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If your dogwood shrub is not planted in the right kind of soil, and/or has experienced lack of water recently, this will leave it more susceptible to disease. The shrub needs 1 inch of water per week during the growing season, either from natural rainfall or hand watering. Overwatering can cause decay or damage root growth, so be sure not to overwater. You can fertilize in Spring and Fall with fertilizer used for azaleas and camellias. Mulching also will prevent growth of weeds and help keep the right amount of moisture in the soil. You can spread 3-4 inches of organic mulch in an 8-10 foot radius, but be sure to keep mulch 3-4 inches away from the base of the shrubs.
It's possible that the shrub with the holey leaves has been affected by dogwood blight, which is a fungal disease that appears on the leaves as small spots with reddish-purple edges. This can kill an infected shrub in 2-3 years. Another disease that may have adversely affected your one shrub is Anthracnose; this can infect dogwood trees and shrubs in cool weather, and when there is increased moisture. It is best to plant flowering dogwood shrubs in an area which receives sufficient sunlight and also, good air circulation. When the weather is wet, diseases such as phytophthora leaf blight, powdery mildew and cankers, may infect flowering dogwood shrubs. If you also notice any dead tips on young twigs, these are from twig borers, so you may have an insect as well as a disease problem going on with this one shrub.
It might be best to replace the affected shrub with a new, healthy one, and make sure all other conditions are optimal to keep your shrubs healthy. Pruning/thinning out the shrubs to decrease density and increase air circulation is also recommended to avoid this type of problem.
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