Powdery Mildew

Powdery Mildew [Golovinomyces cichoracearum (syn. Erysiphe cichoracearum), Golovinomyces ambrosiae, Podosphaera xanthii]

Powdery mildew is a very common disease, but is seldom economically important. Disease occurrence is generally limited until bloom, but exceptions can occur. Commonly, by plant maturity powdery mildew is readily found in the majority of fields in North American production regions.

The fungus survives in resting structures (cleistothecia) or as mycelium on crop residues. Optimal conditions for infection include high humidity and moderate temperatures (68 to 81 F). Infection begins (and disease severity is the greatest) on leaves in the lower canopy, where conditions are most favorable. Powdery mildew is often first observed as small, discrete, powdery white tufts of fungal growth on the top side of leaf tissue (Figs. 1 and 2).

fig 1 powdery mildew — Figure 1. Small white fungal tufts of powdery mildew

fig 2 powdery mildew — Figure 2. Close up of white fungal tufts of powdery mildew

Under favorable conditions, the fungal growth will quickly expand and cover an entire leaf (Figs. 3 and 4) and may also occur on bracts or the back of the head.

fig 3 powdery mildew — Figure 3. Powdery mildew beginning to cover a sunflower leaf

fig 4 powdery mildew — Figure 4. Sunflower leaf completely covered with powdery mildew

Consistent with powdery mildew on other plants, the white fungal growth can easily be rubbed off with a finger revealing healthy to chlorotic tissue (Figs. 5 and 6).

fig 5 powdery mildew — Figure 5. Powdery mildew on cucurbit leaf, before white fungal growth is rubbed off

fig 6 powdery mildew — Figure 6. Powdery mildew on cucurbit leaf, with white fungal growth rubbed off (‘NDSU’) to reveal chlorotic to green tissue

Small pepper flake-sized black structures (cleistothecia) may be produced within the white fungal growth in severely infected leaves. Importantly, the fungal growth only occurs on the upper-side of the older leaves, distinguishing the disease from downy mildew, where the white fungal growth is only observed on the underside of the leaf tissue.

Active management specific to powdery mildew is generally not needed in the Northern Great Plains, however, crop rotation, selection of a less susceptibility sunflower hybrid and foliar fungicide application may be recommended in other production regions.

Images

Figure 1. Small white fungal tufts of powdery mildew (Sam Markell, NDSU).
Figure 2. Close up of white fungal tufts of powdery mildew (Sam Markell, NDSU).
Figure 3. Powdery mildew beginning to cover a sunflower leaf (Sam Markell, NDSU).
Figure 4. Sunflower leaf completely covered with powdery mildew (Sam Markell, NDSU).
Figure 5. Powdery mildew on cucurbit leaf, before white fungal growth is rubbed off (Sam Markell, NDSU).
Figure 6. Powdery mildew on cucurbit leaf, with white fungal growth rubbed off (‘NDSU’) to reveal chlorotic to green tissue (Sam Markell, NDSU).

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Disclaimer statements

Information based in part on and reproduced from Kandel, H., Endres, G. and Buetow, R. 2020. Sunflower Production Guide. North Dakota Extension Publication A1995. Informational updates made possible by the Sunflower Pathology Working Group, and is/was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2018-70006-28883).