What causes diseases of landscape plants?

Diseases of landscape plants are primarily caused by a range of biological agents, which is why the answer focuses on fungi, bacteria, viruses, nematodes, mycoplasmas, and parasitic higher plants. These organisms interact with their host plants in ways that can disrupt normal physiological processes, leading to various symptoms such as wilting, leaf spots, and overall decline.

Fungi are particularly notorious for causing diseases like root rot and powdery mildew; they thrive in moist conditions and can rapidly spread. Bacteria can initiate gumming, leaf spot, and blight diseases, affecting the plant's health and productivity. Viruses often result in stunted growth, discoloration, and can be transmitted by insect vectors or through infected plant materials. Nematodes, which are microscopic worms, can invade root systems, affecting nutrient uptake. Mycoplasmas are also significant pathogens that can cause a range of symptoms, typically associated with vector insects. Lastly, parasitic higher plants, such as mistletoe, can siphon nutrients from host trees and lead to their decline.

While soil quality issues and weather conditions may influence plant health and can contribute to stress or susceptibility to diseases, they are not the primary causative agents of plant diseases themselves. Understanding