Check your plant for mites. If it doesn't have mites, it's almost certainly oedema.
Oedema is a physiological disorder that
can affect all plants. It occurs when the plant takes up more water than it can
rid itself of through the process of transpiration. In afflicted plants,
internal water pressure (turgidity) can become so high that cells rupture or
the cell contents leak into inter-cellular spaces in the leaf. Oedema is driven most often
by excessive water retention in the soil, but cool temperatures, high humidity
levels, low light conditions, or partial defoliation can individually or
collectively be secondary contributors, as can anything else that slows
transpiration. As well, nutritional deficiencies of Ca and Mg are known
contributors to the malady. Symptoms vary by plant, but can include wet or
weepy areas on the foliage, blisters that turn from white or tan to dark brown
or black, then gradually turn corky/ scaly/ ridged, and wart-like or
gall-like bumpy growth.
Al
Q