 1990s. Infection by this pathogen is characterised by the production of angular localised lesions and sparse sporulation on leaves of infected plants. In recent years, a second species of poppy downy mildew has been found in Tasmania, Peronospora somniferi. This pathogen produces systemic host infection, with symptoms of chlorosis and stunting with profuse sporulation. This thesis has the broad aim of identifying the similarities and differences in infection and disease development of these two pathogens. This information is needed for the development of disease management strategies for these two pathogens. The role of soil-borne oospores in the development of poppy downy mildew was examined for Pe. somniferi and Pe. meconopsidis. For both pathogens, oospores in infested soil were shown to initiate infection of host plant roots following germination and production of germ tubes and appressoria. This was initially asymptomatic both within and between cells before symptom expression was evident. Pathogen invasion from soil-borne infections was subsequently observed in leaf, root and stem tissues. The process of infection of both species were similar, although the rate and extent of symptom expression following inoculation with Pe. somniferi was faster than with Pe. meconopsidis. The study also identified the role of herbicide stress in stimulating expression of disease in asymptomatically infected plants. Foliar application of specific herbicides applied at sublethal levels to latently infected poppy plants stimulated more rapid expression of downy mildew symptoms reflecting what has been observed under commercial production. The role of sporangial inoculation of two pathogens in the development of poppy downy mildew symptoms was examined. The study evaluated sporangial germination and growth, appressorium formation and infection dynamics following sporangial inoculation to poppy leaves. Results showed that Pe. somniferi infective structures germinate, develop and invade the host faster than Pe. meconopsidis. Using a spectrophotometric assay, H\(_2\)0\(_2\) production was measured during foliar infection of both pathogens. The localisation of H\(_2\)0\(_2\) in the host leaf tissue was also visualised using a DAB staining method. H\(_2\)0\(_2\) production in opium poppy leaves was significantly higher following Pe. meconopsidis infection than with Pe. somniferi infections. This work suggests the host defence responses against Pe. meconopsidis may be more effective than Pe. somniferi. The results provided an insight into how Pe. somniferi is more aggressive than Pe. meconopsidis during the host-pathogen interaction. Finally, the experimental host range of Pe. somniferi and Pe. meconopsidis within selected weed (Pa. dubium) and ornamental members (Pa. nudicaule, Pa. orientale, Pa. commutatum, Pa. laciniatum, Pa. atlanticum Pa. rhoeas, Pa. paeoniflorum) of the Papaveraceae family was investigated. Closely related Papaveraceae members including weed and the ornamental species, (Meconopsis cambrica) and a Papaveraceae control (Pa. somniferum) were challenged with both pathogens using both foliar-applied sporangia and soil-borne oospore inocula. Other than Pa. atlanticum, considered as putative weak host, all tested plant species were susceptible to both pathogens although expression of symptoms varied between species. The seeds collected from plants infected with either pathogen from several of the species (Pa. somniferum, Pa. dubium, Pa. rhoeas and Pa. nudicaule) was shown to carry the respective pathogen indicating infested seed could be an important means for the global spread of these pathogens. These studies provide comparison and improve our understanding of the pathology of the two poppy downy mildew pathogens, Pe. somniferi and Pe. meconopsidis in opium poppy. While infection processes were similar for the two species, the rate of infection and symptom expression was faster for Pe. somniferi. This could have implications for disease management strategies, such as timing of fungicide applications. This study also identified alternative hosts that may be important as inoculum reservoirs or means of pathogen long distance spread.