People in the cactus society always ask about cactus scabbing. Cacti naturally become corky as they age but environmental factors can help accelerate the process.
Edema (oedema, corky scab) is a common, noninfectious disease of many herbaceous and woody plants. Edemaresults when the plant takes up more water through the roots than the leaves can transpire through the stomates. The excess water accumulates in the leaf cells, causing them to enlarge and often burst.
Symptoms vary depending upon the plant species or cultivar affected. Bumps, blisters, or water-soaked swellings may form on the underside of leaves, petioles, or stems. These blisters are at first small, about 1 to 2 mm in diameter. They then turn tan or brown and become corky. Severely affected leaves turn yellow and drop from the plant. On cacti and other plants, pale yellowish green spots form on the shoots. These spots may remain smooth, greenish white, or watersoaked. However, these spots often result in irregular corky or rusty areas that may later become sunken.The edema generally do little damage to plants, but detract considerably from the appearance plants.
Edema is best prevented by increasing the temperature and ventilation around the plant to increase moisture loss through the stomates. A rise in temperature of the air surrounding the plant will increase the rate of transpiration and thus help prevent engorged cells and blisters from forming. Increased ventilation also hastens transpiration by lowering the humidity at the leaf surface. Avoid cold drafts. Plants that are overwatered as well as plants that are properly watered but located in low light or cool areas are highly susceptible to this problem.