  A devastating fungal disease--the leaf spot Entomosporium maculatum--has completely defoliated many Fraser photinias (often referred to as "redtips"). The Chinese form, Photinia serrulata, is resistant to that leaf spot but is susceptible to a number of other diseases.
 Chinese junipers ( Juniperus chinensis)--here 'Sea Green'--usually exhibit a rugged resistance that's not necessarily shared by other members of the juniper family. Shore junipers ( J. conferta) suffer from dieback. Creeping junipers ( J. horizontalis), widely used as groundcovers, are susceptible to blight and spider mites.
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by Marie Hofer, Gardening Editor, HGTV.com
Garden phlox is a staple of many perennial gardens, but in warm, humid climates, it's highly susceptible to mildew. The flowering dogwood (Cornus florida) is prey to a wide range of insects and diseases, including borers and anthracnose. The European white birch is susceptible to the bronze birch borer.
Lilacs, garden phlox, roses, crabapples, birch and dogwoods—beloved plants sometimes host a lot of problems. In many of our favorite plant families, some members are especially insect- and disease-prone. The good news, though, is that others are not.
For garden phlox, the resistant cultivars include 'David', 'Delta Snow', Natascha', 'Robert Poore', 'Speed Limit 45' and Phlox caroliniana. Certain dogwood cultivars like 'Cherokee Princess' and 'Pluribracteata' (the double-flowered form) that show excellent resistance to anthracnose; and there's always the Kousa dogwood (Cornus kousa), which resists most of the things that plague C. florida. White-barked birches are generally susceptible to borer, particularly if they encounter stress. The much touted Betula platyphylla 'Whitespire' birch is resistant, but only if the tree has been vegetatively propagated from the original resistant strain and not grown from seed. So you if you don't want to risk tangling with the borer, you could consider the river birch (Betula nigra), which offers multicolored, exfoliating bark.
Dogwood Trials
Choosing a Good Crabapple
Easy Roses
If you're shopping for new plants and you'd like them to stay as healthy as possible, consider the following:
Learn which trees, shrubs, vegetables, perennials, annuals and herbs are experiencing pest problems in your area. Which pests or diseases are the culprits? Do a little research before you go shopping. You won't find disease- and insect-resistance mentioned on most plant labels. Consult trusted sources—extension agents, books, online resources—to find the most resistant cultivars in a particular species. Consult local extension specialists and trusted nurserymen. A plant that's pest-prone in one part of the country may be very resistant in another, and visa versa. If you can't find resistant cultivars, consider another species. There are dozens of trees and shrubs that can thrive in a tough environment. Remember that if a plant is stressed for any reason--incompatible soil or light conditions, too much or too little water, etc.--it's going to be vulnerable for attack. The reverse is also true: Plants that are known to be susceptible to pests and disease can sometimes avoid them with extra coddling. 
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