Which of the following is NOT a factor causing diseases in landscape plants?

Soil quality is not typically considered a direct factor causing diseases in landscape plants. Instead, soil quality refers to the health and suitability of the soil for supporting plant growth. Factors like its nutrient content, drainage capability, and structure influence plant health, but they do not directly cause diseases.

On the other hand, fungi, mycoplasmas, and parasitic higher plants are all pathogens or pests that can directly lead to disease in landscape plants. Fungi can cause various infections like root rot or leaf spots; mycoplasmas are known to be infectious agents that can cause a range of plant diseases; and parasitic higher plants, such as dodder or broomrape, attach themselves to host plants, extracting nutrients and often leading to their decline or death. Understanding the distinction between the underlying conditions that foster plant health and the entities that cause actual diseases can help in effective landscape management and plant care.