Downy Mildew of Sunflower: Biology of Systemic Infection and the Nature of Resistance

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Abstract
Microscopic examination of fixed and stained serial sections of healthy and Plasmopara halstedii-infected (downy mildewed) sunflower (Helianthus annuus) seedlings revealed that zoospore encystment and infection occur primarily within or adjacent to the zone of elongation of the radicle. Up to 1000 infection sites/mm of root length were observed. No infection was observed on the hypocotyl or cotyledons. Following infection of epidermal cells, mycelium entered into intercellular spaces and grew toward the apical meristem through the loosely packed inner cortical tissue. Mycelium progressed from the radicle to the apical meristem in 6-11 days in seedlings infected at 3 days of age. In older seedlings, the rate of progression was slower. Zoospores adhered to the surface but failed to penetrate the roots of sunflower lines possessing the PL2 gene for downy mildew resistance; zoospores soon degenerated. Resistance apparently resulted from some physiological property or structural barrier that inhibited effective zoospore encystment or immediate post-encystment development.

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