Dogwood and boxwood are two mainstays of American gardens: eastern flowering dogwood and Pacific dogwood trees offer the beauty of their large white bracts, and boxwood shrubs offer stately versatile evergreen foliage with resistance to deer browse. The popular bedding plant impatiens has provided spreading masses of color in shaded landscapes for decades, particularly since new hybrids were introduced in the 1960s. Each of these plants has been a beloved ornamental, contributing to societal well-being as well as serving as a source of profits for the nursery-greenhouse industry. Toward the end of the 20th century, native dogwood species on both coasts of North America were threatened by an introduced fungus, while at the beginning of the 21st century boxwood encountered an introduced fungus, and impatiens an introduced oomycete, a downy mildew. The ability of each of these plants to provide beauty in the landscape was destroyed by the long-distance movement of a pathogen from a still unknown geographical location. In the case of the two native dogwoods, prior to the introduction of their new fungal pathogen, the trees were supplying ecosystem contributions in woodlands in addition to having an important role in the ornamentals industry. Although global plant movement is regulated, pathogens are microscopic, often internal, and easily overlooked, especially when introduced on a host of low susceptibility that is relatively symptom-free. This dissertation includes an introductory chapter (Chapter 1) that describes the introduction of three new pathogens into the United States: the fungi Discula destructiva and Calonectria pseudonaviculata and the oomycete Plasmopara destructor. These pathogens have had extraordinary impact on dogwood, boxwood and impatiens, respectively. I contributed to developing a proper understanding of each of these new diseases, working closely with others at botanical gardens, plant breeding companies, USDA, and other universities to conduct the needed research and extension. My contributions have been primarily in the areas of diagnosis and management—in particular, I remain keenly interested in solutions to these problems that will come through ever-improving plant breeding and selection. This first chapter provides a current perspective on these diseases, building upon the stories told in the published articles which constitute Chapters 2, 3 and 4 of the dissertation. Chapter 2 is a co-authored invited review article from 1994, which covers dogwood anthracnose from its first appearance on both American coasts until the review’s publication. Craig R. Hibben and I collaborated closely on field, laboratory, and plot studies from the beginning of the discovery of dogwood anthracnose in New York, which was less than 20 years before the article was written. Chapter 3 is an invited review article on boxwood blight prepared more recently, in 2019, in which I covered all aspects of the disease from the literature available and my own experience. Chapter 4 deals with the impatiens downy mildew and is a very different type of publication from the others. It is a synergistic collaboration of five scientists from three states that covered outbreaks of disease in the production industry in 2019 and 2020 that occurred on disease-resistant impatiens. Our goal was to make the flower industry aware that high disease resistance is not sufficient to prevent disease losses when environment and other circumstances strongly favor downy mildew development. Having conducted trials that repeatedly demonstrated the superiority of the new lines of impatiens, it was important to me that the limits of their resistance to downy mildew also be properly understood. In addition to providing more recent information than offered by the earlier published articles that make up Chapters 2, 3 and 4, Chapter 1 includes my interpretation of what we have learned from these three new diseases of ornamental plants. One obvious conclusion is that introduction of new pathogens of ornamentals is not a rare event, so ornamentals plant pathologists should be ever-vigilant.