Investigation into the emerging soil borne disease of peanut – Neocosmospora root rot

Abstract

Neocosmospora root rot is an emerging soilborne disease of peanut (Arachis hypogaea L.) crops in Australia caused by Neocosmospora vasinfecta var. africana. The fungal pathogen was first identified in southern Queensland in 2005 following extensive damage to an irrigated peanut crop, and has since been found in all peanut growing areas in Queensland and New South Wales. Typical symptoms of the disease include extensive chlorosis and wilting of the plant starting from the main stem, and a blackened and decayed root system with reddish-orange perithecia of N. vasinfecta present on necrotic tissue of the main tap root and lateral root system as well as the pods. While the disease has been widely observed in agricultural production areas of Australia and is considered one of the most destructive pathogens of peanut, the biology, ecology and aetiology of N. vasinfecta is largely unknown therefore the biotic and abiotic factors that may influence the infectivity of the pathogen and severity of disease epidemics have not been identified. The purpose of this thesis is to investigate the biology, ecology and aetiology of N. vasinfecta var. africana and understand how these factors may contribute to the pathogenicity of the fungus.Due to limited knowledge of the biology of N. vasinfecta var. africana, a morphological and molecular approach was taken to determine the taxonomic identification and the morphological characteristics of the pathogen. The results of the morphological analysis of isolates collected from a range of peanut growing areas in Queensland identified the fungus to the species level as Neocosmospora vasinfecta. Further morphological analysis of the ascospores confirmed the characteristics of the ascospores to be consistent with the morphological description for the variety africana. Morphological analysis of isolates collected from peanut crops in Georgia were also consistent with N. vasinfecta however a MegaBlast search revealed the species variety to be vasinfecta which may account for the difference in pathogenicity seen between the two countries.Neocosmospora root rot has been identified in all peanut growing areas in Australia, however outbreaks of the disease have been sporadic and contained to isolated areas of peanut crops. An investigation into the behaviour of N. vasinfecta var. africana concluded that the fungus is likely to originate from foci of infected plants where soil inoculum is high and environmental conditions are conducive to infection, and is spread to neighbouring plants. N. vasinfecta var. africana has been identified as the most prevalent fungi isolated where plants have exhibited symptoms such as wilting, chlorosis, plant death and root decay, and therefore is considered to be a primary pathogen. There is the potential for N. vasinfecta var. africana to infect plants in conjunction with other pathogens such as peanut Tospovirus, which was evidenced by the presence of Tobacco Streak Virus (TSV) in the roots of peanut plants exhibiting symptoms of Neocosmospora root rot. Perithecia ...

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