Stagonospora nodorum SN15 is a filamentous ascomycete that is a major pathogen of wheat and related cereals. It is also known as Septoria nodorum and the disease is variously called glume blotch and Septoria (or Stagonospora) nodorum blotch (not to be confused with Septoria leaf blotch which is caused by Mycosphaerella graminicola). The sexual stage is important in the field and the teleomorph is called Phaeosphaeria nodorum.
Stagonospora causes major losses in wheat crops. In Australia, losses of $57M pa are considered typical. Almost all these losses are concentrated in the Western Australian wheat belt. It is a major pathogen in most other wheat growing regions. In some areas, such as parts of eastern, it renders wheat an uneconomic crop.
Stagonospora is a member of the Dothideomycetes, a class of fungi that includes many important plant pathogens such as Leptosphaeria, Ascochyta, Pyrenophora, Cochliobolus, Alternaria and Mycosphaerella. It is the first Dothideomycete genome sequence to be publicly released.
The lifecycle of Stagonospora comprises both sexual and asexual phases. The asexual spores are released from pycnidia and rain-splashed to initiate new infection foci. The pycnidiospores germinate on the surface of the leaf and penetrate through stomata and directly through the epidermal cell walls. Cell wall penetrations are often associated with simple swollen hyphopodia. Once inside, hyphae grow through the leaf blade. After a few days and when the leaf cells have largely collapsed, pycnidia are formed to complete the life cycle.
The macroscopic symptoms are lens-shaped lesions in which brown pycnidia develop in appropriate conditions. Lesions often form in stem nodes, hence the name "nodorum". The most damaging aspect of the disease is infection of the head, leading to the glume blotch symptoms. Sexual development takes place on stubble and takes several months. The fungus is heterothallic. Ascospores are released from pseudothecia and spread on the wind. Seed-borne transmission is also important. Stagonospora can be easily cultured in vitro on simple defined media. Gene manipulation techniques including gene replacement have been developed and are relatively straightforward. However, sexual crossing in the laboratory is problematic.
Pathogenicity has been studied through the analysis of the effects of disrupting the expression of a number of genes. A picture is emerging whereby pathogenicity is seen to be a tightly regulated process in which the fungus battles the plant to maximise both the speed and quantity of sporulation.
Stagonospora nodorum SN15 is both an important pathogen in its own right and a model for the dothideomycetes. The aim of the genome sequencing is to provide the community with the tools to analyse and dissect the genetic basis of pathogenicity. The goal is to understand the function of genes controlling all aspects of pathogenicity. This will lead to novel control methods, based on improved understanding of the lifecycle of the pathogenicity, new targets for chemical control and methods of enhanced plant defence.
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