Here it will be useful for you to review the cell types in wood. Remember that water (xylem sap) is conducted in vessels (hardwoods) and tracheids (conifers).
Wilting is a symptom in which leaves and tender shoots lose their turgor, becoming flaccid and droopy. It may be followed by death of affected parts. Wilting can 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. The disease type is defined variously:
- True vascular wilt: a disease in which the pathogen moves systemically throughout the host in the nonliving vessels without directly attacking parenchyma cells during the early stages of disease .
- Wilts occur as a result of the pathogen in the xylem vessels of the plant. As long as the infected plant is alive, the wilt-causing fungi remain in the vascular (xylem) tissues and a few surrounding cells .
- Wilt is characterized by simultaneous death of part or all of a stem accompanied by wilting foliage. It results from a fungus reducing or inhibiting water conduction .
- Disruption of xylem vessels by wilt pathogens reduces the capacity of the vessels to translocate water from the roots to the top of the transpiring tree. During hot, dry periods, insufficient water is translocated to the leaves, causing them to wilt and die .
As conifers do not have vessels, most of these definitions would technically exclude black-stain root disease, a conifer pathogen which is usually considered a vascular wilt. It is caused by Leptographium wageneri, which meets criteria for a wilt pathogen :
- it colonizes only the actively conducting xylem tracheids, at least while the tree remains alive and reasonably functional,
- it penetrates from cell to cell through bordered pit pairs and does not decompose or penetrate mature cell walls,
- it induces an occasional tylosis (next paragraph) which protrudes into the tracheid from an adjacent parenchyma cell,
- it has large, sheathed hyphae that often almost fill the tracheid lumina,
- at least in seedlings and small saplings, it becomes systemic even to the point of invading the xylem of juvenile needles.
Mechanisms of wilting are many, mostly by plugging:
- plugging by mycelium and spores
- plugging by polysaccharides produced by fungus
- plugging by gels, gums from cell wall breakdown
- parenchyma cells divide, compressing vessels
- parenchyma cells may produce tyloses (ballooning projections into neighboring vessels; may help to stop pathogen invasion but also blocks water flow)
- toxins disrupt parenchyma cells, which disrupts vessels, or they affect leaves, increasing water loss.
While the tree is alive, the fungus is restricted to vascular tissues. At this time, a good diagnostic symptom, though not always present, can be seen by cutting into a branch or twig: 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 fully in water conduction and the only one that gets infected.
Dutch elm disease
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 (see right). The beetle can do this life cycle without the fungus and get along fine. Now study the second cycle to see how the fungus and disease fit in.
Do you see what a wonderful sick relationship the fungus and insects have? By inoculating healthy trees with the fungus, the insect assures that there will be new dying trees for the population to breed in, while also helping the fungus to disperse and find new trees to colonize. The only partner who doesn’t enjoy benefits here is the elm 🙁 .
The fungus has another 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:
- Detection: Use continuous surveillance and communication to detect any dying and dead elms immediately.
- Destruction: Remove such trees and immediately burn or bury the wood. The point is to prevent it from serving as breeding material for the beetles. If there is no vector, there is no disease. Of course this won’t be completely effective if there are significant natural stands of elms in the area.
- Tree resistance: There are new varieties of elm, in most cases hybrids with Asian elms, that are more resistant and are acceptable in other respects. They should be planted. Elm is still a good street tree.
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 should not be the first line of defense. Another approach that may be effective if the disease is detected early is pruning out the infection  (see one of the How-to’s).
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 it gets near the cambium, it can move from one year’s wood to the 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 diagnosis.
There is no vector. When infected material dies and decays, the fungus converts its mycelium into very small, hardy sclerotia, which carry food reserves and are resistant to environmental problems. Sclerotia can survive in soil for long time. When a root grows nearby, it can sense this, germinate, and infect the root. Isn’t that amazing? What a fungus!
There appears to be considerable variation in resistance, especially in maple.
Bacterial leaf scorch
Numerous leaf scorch diseases are caused by Xylella species, also known as fastidious, xylem-limited bacteria (FXLB) and rickettsia-like bacteria. In the xylem vessels they produce gummy polysaccharides that tend to block water flow, so these are wilt diseases. They are covered in more depth in the page on bacterial diseases.
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