Hibiscus syriacus , Shrub Althea at UDBG Hibiscus spp. Open flowers and buds may drop suddenly when one moisture extreme follows another. For example, drought stress followed by adequate moisture may cause such drop. Other extremes in growing conditions may also contribute. Avoid drought stress.
Circular spots occur on leaves which results in their shedding from the plant.
Use appropriate foliar fungicides to prevent damage. Leaves are small and the plants fail to make normal growth. Use caution when planting to be sure that planting stock is not infested. If yard soil is contaminated, plant shrub in a large container of sterilized soil. This is probably the most serious problem experienced by althea.