Biology Reference
In-Depth Information
Fig. 1
Dutch elm disease. (a) branch cut to show discoloration of wood; (b) wound in twig crotch due to beetle feeding;
(c) bark-beetle carrier of the fungus; (d) egg and larval galleries of the beetle engraved on sapwood
have believed that dishes could have anything to
do with killing our elms? Dishes have to be
crated, however, and several times since 1933
English dishes crated with elm wood carrying
bark beetles and
Ceratocystis
have been
intercepted. All American and European elms
are susceptible. Asiatic elms,
Ulmus parvifolia
and
U. pumila
, are resistant. A seedling elm,
named Christine Buisman for its Netherlands'
discoverer, is highly resistant, though not
immune, and is now available. Other promising
seedlings have been tested by the U.S. Depart-
ment of Agriculture.
Symptoms
are apparent from the latter part of
May until late fall. The acute form of the disease
is characterized by sudden and severe wilting.
First the young leaves, then all leaves wilt and
wither, sometimes so rapidly that they dry, curl,
and fall while still green, before they can turn the
usual brown of dead leaves. Sometimes terminal
twigs are curled into a shepherd's crook. Chronic
disease symptoms are gradual, often taking all
summer for complete defoliation. In many cases
individual branches or “flags” appear, the
yellowed leaves conspicuous against the rest of
the tree; but sometimes all leaves gradually turn
yellow. In another type of chronic disease, trees
leaf out late in spring, with sparse chlorotic
foliage and a staghead appearance. When
an affected twig is cut across, the vessels or
water-conducting tubes show dark brown or
black, being clogged with bladderlike tyloses
and brown gummy substances (see
Fig. 1
). The
production of these substances is thought to be
stimulated by a toxin secreted by the fungus and
carried in the sapstream. Symptoms are not
dependent on the physical presence of fungal
hyphae in all parts of the tree. The fungus lives
in the sapwood, fruiting in cracks between wood
and loosened bark and in bark beetle galleries
under the bark. This fruiting is of the anamorph
state, spores being produced in structures called
coremia. These are black stalks about 1 mm high
with enlarged heads bearing vast numbers of
minute, pear-shaped spores embedded in
a translucent drop of sticky liquid. Spores in the
vessels increase in a yeastlike manner. The peri-
thecial stage, not found in nature, has been pro-
duced in culture by crossing plus and minus
strains of the fungus.
Although the smaller European elm bark bee-
tle is chiefly responsible for spread of the patho-
gen, at times the native elm bark beetle,
Hylurgopinus rufipes
, is the agent. When the
adult beetles emerge from under the bark of
dead or dying trees, they bring along sticky
spores on their bodies and deposit them as they
feed in the crotches of young twigs or leaf axils of
nearby healthy trees. Although the beetles feed
on healthy wood, usually within 200 feet of their
