Downy Mildew of Snapdragon

Downy mildew of snapdragon is a common disease that has the potential to be highly destructive. The disease is caused by the fungus Peronospora antirrhini. The disease is more of a problem when the snapdragon plants are in the seedling stage, however, it can also be a problem in older plants especially on the new growth. The disease is most prevalent during periods of cool temperatures and high humidity. The fungus is spread from plant to plant by spores. Spore production is at it's peak when greenhouse temperatures are cool (50-60 degrees F).

Infected plants are stunted, yellow, and have a sickly appearance overall. The underside of the leaves of the affected plants are covered with brownish to purplish fungal spores masses Figure 1, and Figure 2. The growing point of the plants may also be infected. The fungus can be distinguished from powdery mildew by color and the fact that 90 % of the time it first appears on the underside of the leaf. Powdery mildew is usually white and is on the upper surface of the leaf.