Disease Types

Diseases are classified in various ways.

  • By cause.  Here we first recognize abiotic diseases (also called noninfectious or nonparasitic diseases), caused by nonliving agents.  Those are distinguished from biotic (infectious or parasitic) diseases, which can be subdivided by pathogen groups.
  • By part of the tree affected.  Here we can consider root diseases, stem diseases, foliage diseases, etc.  Stem diseases can be further divided into those affecting wood vs. phloem and cambium.
  • By host.  In some cases it is convenient to group diseases by host, so pine diseases, poplar diseases, etc.
  • By maturity or management scheme.  Here we think of nursery diseases, sapling diseases, plantation diseases, and those of mature trees.

We will use a hybrid of the first two approaches, generally basing the classification on the part of the tree affected, but separating out some important or unique causal agents, including parasitic plants, rust fungi, and bacteria and viruses.

Merrill’s Type Diseases

Dr. William Merrill Jr. (1933-2003) was a forest pathologist at Pennsylvania State University.  Sometimes known as “Wild Bill”, he was an independent thinker and a passionate iconoclast who was dedicated to teaching and inspired many students to go into plant pathology.

Among his many contributions is the development of the concept of type-diseases ​[1]​.  He noted that, unlike physicians and veterinarians, a plant pathologist is faced with 100 or so diseases each of thousands of species of plants.  Rather than study and memorize the basic facts of all these diseases, we must group them into types.  If we learn the type-diseases well, we can tackle most diseases that we run into.

Within a type-disease, the specific diseases are caused by similar types of pathogens with similar life cycles, have similar symptoms, and are affected similarly by environmental conditions.  Thus, he focused on the most important type-diseases, recognizing that some diseases are unique and don’t fit into any general type.

Here are many of Merrill’s type-diseases.  Some types have subtypes.

  • Damping-off;
  • Rots of succulent roots
  • Seedling blights, root/crown rots
  • Nematode diseases
  • Mycorrhizae (!)
  • Fungal vascular wilts
  • Stem cankers
    • Perennial target cankers
    • Annual cankers
    • Diffuse cankers
  • Foliage diseases
    • Leaf curls and blisters
    • Powdery mildews
    • Sooty molds
    • Brown felts
    • Snow molds
    • Tar spots
    • Fungal leaf spots
    • Anthracnose/leaf blights
    • Twig and stem diebacks
  • Rust diseases
    • Foliar rusts
    • Gall rusts
    • Stem canker rusts
  • Rots of woody roots
  • Discoloration and decay
  • Phytoplasma diseases (yellows)
  • Virus diseases
  • Parasitic plants

We deal with many of those type diseases on this website, although to help organize them we have the general types mentioned in the previous section.

Disease type pages on forestpathology.org


  1. 1.  Merrill W. 1980. Theory and Concepts of Plant Pathology (published class notes). University Park, Pennsylvania: Department of Plant Pathology, Pennsylvania State University. 515 pp.