There are a great many, probably thousands, of foliage diseases caused by many different fungi (and a few bacteria and viruses). 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 these are not considered economic problems, except with ornamentals, Christmas trees, and in nurseries.
Categories of foliage diseases are loose and not well defined. There are many colorful terms but usually no clear technical meanings.
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!
Virtually all tree species can have foliage diseases under the right circumstances.
By far the great majority of foliage diseases are caused by ascomycetes, though in many cases you will see only their asexual fruiting. Some foliage diseases are caused by rust fungi (basidiomycetes), but those are covered separately under rust diseases.
Foliage pathogens are highly dependent on weather for sporulation, dispersal, and infection. Moisture is needed for these processes, and for a few well-studied pathogens (mostly agricultural) the minimum period of leaf wetness and temperature thresholds are known.
Although we sometimes try to link these diseases to especially wet years, keep in mind that weather is critical only during a short time. If a wet period with suitable temperature occurs at the right time, they can cause a lot of disease even during an otherwise dry year.
This is idealized, and many don't follow it quite, but it gives the general idea:
Infection courts vary among the pathogens.
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.
Sooty mold is common in the Northeast, especially on conifers such as eastern white pine, Scots pine and Mugo pine. According to Dr. Bill Merrill, 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!
Foliage diseases on conifers are often more damaging than those on hardwoods, at least under certain conditions. Why?
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.
Pines, spruces, firs, larches, cedars, hemlocks and Douglas-fir all get needle casts.
Most needle casts are caused by a characteristic group of fungi in the family Rhytismataceae, order Rhytismatales (same group as the pathogen of tarspot, above!). There are at least 40 pathogenic species in this family in the U.S. But some needlecast fungi are in other groups of the Ascomycota.
Needle casts 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).
Needle-cast pathogens in the Rhytismataceae usually have modified apothecia, traditionally 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.
Most needlecasts infect young, current-year needles. Some infect mostly older needles, but they are less serious diseases and verge into the saprobic species.
Symptoms include 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.
Needle casts on Christmas trees are routinely controlled with fungicide sprays. Spraying is not feasible in the forest, nor is it usually necessary.
Many Lophodermium species occur on conifers, but most are very weak pathogens, colonizing needles as they die, or even endophytes, colonizing but causing no symptoms and fruiting as the needle dies. But a few are significant pathogens. [in progress] There is an important monograph of the genus that is indispensable in identifying species. It is impossible to find online so I am hosting it here. It is a 16 MB download.
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.
Swiss needle cast:
Elytroderma needle cast, caused by Elytroderma deformans, is a unique disease that is widespread in North America but usually not very damaging. It is most common on ponderosa and Jeffrey pines, but can also be found on lodgepole pine and pinon. It infects needles as all of them do, but then grows down into the shoot. Much like many dwarf mistletoes, becomes locally systemic and perennial, growing along with the shoot and entering new branches that are formed. Also like dwarf mistletoes, it causes witches' brooms. The pathogen forms long, dark hysterothecia on infected needles.
Elytroderma disease of ponderosal pine:
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 an important foliage disease of conifers to know about. There is a separate page dedicated to this disease (also known as Dothistroma needle blight).
Dothistroma needle blight:
The late Dr. Bill Merrill helped shape my understanding of foliage diseases and helped improve this page, especially regarding the needlecasts.