Biology Reference
In-Depth Information
twisted, brittle, often cracked, yellow. The celery
strain of the virus causes yellowing and stunting
of cucumber, squash, pumpkin; infects gladiolus
and zinnia.
Control
of aster yellows is directed against the
leafhoppers. Asters are grown commercially
under frames of cheesecloth, 22 threads to the
inch, or wire screening, 18 threads to the inch.
In home gardens all diseased plants should be
rogued immediately and overwintering weeds,
which harbor leafhopper eggs, destroyed.
Spraying or dusting ornamentals and vegetables
with pyrethrumwill reduce the number of vectors
but will not entirely eliminate the disease.
Recent work raises the probability that the
etiological agent of aster yellows is
a mycoplasma rather than a virus. Therefore,
treatment with antibiotics, such as chlortetracy-
cline, has suppressed the development of yellows
symptoms. Mycoplasma-like bodies have been
seen in microscopic study of diseased plants and
in transmitting leafhopper vectors, but not in
healthy plants or nontransmitting vectors.
Clover Proliferation On strawberry and onion.
Corn Stunt A dwarfing disease present primar-
ily in the South;
about May 1 from eggs wintered on elm bark and
feed on leaf veins. Adults move from diseased to
healthy trees.
There is hope of propagating elms resistant to
phloem necrosis. Communities should interplant
existing elms with Asiatic or European varieties
or with some other type of tree to provide shade if
and when present elms die.
Peach Western X-Disease Perhaps same as
X-disease but usually treated separately; also
known as cherry buckskin and western-X little
cherry. The pathogen is transmitted by leafhop-
pers (
Colladonus germinatus, Fieberella florii,
Osbornellus borealis
, and others) to peach, nec-
tarine and cherry in western states. Symptoms
vary according to rootstock, but cherry fruit is
smaller than normal. Sour cherries are puttylike,
pinkish; sweet cherries are small, conical, hang
on trees late, fail to develop normal color. Symp-
toms on peach are similar to those of X-disease.
Peach X-Disease On peach and chokecherry,
sometimes cherry in the northern United States
and of major importance in Connecticut, Massa-
chusetts, and New York. Peach trees appear nor-
mal in spring for 6 or 7 weeks after growth starts,
then foliage shows a diffused yellow and red
discoloration with a longitudinal upward curling
of leaf edges; spots may drop out, leaving
a tattered effect. Defoliation starts by mid-sum-
mer. Fruits shrivel and drop or ripen prematurely.
Seed do not develop. Weakened trees are killed
by low temperatures or remain unproductive.
Chokecherry has conspicuous premature red-
dening of foliage, dead embryos in fruit. The
second and third seasons after infection foliage
colors are duller, there are rosettes of small leaves
on terminals, and death may follow. Natural
infection is apparently from chokecherry to
peach (not peach to peach or peach to choke-
cherry) by a leafhopper (
Colladonus clitellarius
).
Elimination of chokecherries within 500 feet of
peach trees provides the best control.
Peach Yellow Leaf Roll; a form of Western
X-Disease; perhaps caused by a more severe
strain of the MLO.
Peach Yellows; Little Peach. First noted near
Philadelphia in 1791 and so serious that in 1796
the American Philosophical Society offered
transmitted by leafhoppers.
Mycoplasma-like
bodies
present;
See
Spiroplasma citri
.
Elm Phloem Necrosis On American elm from
West Virginia and Georgia to northern Missis-
sippi, eastern Oklahoma, Kansas, and Nebraska.
Origin unknown but apparently present since
1882; the disease reached epidemic proportions
in Ohio in 1944, killing 20,000 trees that year
near Dayton and 10,000 at Columbus. The most
reliable diagnostic character is a buttercup yellow
discoloration of the phloem, often flecked with
brown or black and an odor of wintergreen.
Destruction of phloem causes the bark to loosen
and fall away. Roots die first, then the phloem in
lower portions of tree, followed by wilting and
defoliation. American elms may be attacked at
any age; they wilt and die suddenly within 3 or
4 weeks or gradually decline for 12 to 18 months.
This is now thought to be caused by
a mycoplasma-like agent. Transmission is by
the white-banded elm leafhopper (
Scaphoideus
luteolus
) and possibly other species. Nymphs hatch
