Crassula ovata, commonly known as Jade Plant, is a succulent often used as a houseplant. This well-behaved plant blends into the landscape in USDA hardiness zones 9 through 11, where it thrives on neglect. Few pests or diseases affect Jade Plant, but when white powdery mold appears on its leaves, it is often the result of environmental problems. If you have white mold on your Jade Plant, you are more likely dealing with powdery mildew. True to their easy-care nature, though, Jade Plants bounce back from powdery mildew once it is properly treated.
Powdery mildew is caused by several different fungi that grow in a thin layer on plant tissues. In perennial plants such as Jade Plant, the fungal bodies can overwinter in buds, reemerging when conditions are ideal for their growth. Spores are often distributed by the wind to new, uninfected plants or spread in greenhouses from plant to plant. Temperatures between 60 and 80 °F (16 and 27 °C) are required for germination, along with relative humidity above 90 percent. Standing water on leaves is unnecessary and can inhibit spore germination in some cases. Standing water encourages other fungal growth, so it should be discouraged.
Powdery mildew begins as a white powdery coating on the plant's leaf surfaces but soon spreads, penetrating plant tissues and sending out spores. Emerging vegetation may be dwarfed or distorted and covered in white mildew. Jade Plants also develop corky brown lesions where powdery mildew is taking hold. If not addressed, this mildew will spread farther, resulting in leaf drop and the Jade Plant's eventual death.
Cultural control is effective for powdery mildew. Plant Jade Plants in full sun and trim them to encourage better air circulation. Reduce watering if the plant's soil is often moist. Always allow the soil to dry before watering Jade Plants. Powdery mildew spores can be killed with water, but be careful to wet the plant early in the day to dry completely. Stop misting house Jade Plants instead of watering them at the plant's base since humidity around the plant encourages powdery mildew growth.
Several chemicals are available for chronically infested plants or those that are victims of environmental conditions beyond the gardener's control. Test a safer pesticide such as horticultural or neem oil on a small spot on your Jade Plant before spraying the entire plant thoroughly. If an outside Jade Plant yellows or shows signs of sunburn, utilize a sunshade until signs of powdery mildew are gone to prevent serious damage. Never apply horticultural oils when temperatures are above 90 °F (32 °C).
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