Seed infection and latent infection of sunflowers by Plasmopara halstedii

Sunflowers bud-inoculated with Plasmopara halstedii became systemically infected with downy mildew and produced infected seeds at 15 to 30 °C. Oospores were observed in seeds of inoculated and also naturally infected plants from the field. Infected seeds gave rise to symptomless plants. Infected seeds proved effective as inoculum, inducing infection in 14 to 89% of the plants inoculated at 20 °C. Most of the infections (80%) were symptomless (latent).Systemic infection occurred, but latent infection was more frequent, in plants grown in soil containing debris of mildewed plants. Latent infection also occurred in uninoculated plants through belowground contact with systemically infected plants. Seed produced by plants with latent infection may carry the pathogen, and may give rise to more plants with latent infection, accounting for widespread dissemination of the disease before plants with typical symptoms are observed.