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COTTONWOOD LEAF RUST.
The yellow-orange spots on this leaf are
pustules of leaf rust (Melampsora). The
pustules are filled with uredospores ready to be
spread on the wind.
Photo courtesy of James A. Walla.
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This is TAR SPOT on maple,
caused by Rhytisma. The tar spot is
entirely a sign. That is, the sign is also the
symptom.
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Tar spots on maple may be
numerous, as in the picture above, or large and
solitary, as on these leaves of red maple
(Acer rubrum).
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A view of a single tar spot. The
entire structure is a fungal STROMA. The
surface of the tar spot shows a mazelike pattern
of ridges and furrows, indicating where the
apothecia will develop.
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In POWDERY MILDEW, two kinds of
signs are present: 1) the white powdery areas
are covered with conidia, which give the
disease its name;
2) the small black specks are fruit bodies
(CLEISTOTHECIA) which contain the sexual
spores.
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PLUM POCKETS. Developing plum
(Prunus) fruits are infected by the
fungus Taphrina. The infected fruits are
inflated and hollow. In this picture only one of
each pair of fruit is infected.
Photo courtesy of H. A. Lamey.
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TAPHRINA LEAF BLISTER on
chokecherry (Prunus virginiana).
Infection occurs during leaf expansion,
resulting in the raised blisters.
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TAPHRINA signs.
The glistening surface of leaf blisters
(left) and of plum pockets (above) is covered
with spore-containing asci.
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The LEAF BLIGHT of green ash
(Fraxinus) has conspicuous signs on the
underside of leaves. These STROMATA resemble
dirt but don't rub off.When they mature each
will contain spores of the causal fungus
(Mycosphaerella).
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The small dark spots of elm black
leaf spot are slightly raised, giving the
surface of the leaf a pebbly feel. The spots are
actually STROMATA of the causal fungus
(Stegophora).
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The leaf spots of PHYLLOSTICTA LEAF
SPOT of maple (Acer) contain small,
dark PYCNIDIA embedded within the leaf.
These contain the spores.
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The dark, sooty appearance of these
APPLE SCAB colonies is due to the
presence of millions of dark conidia, signs of
the causal fungus Venturia inaequalis.
Photo courtesy of H. A.
Lamey
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