This is a picture of powdery mildew. Both downy and powdery mildew can develop a fungal growth on the lower leaf surface of infected plants. The difference between them:

• Downy mildew produces greyish purple fungal hyphae and black-to-purple sporangia on the lower leaf surface; the fungal growth corresponds to a water-soaked, yellow-to-necrotic lesion on the upper leaf surface.
• Powdery mildew growth is white, becoming off-white as it ages; it is present on both the upper and lower leaf surface; early symptoms appear on the lower leaf with a corresponding pale green to yellow discolouration on the upper leaf surface.