• Fungi are the most common plant disease agents.
  • Fungi are filamentous, non-green plants that are widely distributed in plants, soil and air.
  • The fruiting bodies they produce are called mushrooms, conks and molds.
  • Though most fungi are beneficial, malevolent varieties are the most destructive pathogens, causing cankers, stem rot, root rot, wilts and various leaf diseases.
• Bacterial agents are single-celled organisms that obtain food from organic matter.
  • Bacterial infections on plants cause blights, crown galls and wet wood infections.
• Viruses can only exist and reproduce in living cells and are parasitic.
  • Viruses are seldom fatal, but they will dwarf or weaken the plant to the point where other pathogens will kill it.
  • Most viruses are leaf or fruit based but some do attack stems and branch tips.
• Mycoplasmas share traits with both viruses and bacteria.
  • spread through insects (vectors) or through use of non-sterile pruning equipment
  • nebulous agents often block movement of organic material in the tree’s vascular system.
  • two main diseases caused by these agents are lethal yellows in palms and witches brooms on many species.
  • often fatal if not treated.

Tree diseases are commonly grouped based on the organ of the tree they attack:

Foliage diseases

• Foliage disease over time will weaken the defense system of a tree due to decreased food production.
• In shade trees, leaf diseases decrease suitability of the plant for use in urban areas.
• These diseases include leaf spots, anthracnose, blights, foliage rusts, needlecasts, mildew, mold and leaf blisters.

Stem diseases

• Stem diseases weaken both the vascular system and the structure the tree needs to support itself.
• Chronic stem diseases are almost always fatal, usually by the time they are discovered, the tree is already in serious decline.

Root diseases