Article on Edema:

Pests, Fungi and Bacteria are not always the cause of the damage found on the leaves of your plants. Edema occurs when a plant takes in more water than it is able to process and transpire. The excess water builds up inside the interior cells, causing them to swell and burst. This physiological disorder begins with the appearance of wet patches on the leaves, which then harden into a blistering scab like surface; often thought to be live scale, mite damage or a fungal disease. Oil based sprays like neem oil, pesticides or fungicides can actually cause Edema in some cases, and will only make it more difficult for your plant to release water vapor through its leaves. Identify live pests, eggs, fungus or bacteria before treating your plant with something that may actually make matters worse. Ant plants, Hoyas, Orchids, Aroids, Cacti, Succulents and many other kinds of plants can easily be affected by Edema in indoor or greenhouse environments. Allow your plant’s soil to dry out as much as it can reasonably tolerate before each watering, and only provide enough water to remoisten the soil. The Edema should stop affecting the new growth after the watering and environmental conditions have been properly adjusted. Repotting your plant with fresh media, and providing the proper pot size for its root system can also aid your plant in the recovery process. Avoid high humidity and ensure that your plant is receiving an adequate amount of air circulation. There are many different causes of Edema, so make sure that you are fertilizing your plant correctly based on the current conditions of its environment. Avoid removing leaves that are not severely affected, as the healthy parts of each leaf still go through the process of transpiration.