Host–parasite relationships in dual cultures of Helianthus annuus and Plasmopara halstedii

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Abstract
The downy mildew fungus (Plasmopara halstedii) sporulated on callus tissue of sunflower (Helianthus annuus) in dual cultures on tissue culture medium. The fungus was most concentrated in the seven outermost layers of the callus. No haustoria were found inside the host cells. Healthy callus from both susceptible and resistant sunflowers was successfully inoculated with pieces of the dual cultures, giving rise to mycelium but no spores on the fresh callus. Dual cultures developed at comparable rates in both the compatible and incompatible combinations; the downy mildew resistance of intact plants was not expressed in dual cultures under the environmental conditions used.