FREE ESSAY ON TOMATOE DISEASES

TOMATOE DISEASES

Bacterial Spot (bacterial - Xanthomonas campestris pv. vesicatoria)
Infected leaves show small, brown, water soaked, circular spots about one-eighth inch in
diameter. The spots may have a yellow halo. This is because the centers dry out and
frequently tear. on older plants the leaflet infection is mostly on older leaves and may
cause serious defoliation. The most striking symptoms are on the green fruit. Small,
water-soaked spots first appear which later become raised and enlarge
until they are one-eighth to one-fourth inch in diameter. Centers of these lesions become
light brown and slightly sunken with a rough, scabby surface. Ripe fruits are not
susceptible to the disease.. but the surface of the seed becomes contaminated with the
bacteria The organism may also alternate hosts over winter, by getting on volunteer
tomato plants and on infected plant debris. Moist weather and splattering rains are
ecential to disease development. Most outbreaks of the disease can be traced back to
heavy rainstorms that occur in the area. the Infection of the leaves may occur through
natural openings. The infection of other fruits may occur through insect punctures,
sandblasting and other mechanical injury means. Bacterial spot is difficult to control
once it appears in the field.
Late Blight (fungal - Phytophthora infestans)
Lesions produced on the leaves are at first large, greenish-black, and water-soaked.
These areas enlarge, becoming brown, and under humid conditions, develop a white moldy
growth near the edge of the diseased area on the lower surface of the leaves or on stems.
The disease spreads rapidly under humid conditions, destroying large areas of tissue.
Fruit lesions occur as large, green to dark brown, mostly on the upper half of the fruit.
Also, a white moldy growth may appear on fruits in humid conditions. The fungus produces
an abundant number of spores which may be splashed by rains or be airborne. These spores
infect healthy leaves, stems and fruit if weather conditions are good. Ideal conditions
for late blight development are cool nights, moderately warm days, abundant moisture. Hot
and dry weather reduces disease development.
Gray Leaf Spot (fungal : Stemphylium solani)
First infection appears as small, brownish-black specks on the lower leaves that extend
through to the under surface of the leaf. These spots usually remain small, but may
enlarge until they are about one-eighth inch in diameter. The spots become glazed and the
centers crack. Infected leaves usually die and fall off. spots may also form on the stems
of the host plant.
Leaf-Mold
Causal Agent(s): (fungal - Fulvia (Cladosporium) fulvum)
Leaf mold is usually first observed on older leaves near the soil where air movement is
poor and
humidity is high. At first, diffuse whitish spots appear on the upper surfaces of older
leaves; these
rapidly enlarge and become yellow. Under humid conditions, the lower surface of these
spots
become covered with a gray, velvety growth of the spores produced by the fungus. When
conditions
are proper for fungal development, large areas of the field are infected, plants are
weakened and the
crop is greatly reduced. The fungus produces abundant spores during periods of high
temperature
and very high relative humidity. Infection occurs readily, and the disease becomes
established in the
fields quickly. The best control of this disease is by using a preventative fungicide
program at 7 to 10
day intervals, the same as used for late and early blight control. 
Buckeye Rot
Causal Agent(s): (fungal - Phytophthora parasitica)
This disease occurs on tomato mainly on the fruit, particularly where it touches the
soil. The fungus is
different from the one causing late blight, which affects both leaves and fruit. Buckeye
rot is first
noticed as a light green water-soaked area on the fruit. Later, dark zonate bands can be
seen on the
surface of affected areas. The surface of the lesion is usually smooth and firm. With
time, the entire
fruit will rot. The fungus lives in the soil and it can also affect pepper. The disease
is more
troublesome in heavy, poorly drained soils during prolonged warm wet weather. 
Nailhead Spot
Causal Agent(s): (fungal - Alternaria tomato)
Leaf symptoms are the same as those caused by early blight on fruits; however, spots are
smaller,
with slightly sunken centers and dark margins. As the spots become older, the edges
become
roughened. On ripe fruit, the tissue immediately around the spots often remains green.
Control is the
same as for early blight. 
Anthracnose
Causal Agent(s): (fungal - Colletotrichum sp.)
At first, infected fruit show small, slightly sunken, watersoaked spots. These spots
enlarge, become
darker in color, depressed and have concentric rings. Masses of the pink fruiting fungus
can be seen
on the surface of the lesions in moist weather. Under warm and humid conditions, the
fungus
penetrates the fruit, completely destroying it. The fungus persists on infected plant
refuse in the soil.
Fruit may be infected when green and small, but do not show any marked lesions until they
begin to
ripen. Fruit becomes more susceptible as they approach maturity. Control of this disease
involves the
use of well-drained soil, crop rotation and a preventative fungicide program as
recommended for
other diseases. 
Fusarium Wilt
Causal Agent(s): (fungal - Fusarium oxysporum f. sp. lycopersici)
The first indication of disease in small plants is a drooping and wilting of lower leaves
with a loss of
green color followed by wilting and death of the plant. Often leaves on only one side of
the stem turn
yellow at first; yellowed leaves gradually wilt and die. The stem of wilted plants shows
no soft decay,
but when cut lengthwise, the woody part next to the green outer cortex shows a dark
brown
discoloration of the water conducting vessels. The fungus is soilborne, passes upward
into the xylem
of the stem. Blocking of the water-conducting vessels is the main reason for wilting. The
fungus is
most active at temperatures between 80 and 90 degrees F., seldom being a serious problem
where
soil and air temperatures remain low during most of the growing season. Control can be
obtained by
growing plants in disease-free soil, using disease-free transplants, and growing only
resistant
varieties. 
Verticillium Wilt
Causal Agent(s): (fungal - Verticillium albo-atrum)
The first symptom is yellowing of the older leaves, followed by a slight wilting of the
tips of the
shoots during the day. Older yellowed leaves gradually wither and drop, and eventually
the plant is
defoliated. Verticillium wilt does not show the one-sided effect as does Fusarium wilt.
Leaves from
Verticillium infected plants sometimes show brown dead spots that may be confused with
those
caused by other fungi. However, they are lighter in color and do not show concentric
zones as in
early blight. In late stages of the disease, only the leaves near the tips of the branch
remain alive.
When the stem is cut lengthwise, the base shows a discoloration of the woody tissue
similar to
Fusarium, but is usually darker, and generally it occurs only in the lower part of the
stem. The fungus
enters the plant through the feeder roots and grows into the stem in the woody conducting
vessels
just under the cortex. The fungus lives in the soil for a long time and it is exclusively
the source of
infection. Progress of the disease is favored by cooler temperatures and is retarded by
the high
temperatures that are favorable to Fusarium wilt. Locating seedbeds and fields in
Verticillium-free
soil, and using resistant varieties are the most effective means of controlling the
disease. 
Gray Mold
Causal Agent(s): (fungal - Botrytis cinerea)
Plants become more susceptible to this disease as they become older. It is mostly a
problem in
greenhouses, but it can also affect tomatoes in the field. The fungus first becomes
established on
dead leaves at the base of the plants. A heavy, gray growth of the fungus covers these,
and
numerous spores are soon found, giving the affected area a cottony appearance. Affected
leaves
collapse and shrink. The fungus progresses into the stem, producing cankers. Affected
fruits first
show a watersoaked, soft area in the points of infection. The dark gray growth of the
fungus soon is
seen on these spots. Regular fungicide applications should help in controlling this
disease. 
Botryosporium Mold
Causal Agent(s): (fungal - Botryosporium sp.)
This fungus can often be found on greenhouse tomatoes. It superficially resembles gray
mold. 
Septoria Leaf Spot
Causal Agent(s): (fungal - Septoria lycopersici)
Infection usually occurs on the lower leaves near the ground, after plants begin to set
fruit. At first,
small watersoaked spots are observed, which under ideal conditions will become numerous.
Large
areas of the leaves may be affected but the individual spots can be recognized. The
watersoaked
spots become roughly circular, with dark margins surrounding a light gray center. With
time, black
specks which are spore producing bodies can be seen in the center of the spots. If the
spots are
numerous, the lower leaves will turn yellow, die and progressively drop from the plant
until only a
few leaves remain on the top of the plant. The fungus is most active when temperatures
range from
60 to 80 degrees F., and humidity conditions are high. The disease is usually not serious
during
periods of hot, dry weather. The fungus can overwinter on crop residue from previous
crops,
decaying vegetation and some tomato-related wild hosts. Crop rotation, plowing under
crop
residues, and clean cultivation will reduce the amount of inoculum in tomato fields.
Repeated
fungicide applications will keep the disease in check. 
Southern Blight
Causal Agent(s): (fungal - Sclerotium rolfsii)
The first symptom is dropping of leaves suggestive of other wilts. Wilting progresses and
plants die
quickly. Stems show decay of outer tissues at the ground line. Frequently, they are
covered by a
white fungal mat in which are embedded numerous small, light-brown bodies about the size
of
cabbage seed. The fungus can also attack fruits where they touch the soil. The fungus
does not grow
at temperatures below 68 oF.; it requires abundant moisture for growth. Infection takes
place below
the soil line or close to ground level. Control is obtained by sanitation, crop rotation,
and by treating
infected soil with a soil fungicide prior to planting. 
Seedling Disease
Causal Agent(s): (fungal - Rhizoctonia sp., Pythium sp.)
Seedlings fail to emerge or small seedlings wilt and die soon after emergence. Surviving
plants may
have infected root systems and watersoaked areas on the stem close to the soil line. As
the plants
mature, they become more resistant to damping-off. Avoid excessive moisture in the
seedbed, plant
seed treated with fungicides and use sterilized media for growing transplants. 
Tobacco Mosaic
Causal Agent(s): (viral - TMV)
Symptoms are light and dark green mottling of the tomato foliage, and curling and slight
malformation
of the leaflets. Plants may be somewhat stunted if infected when small, but the plants
and fruit are not
much reduced in size if plants are not infected until they reach the fruiting stage.
Several strains of the
virus are known that can cause different symptoms. The virus is highly infectious and
readily
transmitted by any means that introduces even a minute amount of sap from infected to
healthy
plants. The most common means of transmission is by handling contaminated plants. The
virus may
also be present in certain types of tobacco; therefore, smokers may transmit the disease.
Control
measures are: avoiding handling plants more than necessary, washing hands before handling
plants,
and protecting healthy plants from infection. 
Double Streak Virus
Causal Agent(s): (viral - viral)
Caused by a combination of viruses. Leaves show a light-green mottling, accompanied by
the
development of numerous small, grayish-brown, dead spots which have a thick, paper
appearance.
Numerous narrow, dark brown streaks develop on the stem and leaf petiole. Fruits are
often rough
and misshapen and on the surface they have small, irregular, greasy, brown patches which
render
them unfit to market. The virus is transmitted by workers handling the crop. Avoid
infection by
tobacco mosaic virus, wash hands before starting to work and remove infected 
Spotted Wilt
Causal Agent(s): (viral - viral)
This disease is similar to streak in that it causes streaking of the leaves, stems and
fruits. Numerous small, dark, circular spots appear on younger leaves. Leaves may have a
bronzed appearance and later turn dark brown and wither. Fruits show numerous spots about
one-half inch in diameter with concentric, circular markings. On ripe fruit these
markings are alternate bands of red and yellow. The virus also affects other vegetables
and many wild hosts and ornamental plants. Thrips can transmit the disease from the wild
hosts. For control, eliminate weeds around field edges and turn rows; remove infected
plants when small, and control insects in the field. 
Curly Top
Causal Agent(s): (viral - viral)
Pronounced upward rolling and twisting of the leaflets that expose their under surfaces,
stiff and leathery foliage, and a peculiar dull yellowing of the entire plant are typical
symptoms of the disease. There is also some purpling of the veins and the plant is
usually very stunted. Very few fruits are produced after infection. The virus is not
transmitted through the seed or soil, nor is it spread by mechanical means. The main
vector is the beet leafhopper that becomes infected by feeding on wild or cultivated
plants having the disease. The disease is difficult to control. Keep field surroundings
free of weeds. Controlling insects may effect some control.