But here we are talking about a specific group of diseases known as vascular wilts. They are characterized by disruption of water-conducting elements.

Verticillium wilt

Verticillium spp. not related to the other wilt fungi. No sexual stage known. Verticillium has upright conidiophores with whorls of branches, each of which produces a cluster of conidia. It is microscopic; you won't see it on the host; have to isolate it. Two species but we won't worry about names and differences.

Verticillium wilt occurs on many agricultural crops as well as trees. It is a serious disease in agriculture, but in forests it is not as widespread and devastating as the other wilts. Does not kill a tree rapidly.

Best known on elms and maples. However, many tree species can get it, usually in landscapes. Get discoloration of the xylem in streaks as with other wilts, but it can be greenish.

Fungus infects through fine roots. It moves up by spores in the transpiration stream. Wherever they get stuck, they produce hyphae and more spores. It spreads out as it goes up. If current year ring is infected, symptoms are acute. Death can result. If fungus gets near cambium, it can move from one year's wood to next. Can even kill cambium, causing canker. On the canker, Nectria cinnabarina or Cytospora may fruit. Good example of how some of these weak canker pathogens cause difficulty in proper diagnosis.

There is no vector. When infected material dies and decays, the fungus converts its mycelium into very small sclerotia. These are black irregular bodies of knotted hyphae with thick-walled cells. They carry food reserves and are resistant to environment. Sclerotia can survive in soil for long time. When a root grows nearby, it can sense this, germinate, and infect the root.

There appears to be considerable variation in resistance, especially in maple.

Information provided by: http://www.forestpathology.org