Biology Reference
In-Depth Information
Fig. 2 Verticillium wilt on
tomato
original tree, they breed only on weakened
or dying wood and may fly some distance for it.
The European female tunnels out a brood gal-
lery 1 or 2 inches long in the wood, and when
the larvae hatch, they tunnel at right angles
across the wood ( Fig. 2 ). There is a second
brood in August and September, but the
overwintering one, emerging in May, is most to
be feared. Because the disease often follows traf-
fic routes, automobiles probably account for
a good deal of long-distance spread. So far as
we know, the only other natural means of infec-
tion is by root grafts, made when trees are planted
so close together that their roots touch. This is
another argument for diversified planting,
rather than streets closely lined with but one
type of tree.
Control In the first few frantic years an enor-
mous amount of money (more than $26 million)
was spent on trying to eradicate the disease by
removing and burning diseased trees; and while
this was undoubtedly helpful, it did not stop the
spread of wilt. The Federal government has now
left the control of Dutch elm disease up to the
communities and is restricting its efforts to
research. Many towns have taken a laissez-faire
attitude, thinking that our elms are doomed any-
way, so why waste money? Other, more enlight-
ened communities have proved that a sustained
control program keeps the disease down to
a negligible 1 or 2 %, or less, and that the cost is
far, far less than that of continuous removal of
dead trees.
The Midwestern Chapter of the National
Shade Tree Conference, in its Guide for Commu-
nity-Wide Control of Dutch Elm Disease ,
suggests:
1. Survey of the total elm tree population to be
protected.
2. Symptom scouting for detection of diseased
trees and sanitation scouting for badly weak-
ened elms and wood piles containing elm
wood.
3. Destruction of known sources of elm wood
actually or potentially hazardous for spread
of disease. Elm wood piles should be
destroyed completely, or each log stripped of
bark and the bark destroyed. Diseased trees
should be burned, on site if possible, or thor-
oughly sprayed. Wood chips from diseased
elms may still carry the fungus; material
should be burned, not used for mulches.
4. Spraying of healthy trees to prevent infection.
5. Maintenance of elms in healthy condition to
prevent invasions of bark beetles. This
includes proper watering and fertilizing,
spraying to control summer foliage pests if
necessary.
A single annual DORMANT SPRAY is now
considered sufficient to protect healthy elms from
bark beetles if enough material is used and com-
plete coverage is obtained. This spray was origi-
nally a very heavy dosage of DDT, which caused
some birdmortality and other environmental prob-
lems. Some communities, of which Greenwich,
Connecticut is a good example, figure that they
Search WWH ::




Custom Search