Biology Reference
In-Depth Information
and finally, some plant diseases formerly thought
to be caused by viruses have now been deter-
mined to be caused by spiroplasma, such as citrus
stubborn disease. Thus the field of virology has
changed somewhat in recent years. In order to
simplify the discussion of these viruses and virus-
like agents and the diseases they cause, these
agents are grouped under virus diseases, since
the symptoms which they cause in plants are
similar.
Some viruses attack a large number of differ-
ent plants and are of great economic importance;
others are confined to a single host. Virus symp-
toms fall into several categories, but commonly
there is loss of color due to the suppression of
chlorophyll development. Foliage may be mot-
tled green and yellow, mosaic, or have yellow
rings (ring spot); or there may be a rather uniform
yellowing (yellows). Stunting is common. The
reduction in manufactured food from the chloro-
phyll loss leads to smaller size, shorter inter-
nodes, smaller leaves and blossoms, and
reduced yield. There may be various distortions
of leaves and flowers, witches' brooms, or
rosettes. There may be necrotic symptoms with
death as the end result, and sometimes symptoms
are “masked,” not showing up under certain con-
ditions, such as hot weather, or latent, not
appearing until another virus is also present.
Viruses are transmitted from plant to plant by:
insects, mites, fungi, and nematodes; rubbing,
abrasion, or other mechanical means (sometimes
handling tobacco and merely touching a healthy
plant spreads mosaic); grafting or propagation by
cuttings and bulbs; occasionally seeds; some-
times soil and water; and dodder, parasitic vines
whose tendrils link one plant to another. About
half of the insect vectors are aphids; a third are
leafhoppers. Mealybugs and whiteflies transmit
some viruses, and six, including tomato spotted
wilt, are transmitted by thrips. In some cases the
virus multiples within the insect as well as in the
plant. Some viruses have many different vectors,
50 being recorded for onion yellow dwarf, and
some have but a single known vector.
Control of virus diseases starts with obtaining
healthy seed, cuttings, or plants. “Certified”
means that plants have been inspected during
the growing season and found free of
certain diseases. Virus-free foundation stock
can be built up from heat treatment -rowing
plants at high temperatures for weeks or even
months - and/or meristem tip cultured plants.
Virus-free stock is tested by “indexing”, bioas-
says and/or serological assays, before using stock
for propagating. Controlling insect vectors (by
spraying plants or treating soil with systemic
insecticides), eliminating weed hosts, roguing
diseased plants before insects can transmit the
virus, and using resistant varieties are all ways
of combating virus diseases.
This handbook does not deal predominantly
with the characteristics of the causal viral agent,
but with the disease caused by the virus, viroids
and phytoplasmas.
Viruses are classified now in the traditional
taxonomic system (family - genus - species).
Formal use of a virus species name should be
printed in italics with the first word capitalized;
an acronym, when used, should also be capital-
ized. In this topic however, we used bold font for
the generic name of viruses. Generally, the spe-
cies name consists of the vernacular plus the
generic names. For example,
Tomato spotted
wilt tospovirus
is the species name, tomato
spotted wilt virus is the vernacular name and
Tospovirus
the generic name. The scheme of
virus and viroid classification (shown below)
according to Murphy et al. (1995) and Brunt
et al. (1996) include the following data: genus
of virus, family (if designated), kind of nucleic
acid in genome, shape of virions, presence of
envelope and the type species:
Alfamovirus
Bromoviridae
; (+)ssRNA; isometric particles;
not enveloped; alfalfa mosaic virus.
Alphacryptovirus
Partitiviridae
; dsRNA; isometric particles; not
enveloped; white clover cryptic virus 1.
