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

Root-knot of boxwood is an example of a nematode disease affecting ornamentals because it is specifically caused by plant-parasitic nematodes, such as Meloidogyne spp. These nematodes invade the root system of the boxwood, leading to the formation of galls or 'root knots' that disrupt normal growth and nutrient uptake. This condition can result in significant damage to the plant, including stunted growth, yellowing leaves, and even plant death if left untreated.

In contrast, the other options listed involve different pathogens or conditions. Powdery mildew is a fungal disease that typically affects the foliage of plants and does not involve nematodes. Leaf spot disease can result from fungi or bacteria that cause discoloration and decay of leaves, but again does not involve nematodes. Black rot is primarily associated with certain plant diseases caused by bacterial pathogens, particularly in crops like grapes, and does not relate to nematode infestations. Thus, root-knot of boxwood is distinguished as the only option directly involving nematode pathogens affecting ornamentals.