P&PDL Picture of the Week for
August 26, 2013

Sunflower Downy Mildew

Gail Ruhl, Sr. Plant Disease Diagnostician, Purdue University

This sunflower is infected with a soil-borne fungal-like disease known as downy mildew, caused by the pathogen, Plasmopara halstedii (Figure 1).

The visible yellowing (chlorosis) and mottling bordering the veins on the topside of the leaves (Figure 2) is what first draws one’s attention to infected plants. When the leaf is turned over, the typical signs of downy mildew, white cottony masses of fungal mycelium and spores (Figures 3, 4), are readily apparent, different from Powdery Mildew seen on the upper leaf surfaces of many other plants, such as peony (Figure 5).

Plasmopara halstedii survives in soil as thick-walled oospores, which are formed in root tissue shortly after infection. These oospores are very resistant to severe environmental conditions and can remain dormant in the soil for up to 10 years. Infection of plants is favored by cool, wet weather. Practicing good sanitation (removing diseased plant materials from planting beds) and crop rotation are considered the most efficient control for sunflower downy mildew. Fungicides are not considered effective for control of the foliar portion of this disease, and are not recommended.

BP-68-W Diseases of Landscape Plants: Downy Mildew (pdf file) - Purdue University Extension

Click image to enlarge

Figure 1. Sunflower leaf shows symptoms on top and signs underneath

Figure 2. Symptom of downy mildew infection - top of sunflower leaf

Figure 3. Signs of downy mildew infection - underside of sunflower leaf

Figure 4. Signs of downy mildew sporulation along veins on lower leaf surface of sunflower

Figure 5. Powdery mildew visible on the TOP of peony leaf

Purdue Plant & Pest Diagnostic Lab Purdue Cooperative Extension Service