Forest and Shade Tree Pathology
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Forest and Shade Tree Pathology Foliage Diseases |
Outline:
This might be a good time to review (or view!) concepts of disease, names of diseases and pathogens, and information on signs and symptoms. If you have a good grip on that, plow ahead!
This is idealized, and many don't follow it quite, but it gives the general idea:
Infection courts vary among the pathogens.
There are a great many, probably thousands, caused by many different kinds of fungi (and a few bacteria and viruses). Hardwoods are usually not seriously affected. The diseases are common, but they don't often seriously affect the trees. The ones that have more potential for serious damage are ones that cause defoliation. Generally not considered economic problems, except in ornamentals, nurseries.
Categories of foliage diseases are loose and not well defined. There are many colorful terms but usually no clear technical meanings.
Many rusts cause foliage diseases, but we consider them separately under rust diseases.
Not uniformly defined, but these diseases tend to have:
 Anthracnose on eastern hardwoods |
 Dogwood anthracnose |
A good example, caused by Apiognomonia veneta on London plane (Platanus Xacerifolia).
Weather during leaf expansion is critical - wet springs. Sometimes severe, but sycamores keep producing leaves. Conidial state looks different in cankers vs. leaves - shows how fungi defy attempts at classification.
There are many, nondescript leafspot diseases. Many are caused by fungi that form pycnidia. If the spot is sharply delimited, dry and necrotic, it may tend to fall out. Such diseases are often called "shothole."
Rhytisma spp. A well-known example is Rhytisma acerinum on maples. It is common in northeastern U.S.
Spots begin faintly chlorotic, eventually one or more thickish black stromata develop on the upper surface. Conidia (probably male spermatia) are formed in them during the summer. Leaves fall, then apothecia develop in the stromata during fall and early spring. In May and June, ascospores infect new leaves.
Sooty mold is not a disease. It looks like black soot on leaves and branches because of dark, superficial mycelium. It usually results from insects, especially aphids and scale insects, that secrete excess materials as honeydew, a sugary liquid. Honeydew is the primary substrate for the fungal growth, and the plant is not penetrated. In some cases plant exudates are the substrate.
The fungi are mostly Loculoascomycetes, quite a variety. One common genus is Capnodium. Usually most severe in areas with mild climate.
Dr. Bill Merrill reports that sooty mold is a common problem in the Northeast, especially on conifers such as eastern white pine, Scots pine and Mugo pine. Cinara and spotted pine aphids, or scales, particularly the pine tortoise scale and the striped pine scale, are common insect associates. Late summer build-up of insect populations leads to blackening of trees in early fall, making Christmas trees unsaleable.
So called because they look like a powdery whitish material on the leaf surface. Need a lens to be sure it isn't dust or something.
These are unusual fungi and diseases for several reasons:
Also obligate parasites. They cause the host to overgrow in infected areas. Lead to blister, puckering, curling, expansion.
This pathogen is the only member of Hemiascomycetes we will deal with. Naked asci - no ascoma. Asci are produced on leaf surface. The ascospores keep dividing so the asci have lots more than 8 spores.
Peach leaf curl is an important disease in orchards, caused by Taphrina deformans.
One curious one is on female catkins of alder - what a specific habitat! It causes the bracts to grow much longer than normal so they look like tongues sticking out. Even more curious, there is a powdery mildew that also is restricted to the female catkins of alder. There must be something good happening in those catkins that we don't know about!
I said that foliage diseases on hardwoods don't cause much impact and are usually not a serious problem. They are more often serious in conifers, at least under certain conditions.
Why are they more severe in conifers?
However, most either infect foliage of current season or older foliage, not both, so mortality is rare. Under what conditions are they damaging?
Let's address here several generalizations you often hear about diseases from non-pathologists:
Many of the foliage diseases are pretty straightforward and perhaps require no further elaboration here if you have a source of information on specific foliage diseases in your area.
This name is obviously used because needles are often lost, or cast, prematurely. However, there are some known (for instance on larch) where the needles are kept longer than normal.
Needlecasts have only one infection period per year and per generation (needle blights, in contrast, typically can infect multiple times whenever temperature and moisture are favorable). Most are caused by a characteristic group of fungi in the family Rhytismataceae, order Rhytismatales (same group as the pathogen of tarspot, above!). But some needlecast fungi are in other groups of the Ascomycota. There are at least 40 species in U.S.
Needlecast pathogens in this family usually have modified apothecia called hysterothecia. Hysterothecia typically are elongated and have a covering (clypeus) over the hymenium. The clypeus develops a longitudinal slit in the middle. Special cells at the outer edges of the clypeus absorb water under wet conditions and force the slit open to expose the hymenium. When the weather is dry, they close again. Neat! They function like a biological hinge, opening the clypeus like outside basement doors in old houses, or bomb bay doors.
Ascospores are usually long and narrow, which may increase the likelihood of hitting a needle. They have a sticky sheath that helps them stick to needles. Sometimes pycnidia are produced, but we think their spores don't cause infections. Such spores may act as male fertilizing elements (spermatia) to produce the ascomata.
Pines, spruces, firs, larches, cedars, hemlocks and Douglas-fir all get needle casts.
Most needlecasts infect young, current-year needles. Some infect mostly older needles, but they are less serious diseases and verge into the saprobic species.
Symptom: red to brown discoloration, may turn to gray. Discoloration is often regular, the needle dying and turning color uniformly. In some cases, needles retain short green basal portions; in others ,irregular discoloration occurs. Not all needles are affected. The irregular distribution of affected needles within a year may help in distinguishing needle casts from abiotic diseases that affect needles.
From the surface, the hysterothecia may appear in various colors such as black, gray, reddish orange, and creamy white. The depth at which the hysterothecium forms (subcuticular to subhypodermal) determines in part how light or dark it appears.
Most release spores around the time of bud break and infect the current-year needles. They show no symptoms until the following spring. The hysterothecia may appear during that summer and then the needles fall off, or the hysterothecia may take a second summer to mature.
There are a couple of noteworthy ones:
Needle casts on Christmas trees are routinely controlled with fungicide sprays. Spraying is not feasible in the forest, nor is it usually necessary.
A separate page on Swiss needle cast tells the story of a disease that has traditionally been common but not severe in forest conditions, but is now causing an unprecedented epidemic on the coast of Oregon.
This is another nifty disease. It is caused by pseudothecial fungi that grow on the foliage under snow in spring. After snowmelt the dead shoots can be found covered by a thick felt of brownish mycelium, often studded with small pseudothecia.
A similar disease, snow blight, is caused by unrelated apothecial fungi, especially Phacidium infestans. It is sometimes damaging in nurseries, attacking foliage under a heavy snowpack. A thin, ephemeral, white mycelium may be found on the soft, dead foliage as the snow melts. Late in the summer, small dark apothecia begin to appear on the undersides of the dead needles.
Caused by Mycosphaerella dearnessii, it is best known on longleaf pine. Longleaf pine is adapted to ground fires. Its older and even newer leaves can be burned off without ill effect. This is one disease that can be controlled to some degree with fire. A ground fire burns the outer dead needles that provide inoculum.
Brown spot needle blight
This is a final foliage disease of conifers important to know about. It is caused by a fungus with a sexual stage in the Loculoascomycetes, Mycosphaerella pini. But usually only the pycnidia are found, so we call it by the asexual stage name, Dothistroma septospora.
Monterey pine is native to small coastal portions of California. The pathogen is known from the native range but damage is mild. It is known to be severe when the trees are grown in plantations in northern California, where the weather is much cooler and wetter.
Monterey pine is a major plantation species in many areas of the southern hemisphere, where it is usually called radiata pine. Eventually the pathogen got to those plantations and became a serious problem.
Most serious at <10 yr old. Hits older foliage first, but in wet years it goes after younger foliage and can be devastating. In this case, the pycnidia really do produce infective conidia. Infection can occur all through the growing season, so it is explosive in wet years.
Now, it is the only forest disease that is actually controlled by fungicides. Plantations are protected this way until they get older, then they are safe. Can also be serious on other pines planted in the West.
Dothistroma needle blight
Many thanks to Dr. Bill Merrill of Pennsylvania State University for helping with this page, especially with the needlecasts.
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"I used to think trees were nice. Tree diseases are even better!" | |
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