Bacteria are single-celled organisms that lack a true cell nucleus and have a single chromosome instead. There are only a few pathogenic bacteria that attack trees.
The majority of forest pathogens are fungi, which generally belong to one of the following divisions: Basidiomycotina, Ascomycotina or Deuteromycotina.
Deuteromycotina reproduce asexually by producing conidia on conidiophores (Figure 1) or within special structures, such as pycnidia. Ascomycotina reproduce sexually by producing 4, 8, 16 or 32 ascospores inside sacs, or asci (Figure 2), within structures called ascomata. These ascomata may be cup-shaped (apothecia), bottle-shaped (perithecia), or balloon-shaped (cleistothecia).
Basidiomycotina reproduce sexually by forming basidia, which produce four basidiospores (Figure 3) on a structure called ascoma. The basidia develop in gills, pores, teeth or other structures. Rusts (Uredinales) are Basidiomycotina that function as obligate parasites and have a complex life cycle that generally requires an alternate host.