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
Many rusts cause
foliage diseases, but we consider them separately under rust diseases.
Hardwood Foliage Diseases
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
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.
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
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
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
- 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
- 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
- Sensitive to sulfur, and noticeably
absent from areas of intense SO2 pollution.
Leaf blisters and other diseases caused by
Also obligate parasites. They cause the host to
overgrow in infected areas. Lead to blister, puckering, curling,
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
- 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.
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.
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.
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
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.