THE SUNFLOWER - PLASMOPARA HALSTEDII PATHOSYSTEM: NATURAL ANDARTIFICIALLY INDUCED COEVOLUTION
Department of Plant Protection, Godollo University of Agricultural Sciences, 2103 Godollo,
Background and objectives
Downy mildew, incited by Piasmopara haistedii, is one of the major diseases of cultivated sunflower worldwide. Since both the fungus and its main host are assumed to have originated in the central portion of North America , they are likely underwent a coevolution in the past. In fact, open-pollinated sunflower cultivars released from the late 1960s highly supported fungus spread wherever the crop was grown. Though breeders have produced a number of resistant cultivars, soon after their introduction new virulent forms (pathotypes) of P. haistedii appeared . In the recent years, efforts are being continued to detect new sources of resistance in order to combat with the more diversity in virulence occurring within local fungus populations in many countries.
Materials and methods
Field isolates of P. haistedii collected from either cultivated or volunteer sunflowers and from Xanthium strumarium were the subject of host range studies on a series of annual and perennial Helianthus species, as well as on other members of the Asteraceae. Tests for virulence and aggressiveness were carried out by using a methodology described earlier .
Results and conclusions
The Xanthium isolates hardly infected sunflower at first but they were as pathogenic as those from cultivated sunflower after a few reinoculation (subculturing). Besides Helianthus annuus, five additional Heiianthus species (H. argophylius, H. debilis, H. divaricatus, H. grosseseratus, H. petiolaris), as well as another composite, Artemisia vulgaris have successfully been inoculated, although sporulation was restricted to cotyledon leaves only. Over a period of eight years (1989-1996) six pathotypes of P. haistedii, each with distinct virulence character have been identified, the majority from Hungary but some received from Bulgaria, Italy, Romania and Yugoslavia. Some of our single spore isolates obtained from a particular pathotype differed in their virulence character as compared to the parent isolate, with no respect the isolate has been deep-frozen or regularly subcultured on susceptible sunflower prior to virulence test.
Our results support the idea that Piasmopara haistedii has been evolved in several steps: started as a complex biological species living on a wide range of composites, including Helianthus annuus, it became a severe pathogen of cultivated sunflower and, with the release of resistant cultivars, the fungus was induced to become more diverse in its virulence performance.
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