Biology Reference
In-Depth Information
cannot afford not to spray, for it costs less to spray
for control than to remove a dead tree. Where
dormant spraying and sanitation have been com-
bined consistently, the annual loss fromDutch elm
disease has been kept to 1 % or less.
Chemotherapy, injection of chemicals that
will inactivate the fungus, has been a promising
line of research for many years. A parasitic
European wasp is now being bred at several lab-
oratories for release against the bark beetles. To
have elms in our future we must keep on planting
them. Some forms, such as the Christine Buisman
and Groeneveld elms, are quite resistant although
not immune. Chinese and Siberian elms are
resistant.
soil. The inclusion of a fungicide in sprays for
elm-leaf beetles or cankerworms might be
helpful.
Fusarium
Rots .
Fusarium annuum ( F. solani ). Fusarium Wilt
of chili pepper. Underground stems are dry,
brown, but the roots soft and water-soaked; plants
wilt and die rapidly. Spores are spread in irriga-
tion water and with wind-blown particles of soil.
Avoid heavy, poorly drained soils.
Fusarium foetens Wilt on begonia.
Fusarium oxysporum Wilt on pyracantha and
basil. Blight and Wilt on purple coneflower
( Echinacea ).
Fusarium oxysporum f. sp. apii Celery Wilt ,
Yellows , general in northern celery districts.
There are three strains of the fungus, all causing
stunting, vascular discoloration, crown and root
rot, but one form causes the entire plant to turn
yellow at high temperatures, producing brittle
stalks with a bitter taste. Another strain causes
downward curling of young heart leaves, and the
third produces no above-ground symptoms
except stunting. The fungus persists indefinitely
in soil. Golden, self-blanching varieties are more
susceptible. Grow green petiole celery or some-
what resistant Michigan Golden, Cornell 19, Tall
Golden Plume, Golden Pascal or Emerson Pascal.
Fusarium oxysporum f. sp. asparagi Fusarium
Wilt of asparagus, a major factor in asparagus
decline in California, found in most plantings.
The fungus lives in soil and may be distributed
on seed.
Fusarium oxysporum f. sp. barbati Fusarium
Wilt of sweet william. New growth is yellowed;
plants are stunted; leaves point downward and are
tinged with tan as they die. Roots and lower stem
are discolored brown. Plant in new or sterilized
soil.
Fusarium oxysporum f. sp. batatas
Dothiorella
Cankers and Diebacks .
Dothiorella ulmi Dothiorella or
Cephalosporium Wilt of elms. Dieback , rather
common on American elms, occasional on slip-
pery and Siberian elms in central and eastern
states. The names are confusing. In culture the
fungus develops spores as in Cephalosporium ,
but in nature Dothiorelia -type pycnidia are devel-
oped on bark of killed twigs. The fungus has also
been classified as Deuterophoma . Spores are
extruded in a sticky mass and are disseminated
by wind, rain, possibly insects. Infection is
through insect or other wounds on foliage. The
mycelium proceeds from leaf petioles into wood,
where it is confined to the vessels. The foliage
wilts and yellows; there are gradual dying back of
the crown and a brownish discoloration in outer
rings of the wood. Without laboratory diagnosis
the disease cannot be positively separated from
Dutch elm disease, but the elliptical cankers on
the stems, with small black specks of pycnidia,
provide one diagnostic symptom. Older trees die
3 to several years after first symptoms; nursery
trees, in 1 or 2 years. Some trees recover, and
some remain infected for many years without
showing much effect.
Control Prune out infected branches a foot or
more below the lowest point of discoloration.
Promote vigor by feeding, watering, aerating
Rots .
Fusarium oxysporum f. sp. betae Fusarium
Yellows on sugar beet.
Fusarium oxysporum f. sp. callistephi Aster
Wilt , one of the most serious diseases of China
 
 
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