- Disease Cycle
- Host Specialization
- Root and butt rots
- Cortical Root Rots
- Vascular Wilt
We’ll go through that with each disease, as it varies slightly. But one thing you should keep clearly in mind: in addition to airborne dispersal by spores (which most of them can do), they can usually grow as mycelium from one tree to another, across root contacts or grafts. So generally there are two kinds of inoculum: airborne (spores) and soilborne (infested stumps and root systems).
This is a fascinating aspect of root pathogens, especially root- and butt-rot fungi, that we have only learned in the last 10-20 years. In many of the major pathogens, we have been finding that there are host-specialized groups within what has been considered a single, uniform species. In most cases these groups are morphologically very similar. But there are different populations, which often cannot interbreed, on different host groups. The details vary with disease, so we’ll consider it as we discuss examples. Keep this phenomenon in mind and try to imagine explanations in terms of biology, ecology, genetics, etc.
Root and Butt Rots
These are wood-decay diseases, and overlap with stem decays. Most are caused by Basidiomycota. They may get in through wounds in the lower part of the tree or penetrate roots directly. They involve the roots and in many cases the butt also.
They may be found killing cambial tissues and/or growing in the inner wood:
- For cases where the fungus kills cambium at the root collar, the tree will die when the root collar is girdled (killed all the way round). This is quickly lethal.
- In other cases, the fungus decays the inner wood at the base of the stem (heartwood). This is called butt rot. A tree can live with butt rot for many years, just as it can with stem decay in the main stem. Very often the tree dies because it is uprooted or snapped (physical failure) rather than direct mortality from killing tissues.
Imagine you are in the forest and come upon an opening. You observe:
- Some old rotten stumps or broken old snags (standing dead tree) in the center.
- Some trees uprooted or snapped at the base.
- Some recent mortality near the margin.
- Live trees at margin that look sick.
You have just entered
the twilight zone a mortality center due to root and butt rot. How do you know?
How do you diagnose root rot? How do you distinguish a tree that went down from root rot vs. windthrow? Think of your observations as you stand among the trees. First, if there is a mortality center, look for evidence of a chronosequence of mortality (i.e., trees died long ago near the center and more recently as you approach the margin). Centers of mortality from bark beetles, lightning, wind, etc. won’t have that. Look for trees that have snapped at about stump or soil height or uprooted due to root and butt rot. The center may be a few trees or a few hectares.
Then, whether there is a center or not (there often isn’t!), look at the uprooted ones closely. Wood that was sound when it broke has jagged, splintery breaks, whereas wood that was decayed when it broke has “brash” breaks, where failure is across the grain and less splintery. This difference is less noticeable in hardwoods, which have relatively short fibers and therefore less splintery breaks of sound wood.
|Chronosequence of mortality.||Simultaneous mortality (blowdown, patch kill by bark beetles, lightning).|
|Butt-rotted trees have evidence of stem or root failure. Lower stem snaps or roots fail near root collar.||Wind-thrown trees don’t usually have failure of main roots or stem. Instead the soil fails, and some usually adheres to the roots. Snapping of sound stems near the base is rare.|
|Failures of roots or snapped stems have evidence of decay on the broken surface (brash failure, breaks easily across the grain), even long after the failure and after subsequent decay occurs.||Wood that does break is sound on the broken surfaces, leaving evidence of a splintery fracture surface.|
|In a mortality center due to root and butt rot, downed trees usually point in all directions like “pick-up-sticks.”||When trees are windthrown in a group (blowdown), they are usually oriented in more or less the same direction.|
For confirmation, fell a dying or recently killed tree, or cut a fallen tree, and look for decay. You may need to excavate some roots and cut into them.
A useful tool for investigating root diseases is the pulaski. The head has a chopping blade on one side and a narrow mattock (hoe) on the other. One can scrape away soil and chop into roots with a single tool. Although versions of it were apparently invented earlier, it was largely developed by Edward Pulaski in the early 20th century.
A long-time Forest Service employee, he designed it for firefighting. In his blacksmith workshop, he started with a tree-planting tool but essentially developed the idea of a double-bladed axe with one blade twisted around. The full story of the invention is here (see page 19). But Pulaski is even more famous for saving the lives of a large crew of firefighters during an incredibly severe firestorm in Idaho, the story of which you can read here, or better yet in Pulaski’s own words.
Bark beetles: If you find bark beetles or ecologically similar insects in a tree, consider the possibilty that they killed it. But recognize that, although many bark beetles can kill healthy trees, a tree that dies from other causes usually has them too. In conifers the resinous response to beetle attacks and the presence of mass attacks can be used to diagnose bark beetle mortality.
Crown symptoms may or may not be seen, and are not diagnostic (specific):
- Growth loss: In many conifers, a flattening of the top as top growth slows is a typical indication of growth loss. It doesn’t always happen.
- Stress crop of cones (numerous, smaller than normal).
- Thin foliage; chlorosis (yellowing), smaller leaves.
- Dieback in hardwoods.
Diagnosis of specific diseases: One can learn to recognize the decays based on type, texture, zone lines, visible mycelium, etc. Of course, the easiest way is to find a conk. You should be so lucky!
The pathogens in these cases are almost all basidiomycetes, usually polypores. But there is one important one that forms gilled mushrooms (Agaricales), and even a few in the Ascomycota (perithecial), such as Kretzschmaria deusta (Hypoxylon deustum).
In many areas these are the most important diseases.
For root diseases in general, impact is often unrecognized. Slow, steady, consistent, not epidemic outbreaks. Widespread, well distributed. Detection difficult and uncertain. Levels of root rot are always higher than can be estimated by above-ground symptoms.
Root and butt rots have three distinct kinds of economic impact: mortality, cull, and growth loss.
- Mortality occurs either by girdling (growing all the way around) the root collar, OR indirectly by tree failure (uprooting or snapping).
- Cull occurs as butt rot (decay in the heartwood of the living lower stem, the most valuable part of the tree).
- Growth loss occurs most likely because of root killing or killing of sapwood from the inside of the stump region, but the fungi may also produce toxins that interfere with growth. Growth loss is especially difficult to quantify, and can be present without above-ground symptoms.
Their ecological impact is also important.
Cortical root rots
The cortex is the succulent outer tissue of young roots. As the root becomes woody, the cortex gets squashed, and instead we have cambium and phloem (inner bark), which is also succulent and rich in nutrients. Many of the fungi that can attack the cortex of young roots can move into the cambium and phloem of woody roots.
This group overlaps with nursery diseases, many of which are cortical root rots. Here we just consider those that are known to be important in big trees.
Most of the well-known pathogens of mature trees in this group are water molds (Oomycota).
A vascular wilt is a disease in which the pathogen moves in the active xylem (sapwood), disrupting the flow of xylem sap, causing wilting and other drought-like symptoms.
These could be better grouped with the other vascular wilts that we’ll learn about later, but some infect through the roots and can spread from tree to tree by mycelial growth the way many root diseases can, so we think of them more as root diseases. They do not cause root “rot.”
There are few such diseases. We will consider one group, black-stain root disease.