Secondary infection of Plasmopara halstedit was studied at several growth stages under controlled conditions or under netting cages in the field. Infection was conducted by placing fresh zoosporangia of race 100 on flower bud or on top of shoot with a micropipette. Percent of successful infection, symptom development and agronomic characters of seeds were observed macroscopically. Progression of fungus in organs of infected plants after inoculation was observed microscopically.

Sunflower plants naturally infected by downy mildew were observed at flowering and classified into four categories according to the intensity of symptoms. Capitula of infected plants were harvested for agronomic character analysis. These results were compared with those obtained with artificial infections.

Secondary infection can occur on leaf tissues or on the flower bud; and symptoms after artificial infection on 28- and 42- day-old plants correspond with symptoms observed in naturally infested sunflower at flowering.

The economic risks of secondary infection of P. halstedit can be divided into three categories: 1) loss of plants infected at the age of 7 and 14 days, 2) plants infected at the age of 28 days may produce no seed, 3) plants infected at the age of 35 or 42 days may produce a reduced quantity of contamined seeds.