Author
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Gulya Jr, Thomas |
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Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/5/2002 Publication Date: N/A Citation: N/A Interpretive Summary: The same fungus that causes "downy mildew" on sunflower is reported to infect many different genera of ornamental flowers and weeds in the "Composite" family. In very few instances, however, has anyone tried to see if the fungus causing mildew on these other plants able to infect sunflower. This summer, we found the weed marshelder with downy mildew symptoms. Spores taken from the weed were able to infect cultivated sunflower and cause typical mildew symptoms. This proves that common North Dakota weeds, such as marshelder and cocklebur, may serve as "reservoirs" for the downy mildew fungus which can then infect sunflower. Thus, weed control in fields and ditches is important in managing this sunflower disease, especially since the fungus has recently developed resistance to the chemical seed treatment (APRON) that has been the main protection for the sunflower crop. Technical Abstract: Marshelder (Iva xanthifolia Nutt.) plants were observed in May, 1999 in roadside ditches near Fargo, North Dakota with chlorotic upper leaves and abundant sporulation on the abaxial surface. Microscopic examination confirmed the spores were zoosporangia matching the size range of Plasmopara halstedii. Zoosporangia from marshelder plants were used to attempt sunflower infection by immersion of 3-day seedlings. Systemic chlorosis, stunting and sporulation were observed on sunflower plants 12 days later. Isolates from two individual marshelder plants were inoculated onto a standard set of nine sunflower downy mildew differential lines, and identified as race 3 (virulence pattern 700) and race 4 (virulence pattern 730). Both isolates were also characterized as metalaxyl-insensitive, using a soil drench inoculation method. While many Compositae genera are reported to be hosts for P. halstedii (1), this is only the third report of pathogenicity on sunflower of zoospores originating from other genera, the other two being Dimorphotheca (cape marigold)(2) and Xanthium (cocklebur)(3) The cross-infectivity of the P. halstedii from Iva and Xanthium suggests that Compositae weeds may serve as a reservoir of P. halstedii to infect cultivated sunflower. |
