The downy mildew fungus, Plasmopara halstedii (Farlow) Berlese & de Toni, was cultured axenically in association with sunflower roots derived from petioles inoculated with Agrobacterium rhizogenes. An axenic zoosporangial suspension, obtained from sporulating cotyledons, was added to A. rhizogenes-induced root pieces on agar or in liquid nutrient medium. Roots showed profuse P. halstedii sporulation after 1 week. A comparative study in infection morphology with excised sunflower roots (not induced by A. rhizogenes) of the same cultivar showed no difference, both of which resembled P. halstedii infection in roots from intact plants. The dual-member cultures with A. rhizogenes induced roots were a good source of axenic P. halstedii inoculum, and zoosporangia remained viable for up to 2 monhts in dual-member cultures maintained at 18–20 °C. Noteworthy features were the balanced state of host and parasite and the prolific production of P. halstedii oospores. Key words: downy mildew, sunflower, Plasmopara halstedii, Helianthus annuus, root culture, Agrobacterium rhizogenes.
Nonhost wild lettuce as a donor for resistance to downy mildew in cultivated lettuce
