basil downy mildew

Symptoms

Yellowing between veins, grey-purple fuzz on underside.

Easily confused with

• basil Fusarium wilt

  How to tell them apart: Fusarium oxysporum f.sp. basilici (Fusarium wilt) and Peronospora belbahrii (basil downy mildew) both start as leaf yellowing, but the location and partner signs differ. Fusarium yellowing is often one-sided on the plant, paired with sudden wilting, brown stem cankers, and — when you slit the stem lengthwise — dark brown streaks inside the vascular tissue; the underside of the leaf stays clean. Peronospora chlorosis is a diffuse, blocky yellowing on the upper leaf surface sharply hemmed in by the leaf veins, and turning the leaf over reveals a grey-to-purplish velvety fuzz on the underside, worst in humid, cool nights. Wilt with a clean leaf back means Fusarium; angular yellow patches with grey furry undersides mean downy mildew.