Which of the following is an example of a nematode disease affecting ornamentals?

The example of a nematode disease affecting ornamentals is indeed the root-knot of boxwood. This disease is caused by nematodes, specifically the root-knot nematode species, which invade the roots of the boxwood plant. As these nematodes feed, they create galls or knots along the roots, leading to reduced nutrient and water uptake, which can eventually cause stunting, yellowing, and decline of the plant.

In contrast, the other options pertain to diseases caused by fungi or environmental factors rather than nematodes. Powdery mildew and downy mildew are fungal diseases that affect the leaves, creating a powdery or downy appearance, respectively, and are not linked to nematodes. Leaf spot disease can arise from various pathogens and conditions but does not specifically involve nematode action. Thus, the correct identification of root-knot of boxwood as a nematode disease highlights the specific role that nematodes play in affecting plant health in ornamental horticulture.