thick leaves

My brother's tomato plants have very thick leaves - they don't look like leaves at all. There are no insects visible.

It is only plants from his seeds from last year's tomatoes, that are affected. Tomato plants that I gave him are fine, as are different vegetables, like lettuce and chard, growing nearby.

He has grown tomatoes using his own seeds for many years and has never had problems, except for late blight some years. He does not ferment his seeds - just dries them straight out of the tomato. Last year's tomatoes had no diseases.