Spots may be surrounded by a yellow halo. Yellowing, leaf drop and defoliation may occur in severe cases. The fungus can survive from year to year on certain weeds, so weed control is important. Leaf moisture from rains or dew increases disease severity. Fungicides may be used as recommended.
Many varieties are resistant.
Powdery mildew strikes older leaves, causing a yellow spotted appearance and a whitish-gray powder on the surface. The leaves eventually die but usually remain attached to the stem. The disease is worse under warm, dry conditions. Sulfur dusts or wettable sulfur sprays are effective preventative controls. Once powdery mildew is established, a labeled mildew fungicide will be needed for control.