General Comments on Foliage Diseases

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.

Hardwood Foliage Diseases

Anthracnose

Not uniformly defined, but these diseases tend to have:

• irregular-shaped necrotic areas (often along veins)
• pathogen produces acervuli
• may overwinter in small twig lesions, can cause twig blight when severe

Sycamore Anthracnose

A good example, caused by Apiognomonia veneta on London plane (Platanus Xacerifolia).

Dormant season: small cankers on twigs may kill buds and twigs. In lesions on fallen leaves, perithecia mature (not usually seen).

Spring: pycnidia form in twig cankers, perithecia release spores from fallen leaves. Infections of new shoots and leaves. May get shoot blight, rapid death of expanding shoots.

Spring and summer: leaf infections, leaf blight. Lesions usually along midrib (water accumulates?). Even small lesions can cause defoliation, apparently when near petiole (resistance mechanism?). Small creamy acervuli on underside, cause more infections if weather suitable.

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.

Leafspots

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."

Tar spot

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

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.

Powdery mildew

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:

• These are the only fungi we will consider that form cleistothecia. They are an order unto themselves: Erysiphales.
• They are obligate parasites and biotrophs. This means they are very highly adapted for their host. They cannot be cultured on laboratory media. They are apparently tuned into complex organic factors that they get only from their host.
• Probably as a result, they are usually host-specific. There are powdery mildews on thousands of different angiosperms, and for the most part each one is a different fungus species and will not grow on another host. In crop plants, they are even specific to cultivars: each cultivar has a certain mildew race that can grow on it. Very sophisticated interlock with host.
• Most are entirely superficial, except for the haustoria in the epidermal cells. No hyphae go inside the leaf. They are so delicate in their relation to the host.
• During summer, they produce conidia in chains on short, straight conidiophores. Oidium spp. They are airborne, and carry an unusual amount of water with them, so these fungi are somewhat less dependent on wet conditions for infection than many others.
• But most are known by the name of the sexual stage if one is known. There are half a dozen common genera, simple keys to which can be found in many books. Cleistothecia usually appear in late summer, early fall as mycelium collapses. Usually overwinter as cleistothecia, which discharge ascospores in spring.
• Sensitive to sulfur, and noticeably absent from areas of intense SO₂ pollution.

Leaf blisters and other diseases caused by Taphrina spp.

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!

Conifer Foliage Diseases

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?

• Conifers cannot refoliate like hardwoods. A defoliated hardwood will have a full complement of leaves the next year if not the same year.
• Conifers depend on several years of foliage, so they are severely impacted if they lose some. Growth can come to nearly a complete stop.

However, most either infect foliage of current season or older foliage, not both, so mortality is rare. Under what conditions are they damaging?

• Trees "off site" (wrong type of site for the species) or out of native range
• Pure stands
• Dense stands
• Seedling, sapling, small pole stages, under about 30 years old, often more susceptible
• Christmas tree plantations usually fit all four of these criteria!! For Christmas trees, the problem is doubly severe because appearance (full complement of foliage) is at least as important as good growth.

Let's address here several generalizations you often hear about diseases from non-pathologists:

• One is that, "it's not nice to fool with mother nature," the idea being that diseases are more severe in artificial, managed systems. Well, in this case the generalization seems to hold pretty well. Pure stands, even-aged stands, trees planted where they don't grow naturally, all these things can be associated with increased damage from foliage diseases (certainly not always).
• Another generalization is that "diseases are nature's way of taking out the old and unfit, weak, cleaning up the forest, and making way for the young." Leaving aside the assumption that nature has some guiding purpose or intent, foliage diseases of conifers clearly don't fit that generalization. Trees are often more susceptible when they are young and in their most vigorous stage. As far as I know, foliage diseases are never involved in the death of overmature trees. Stress and suppression seem to make no difference to some of them. Rhabdocline needle cast of Douglas fir is an example.

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.

Needle casts in general

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:

• Rhabdocline pseudotsugae on Douglas fir, one-year life cycle.
• Lophodermium spp. [This section is under repair]
• Elytroderma deformans causes a severe disease of ponderosa and Jeffrey pines, even in natural stands. It is unusual in that it invades bark of twigs and small branches and survives there for many years. This kind of extensive growth inside a major part of the plant, without killing it we call systemic infection. In those branches you get a symptom we have not discussed yet, witches' brooms. These are clusters of profuse branching.

Needle casts on Christmas trees are routinely controlled with fungicide sprays. Spraying is not feasible in the forest, nor is it usually necessary.

Swiss Needle Cast

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.

Brown Felt Blight

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.

Brown Spot Needle Blight

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.

Dothistroma 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.

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