Hosta leaves can turn yellow from frost, sunburn, heat stress and disease. Like people, trees and plants are not immune to disease, injury and harm from the elements. Hosta plants are no exception, and like many plants and trees that sport foliage, they express physical harm and stress through the condition of their leaves.
Extreme heat and cold can easily cause Hosta leaves to turn yellow, wither and die. In the event of a frost or sudden plunge in temperature, buds, stems, crowns and even entire plants and trees can suffer tremendous, sometimes irreversible, amounts of damage. When Hosta plants are exposed to frost, the cold damages the soft and actively growing tissues. This process is similar to frostbite in humans, and just as people may lose fingers and toes, the affected leaves often fall off. Hosta leaves are also prone to sun damage: yellowing leaves are often the first indications that Hostas are getting too much sun exposure. Sunburn often first affects the leaves on the top of the plant and in the direct path of sunlight; damaged leaves should be trimmed and the plant removed from direct sunlight until its leaves return to their normal color. Lastly, diseases such as Hosta virus X and infestation with nematodes and parasites can cause Hosta leaves to turn yellow as well.