Lavender is a perennial shrub that flowers in the summer, producing dark purple to pale blue flower spikes. Some lavender varieties will produce white, green and pink flower spikes. It thrives in full-sun desert conditions with rocky and alkaline soils and only needs occasional watering once rooted. While lavender is a hardy plant, it can pick up fungal, bacterial and viral diseases that will turn its leaves yellow.
Alfalfa Mosaic Virus
Aphids that infect lavender bushes carry Alfalfa Mosaic Virus. It is one of the most common lavender diseases and causes the leaves and new growth to become blotchy with yellow patches, often causing malformations. It usually will won't kill the plant, but it must be uprooted and burnt to keep AMV from spreading to other lavender plants. Also, disinfect gardening tools with a one part bleach to two parts water solution to prevent spreading AMV.
Lavender Leaf Spot
Lavender plants in high humidity and wet conditions such as a greenhouse or in a wet climate may develop lavender leaf spot. The disease starts with yellow spots on the leaves of the plant and quickly turns them yellow. The leaves may actually drop from the lavender bush, but the plants typically do not die from lavender leaf spot if the humid and wet conditions are corrected when first signs of the disease appear.
Shab is a fungal disease that attacks European lavender plants. The lavender plant wilts with all parts turning yellow, and the new growth on the plant dies off first. Eventually, the whole plant will die if infected with Shab. Shab does not affect "L. angustifolia" lavender cultivars. "L. x intermedia" lavender cultivars are more resistant to Shab when they are planted in well-drained soil with full sun and room for full ventilation.
Yellow Decline is another European lavender disease that affects the leaves of the lavender bush. The disease is caused by a mycoplasm bacterial infection spread by leaf hoppers. Yellow decline cause large yellow blotches on the leaves of the lavender bush and can wipe out whole lavender harvests. One cultivar of lavender, "L. x intermedia Grosso," is not affected by yellow decline and is one of the most commonly grown lavender cultivars in the world.