Tomatoe Diseases Essay, Research Paper
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.
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.
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.
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.
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
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
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.
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.
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.
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.
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.
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
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.
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.