There are several types of brown spots that can occur on the leaves of a Jade
plant. If the brown spots are soft, it is caused by leaf spot disease. To control it...water with a systemic fungicide and improve the ventilation. If the brown spots are dry, it is caused by underwatering. Jade
plants require generous amounts of water in the summer.
Sooty Mould is another spot that can occur. It is a black fungus which grows on the sticky honeydew depostited by aphids, scale, whitefly and mealybugs. This mould does not harm the
plant, but will reduce the growth and vigour. It will block the pores. To remove it wipe the leaves with a damp cloth and rinse with clean warm water. Make sure (if there are insects) you control them.