Brocolli leaves deformed from underneath, white powdery substance

by Alison
(Cape Town)

My broccoli leaves have got infested with a strange white substance.

The leaves look lumpy and bubbly when viewed from above (see image), and underneath they have this white powdery stuff in clumps under the bumps.

Any idea what it is? I thought of a form of powdery mildew, it's nothing like the usual mildew that afflicts my tomato and squash plants!

The prevention? The cure? My brocolli seems now reluctant to make stalks and produce - starting to look more like spitz cabbages!