Department of Plant Pathology |
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| TOMATO DISEASES: VIRUSES AND HERBICIDES |
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Viruses and Herbicides often cause leaf deformation
that is most severe on new growth. Tobacco mosaic, cucumber
mosaic, and 2,4-D are three common causes of these symptoms.
Tobacco Mosaic
Cucumber Mosaic
Double-Virus Streak
Spotted Wilt
2,4-D Herbicide Injury
Lightening Injury |
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Tobacco Mosaic is caused by a virus that affects tomatoes, eggplants,
peppers, and related plants. Symptoms on tomato foliage include
light- and dark-green mottling with curling and slight malformation
of leaflets. Sometimes green fruit are mottles. Affected plants
may be stunted. The virus is very persistent and infectious,
and can be spread by merely brushing against plants. The
virus is not spread by aphids. |
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Cucumber Mosaic is caused by a virus that affects hundreds of
unrelated plant species. Infected tomato plants are stunted,
have short internodes, and may have extremely distorted and
malformed leaves. Very narrow leaves are referred to as the
shoestring symptom. Cucumber mosaic virus is not persistent
in refuse, is more difficult than tobacco mosaic virus to
transmit by rubbing, and usually is spread by aphids. Since
aphids are responsible for most spread, infected plants may
be widely separated within a field. |
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Double-virus Streak is caused by a combination of tobacco mosaic virus
(TMV) and potato virus X (PVX). Leaf symptoms include first
a light-green mottling of leaves accompanied by development
of numerous small grayish brown thin dead spots; severely
spotted leaves may die. Later growth is mottled green and
yellow, dwarfed and curled, with small irregular brown spots
on the leaves. Numerous narrow dark brown streaks develop
on stems and petioles. Shoot tips may die and infected plants
are stunted. Fruit set is reduced; these fruit are rough
with small irregular greasy-appearing brown patches. TMV
overwinters in tobacco products; PVX overwinters in potato
tubers. Both are spread by workers. |
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Spotted Wilt is caused by a virus that usually is spread by thrips.
Foliage symptoms are similar to those of double-virus streak,
except that spotted wilt symptoms are more severe. Growing
tips usually are severely affected. Characteristic fruit
spots are about 1/2 inch wide, have concentric rings, and
usually are numerous. The rings may be alternately red and
yellow and the center of the spot may be raised, which gives
the fruit a rough appearance. The spotted wilt virus can
overwinter in several weeds and ornamental plants. |
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2,4-D Herbicide Injury occurs in greenhouses and fields. Symptoms
include downward bending of leaves and of growing points.
New leaves do not expand normally; they are narrow and elongated,
twist at the margin, and have abnormally pointed tips. Veins
are prominent, light colored, and may appear parallel. Source
of the herbicide often is not readily apparent. Possible
sources include i) drift, ii) pesticides or fertilizer contaminated
during storage, and iii) soil, equipment, tools, containers,
clothing and gloves contaminated from a previous herbicide
use. |
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Lightning Injury appears as circular areas 10 to 60 feet in diameter.
Injury is most severe at the center of these areas. Plants
other than tomatoes, including weeds, are affected similarly
within the area. First symptoms may appear within a few hours
of injury. Leaves begin to droop and in the most severe cases
the wilting is progressively rapid and permanent, followed
by death. Stems, branches, and petioles show various degrees
of shrinkage due to collapse of the pith. In many plants,
stems become flattened and the internal stem tissue assumes
a ladder-like appearance. |
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