Forest and Shade Tree Pathology
Wilting is a symptom in which leaves and tender shoots lose their turgor, become flaccid and droopy. It may be followed by death of affected parts. Wilting be caused by many things: lack of water, root problems, a canker that killed a branch, etc.
But here we are talking about a specific group of diseases known as vascular wilts. They are characterized by disruption of water-conducting elements.
Here it will be useful for you to review the cell types in wood. All we'll cover here are hardwood diseases, so we're talking vessels and axial parenchyma mostly.
Mechanisms of wilting are many, mostly by plugging:
While the tree is alive, the fungus is restricted to vascular tissues. At this time, a good diagnostic symptom that can be seen by cutting into a branch is discolored brown areas along infected annual rings. For a ring-porous tree, like elm and oak, the outer ring is the only one that functions in water conduction and the only one that gets infected.
The disease is so named because it was described from the Netherlands, in 1921. It is not a disease of a Dutch elm tree, nor is it originally from the Netherlands.
It is, however, yet another introduced disease, first found around 1930's in the U.S. It is probably from Asia.
The pathogen is Ophiostoma (Ceratocystis) ulmi. Remember the blue-stain fungi? It is closely related, and the sexual stage is very similar.
Also like the Ophiostoma spp. that cause blue stain, this fungus is vectored by bark beetles. In the U.S. there are two beetles that do it, the native elm bark beetle and the European elm bark beetle. The European elm bark beetle was introduced before the pathogen.
The disease cycle can be hard to grasp the first time around. It's best to start with the beetle cycle by itself:
The beetle can do this life cycle without the fungus and get along fine. Now let's see how the fungus fits in:
The fungus has one other opportunity for spread. It can move through root grafts from one tree to the next.
This disease is one of the those that have been relatively big in the public eye because of the damage in cities and towns. Elms were the premier street tree in the USA in the early 20th century. They are big, have beautiful form, and tolerate poor soil conditions. The best neighborhoods often had gorgeous rows of mature elms. There was a lot of anguish and excitement when they began to die in large numbers. The disease is now nationwide.
Dutch elm disease is best controlled with DDT. Most pathologists agree on this and recommend it to municipalities. (Note: this is a joke ). DDT here stands for
Spraying to protect trees from feeding by beetles is sometimes done. Also, therapeutic injections of fungicides into infected trees can be done with high-value trees. But these are very expensive, cause tree damage around the injection holes, and certainly should not be the first line of defense. Another approach that may be effective if the disease is detected early is pruning:
See separate page on oak wilt.
Verticillium spp. are not related to the other wilt fungi. No sexual stage is known. Verticillium spp. have 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; it must be isolated. There are 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. It normally does not kill a tree rapidly.
The disease is best known on elms and maples. However, many tree species can get it, usually in landscapes. An internal symptom is discoloration of the xylem in streaks as with other wilts, but it can be greenish.
The 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 the current year's growth ring of xylem becomes infected, symptoms are acute. Death can result. If fungus gets near cambium, it can move from one year's wood to next. It can even kill the cambium, causing canker. On the canker, Nectria cinnabarina or Cytospora spp. may fruit. This is a good example of how some of the 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, which 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.
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