Plant Viral Diseases

 

Tomato Yellow Leaf Curl Virus (TYLCV) is a major problem on tomato. Once the disease has occurred, it will spread fast and drastically reduce the yield.

Symptoms of infection yellowing of young leaves, upward and downward leaf curl , stunting, bushy appearance, and flower drop, which occurs prior to fruit set and dramatically reduces fruit yield.

Affected plants tend to be distributed in isolated patches. This virus can cause significant yield losses from 80-100%

Pathogen

TYLCV is of the genus Begomovirus and family Geminiviridae.

Leaf curl & Interveinal and marginal chlorosis on young tomato leaves infected with TYLCV

Host Plants

TYLCV predominately causes disease in tomatoes. It can infect other hosts in the Solanaceae family (pepper, eggplant, potato, tobacco, jimsonweed), as well as common bean (Phaseolis vulgaris L.), and ornamentals including petunia and lisianthus. In the absence of symptoms, these hosts may serve as a reservoir of the pathogen.

Signs and Symptoms

On tomatoes, the primary symptoms of TYLCV are interveinal and marginal chlorosis of young leaves, an overall crumpled appearance of the leaves, and upward and downward leaf-curling. Plants infected with TYLCV will also be stunted in height and appear bushy due to shortened internode length. Flower drop will occur with an accompanying reduction in yield. When young plants are infected, the disease may be so severe that no fruit is produced. Symptoms can take up to three weeks to develop after infection.

  

Look-alike conditions

Initial symptoms of TYLCV can resemble other disorders that affect tomatoes, including potassium deficiency, magnesium deficiency, micronutrient deficiencies, and leaf curl. In general, any of these nutrient deficiencies are likely to be consistent across the entire crop whereas plants infected with TYLCV will be patchy across the field and not uniform. This pattern may be used to help diagnose the problem. TYLCV differs from symptoms of nutrient deficiencies in that the marginal and interveinal chlorosis first appears on young leaves. Curling of new leaves may also be caused by excessive aphid feeding or other viruses. It is highly recommended to confirm the diagnosis by a plant disease clinic or extension specialist since there are several lookalike diseases and disorders.

Disease Cycle and Epidemiology

TYLCV is transmitted by adult whiteflies (Bemisia tabaci). A whitefly can acquire the virus within their salivary glands after 15-30 minutes of feeding on an infected plant. After a latent period (time between acquisition and transmission of virus) of ~6 hours, the whitefly is able to transmit the virus to another plant within 15-30 minutes of feeding. Adult whiteflies can retain and spread the virus for several weeks after initially feeding on infected plants and can travel over distances of at least 10 miles.

The virus has been detected on seed collected from infected plants. Yet, the virus survives only on the surface of the seed and is not transmissible to seedlings following seed-surface disinfestation. 

Following infection it may take up to three weeks for symptoms on tomato to appear due to the movement of symptomless infected plant material spreading the disease over long distances. A high population of white flies in mature stages of field-grown tomato production may serve as an inoculum source for younger plantings nearby.

Bemisia tabaci

 General Disease Management

• ·       Use disease resistant cultivars
• ·       Destroy old infested crop after harvest.
• ·       Raise barrier crops – cereals (3 rows of sorghum, cumbu and maize) around the field.
• ·       Remove weeds as these may harbour the virus.
• ·       Adopt polythene mulching technology during planting to control weed hosts.
• ·       Physically protect the nursery from the vector, e.g. net house or green house.
• ·       Keep yellow sticky traps to monitor the white fly.
• ·       Spray insecticides to control the vector.

Banana Bunchy Top

Banana bunchy top is a viral disease caused by a single-stranded DNA virus -banana bunchy top virus (BBTV), of the family Nanoviridae. It is aphid transmitted and infects banana plants and other crops. It was first identified in Fiji in 1879.

BBTV was named after the symptoms seen, where the infected plants are stunted and have "bunchy" leaves at the top. The disease is transmitted from plant-to-plant in tropical regions of the world by banana aphids, which is an important factor in control of the disease.

There are no resistant varieties, so controlling the spread by vectors and plant materials are the only management methods.

Host

Banana bunchy top disease affects the banana fruit and foliage. BBTV can infect species of the family Musaceae, which includes bananas, plantains, abaca, and more. The aphids also feed on Heliconia and flowering ginger.

Symptom

The pathogen causes cytopathological effects in the phloem tissue, due to damage of the host cells by the virus. The name of the disease is due to the symptom in older plants, in which the new leaves that are produced are narrower than normal, yellow, and flat, which causes a "bunchy" appearance at the top of the tree. 

If any fruit is produced, which is unusual, it will be deformed. One of the most distinctive symptoms is "Morse code streaking" causing irregular spots and dashes on the leaves which are easier to see when the waxy coating over the petiole is rubbed away. The infected cells die and are lighter in color hence this appearance.

Initially, dark green streaks appears in the veins of lower portion of the leaf midrib and the leaf stem. They appear to be “bunched” at the top of the plant, the symptom for which this disease is named. Severely infected banana plants usually will not fruit, but if fruit is produced, the banana hands and fingers are likely to be distorted and twisted. 

 

 

 Disease cycle

BBTV is the sole member of the genus Babuvirus in the family Nanoviridae.

It is known that Banana aphid (Pentalonia nigronervosa) transmits the virus from infected to healthy plants by feeding. Aphids feed on the plant phloem tissues by injecting their thin, flexible stylet into the epidermis of the plant tissue until it reaches the phloem of the leaves. Then the aphid injects saliva, sucks the cell contents and introduces virus in the process.

Vector transmission of the BBTV is non-propagative, and the virus does not replicate within the aphid’s midgut. Acquisition of the virus by the banana aphid requires about 18 hours of feeding and then the aphid can retain the virus for approximately two weeks. It takes about a month for the BBTV symptoms to appear after infection.

 To infect, the carrier aphid can feed on the banana plant for as few as 15 minutes, but more often a couple hours, as the longer feeding time will increase the odds of transmission. The suckers produced on infected plants will also be diseased thus the disease can spread from year to year.

Banana aphids also have the capability to feed on Heliconia and flowering ginger; however, these alternate hosts of the aphid vector are not hosts of the virus. The ability of banana aphids to feed on alternate hosts is important to keep in mind when attempting to control the virus.

 Management

• There are no resistant varieties of banana against BBTV, so the most common method of control is chemical control of the aphid vectors.
• Another way to help control the virus is to remove and destroy any infected plants before the virus can spread, which is a practice known as roguing.
•  Quarantines are also implemented to prevent the import of any potentially infected plant materials
• Fruit is not often produced on infected plants, but if it is, the fruit will be deformed, which easily identifies if there is any virus present in the fruits to comply with quarantine regulations.
• Since bananas are not the only host, the alternate hosts for both the virus and the aphid must also be monitored for disease, and sprayed with pesticides to control the aphids more.
• When planting at the beginning of the season, the seed material or suckers should be obtained from BBTV free areas of the world or from cultures that are grown and developed to be free of the virus.
• Control of banana bunchy top is achieved by killing the banana aphids then destroying all infected material. First, the aphids should be killed on the infected banana material, and then all the plant material should be destroyed to prevent the spread of the virus. 

Importance

Banana bunchy top disease is the most serious virus disease of banana worldwide. Diseased plants rarely produce fruit and when they do, the fruit is stunted and twisted.

BBTV certainly has a huge impact on the industrial scale of banana production and affects the livelihood of farmers.

Once established, it is very difficult to eradicate and manage the disease. 

First of all, the disease is caused by a vector-transmitted virus and this virus is not completely understood yet. Secondly, all bananas are susceptible to the disease and no resistant varieties have been discovered or made commercially available. Lastly, the control methods are quite demanding, including chemical treatment for the aphid vectors, removal of all infected tissue, quarantining plants and monitoring alternate vector feeding sites.

Fungal Plant Diseases- Red rot of sugarcane, Early blight of potato

 Red rot:

Organism: Glomerella tucumanensis/ Colletotrichum falcatum

Red rot disease is caused by the fungus Glomerella tucumanensis, previously called Colletotrichum falcatum. This is one of the most severe of the known diseases of sugarcane. It is widely distributed throughout the sugar­cane-growing countries of the world.

 

Symptoms

• Infection in the stem is internal and the presence of the disease is not visible externally.
• Upon splitting a diseased cane during the early stages of the disease, it will be found that the fibro-vascular bundles near the base are reddish in colour. A gummy dark-red material oozes out of the cells filling the in­tercellular spaces. The soluble pigment present in this ooze, is absorbed by the cell wall producing the characteristic red rot appearance.

·       As the disease advances the red colour spreads to the surrounding tissues and irregular discoloured blotches are formed, which may be reddish or yellowish or white with red margins.

• When the infection becomes old, the blood-red colour changes to straw colour. The stem shrinks and split cane gives sour smell and shows red tissue with white cross-bands.
• Sometimes, the pith inside the cane is filled with blackish brown liquid and exhibits alcohol odour.
• About this time the upper leaves of the stem turn pale and gradually droop down. Ultimately the entire plant withers and droops down. The first external evidences of disease are the drooping, withering, and finally yellowing of the upper leaves, followed by wilting of the entire leaves, and finally the entire plant.
• The external symptoms appear only after16 - 21 days after infection and entire cane dries off in  10 days time.

 

In areas where the disease appears in a severe epidemic form, the en­tire crop withers and droops resulting in a complete loss of crop. Though the fungus attacks all parts of the host above ground, stems and midribs of leaves are more susceptible to fungal attack.

Reddish lesion on leaf midrib  Orange to yellow leaf         Inner reddish region

Management Removal of the affected plants at an early stage and soil drenching with Carbendazim The cut ends and entire plant should be dipped in a fungicide solution (Bordeaux mixture) If the disease is noticed in the field, the leaves and canes should be collected and destroyed by burning. Treatment with Carbendazim before planting should be done Use fungitoxic chemicals like Bavistan, Benomyl, Topsin and Aretan for complete elimination of rot infection. Plant healthy crops in a disease- free area. The red rot affected field must be rotated with rice for one season and other crops for two seasons. Growing of recommended resistant and moderately resistant varieties

Red rot of sugarcane is hard to control because the stalk from which seeds are prepared has been largely affected from the time of planting, and fungicides cannot reach the infected tissues inside a diseased seed sett. Therefore careful selec­tion of red rot-free seed setts is recommended for planting. Seed should always be taken from disease-free nurseries and examined regularly. Before planting, it should be carefully examined and those which show reddening should be discarded.

 

The spread of the red rot can be prevented during the growing season by timely inspection and burning of the affected clumps followed by sanitation. Hot water treatment of seeds and treating seed with fungicides is often effective. The use of sugarcane varieties resistant to red rot is also recommended. The possibilities of an epidemic is very much minimized with the practice of long crop rotations (2 to 3 years) where planting is done in plots. One of the best ways to reduce the incidence of the disease is to raise healthy stock for planting in plots especially fertilized, cultivated, and kept disease-free by constant care.

 

Early blight of potato

Organism–Alternaria solani,

 

Early blight is a common foliage disease of potato and tomato. It is of common occurrence both in cold as well as in warm regions in India and abroad where, potatoes and tomatoes are grown. The pathogen causes injury to the leaves as a leaf spot disease and lead to premature defoliation. Finally it causes tuber rot of potato and fruit rot of tomato.

 Symptoms: 

• The disease appears on the leaflets, 3-4 weeks after the crop is sown as small, isolated, scattered pale brown to dark spots, oval or angular in shape mostly up to 3 or 4 mm in diameter.
• The chlorotic zone increases with the increase in size of the spot. As a rule the oldest (lowest) leaves are affected first and the disease progresses upwards.
• The necrotic tissue of the spot often shows a series of concentric ridges, a symptom characteristic of this disease
• The number of spots on the leaflets may be a few but if the conditions are favourable the spots increase in number and size involving the entire leaf surface.
• In the advanced stage when the number of spots is numerous, the leaf shows signs of old age and droops.
• The spots become hard in dry weather and the leaflets curl. Under humid conditions, the diseased areas coalesce and big rotting patches appear on the leaf surface.
• In severe cases of infection the leaves dry up, shrivel and drop off.
• Falling of leaves starts with the older (lower) ones until a few remain at the top. 
• Stems and petioles may also develop brown to dark lesions which may finally lead to collapse of the entire over-ground portion of the plant.
• Further, there isinfection and rotting of potato tubers. The surface lesions on the potato tubers are a little darker than the healthy skin and slightly sunken. They are irregular or circular in shape. The older lesions develop fissures.
• The injury to the foliage may lead to premature defoliation reduces photosynthetic activity of the plant. Consequently the tubers formed remain small and a few in number. This reduces crop yield. The disease alos result in rotting of tubers.

 

Symptoms on Leaves: The disease can be seen as small, scattered, pale brown spots on the leaflets. These spots are covered with growth of the fungus.

Necrotic Area: In the necrotic area of the spots concentric ridges develop to produce a characteristics board effect.

Target Board: ‘Target Board’ is the most characteristics symptom of early blight of potato.

Vein Chlorosis: Later, vein chlorosis may also occur.

Curling of Leaves: In dry weather the spots turn hard and the leaves curling can be seen.

Rotting of Leaves: In humid weather big totting patches appear on leaf surface.

Severe Attack: Leaves shrivel and fall off. Stems show brown to black necrotic lesions on the skin. Rotten plant may collapse.

 

Transmission

 

Early Blight of Potato is very common disease of potato in India. The occurrence of the disease can be seen few weeks after sowing the tubers. But, the damage to the crop is more during the tuber formation. The disease can be seen both in the cool and warm areas. Farmers of hills and plains, both face this serious disease.

·       The fungus remains viable in infected leaves for many years.

·       Conidia and mycelium survive in the soil on plant debris to cause primary infection to next crop.

·       Contaminated tubers also act as a source of primary infection. 

• Climatic or soil conditions which are unfavourable to the host and reduce its physiological vigour tend to promote the disease.
• The disease spreads and becomes serious when the season begins with abundant moisture followed by high temperatures because these conditions are unfavourable to the host.
• Weaker plants are more susceptible to disease than the healthier ones.
• High soil fertility tends to reduce the severity of disease.
• Periods of continued drought also check its spread.

 Management

 

·       As in the case of other soil borne diseases, crop rotation and field sanitation are the two rational measures which provide effective check against primary infection from spores in the infected plant debris.

·       Infected dry leaves and dead plants should be burned immediately after harvest.

·       Application of regular sprays (with Bordeaux mixture or other suitable fungicides) throughout the period of plant growth effectively control the early blight.

 

 

Bordeaux mixture is a fungicide made of copper sulphate and calcium hydroxide (lime). Bordeaux mixture—a combination of copper sulfate, lime, and water—is an effective fungicide and bactericide used for decades to control diseases of fruit and nut trees, vine fruits, and ornamental plants.

It is widely used to control potato blight, peach leaf curl and apple scab.

Bordeaux mixture has copper ions (Cu²⁺) which inhibit fungal enzymes and prevent germination. It is used preventively, before the fungal disease occurs. It is sprayed on the plants usually once a year.

Because the copper ions build up in the soil, continuous use will cause heavy metal pollution. Copper also bioaccumulates in organisms. So its use is restricted in some countries

 

Fungal Diseases - Fusarium Wilt, Wilt of cotton

 

Diseases are a major limiting factor for crop production. Diseases can be caused by infectious microorganisms that include fungi, bacteria, viruses and nematodes. These diseases are contagious and can spread from plant to plant in a field, often very rapidly when environmental conditions are favorable.  

1) Fusarium Wilt

Organism - Fusarium oxysporum f.sp. lycopersici 

Wilt diseases are caused by pathogens that invade the vascular system (xylem tissue) and disrupt water flow through the plant. Fusarium wilt is the major wilt disease of tomato.  This is a warm-weather disease caused by the fungus Fusarium oxysporum.

Symptom

• The first symptom of the disease is drooping and wilting of lower leaves with a loss of green colour (chlorosis)
• Affected leaflets wilt and die. The younger leaves may die in succession and symptoms continue in subsequent leaves.
• Often leaves on only one side of the stem turn golden yellow at first.
• The disease progresses up the stem. At later stage, browning of vascular system occurs.
•  The stem of wilted plants shows no soft decay, but when cut lengthwise, the lower stem will have a dark brown discoloration of the water-conducting vessels.
• Soon the petiole and the leaves droop and wilt until all of the foliage is killed and the plant becomes stunted and dies.

Transmission

• The fungus is soil-borne and passes upward from the roots into the water-conducting system of the stem. Blocking of the water-conducting vessels is the main reason for wilting.
• The fungus survives and persists indefinitely in field soil.
• Invasion occurs through wounds in roots growing through infested soil.
• Long-distance spread is through seed and transplants.
• Soils become infested by planting infected transplants and from movement of infested soil by wind and water erosion or on farm implements.

Prevention & Treatment:

• The affected plants should be removed and destroyed.
• For control, grow plants in pathogen-free soil
• Use disease-free transplants
• Grow only cultivars with resistance to Fusarium wilt. Growing tomato varieties resistant to Fusarium wilt is the most effective means of control.
• Crop rotation with a non-host crop such as cereals is also an effective control method
• Spot drench with Carbendazim (0.1%)

 

2) Wilt of cotton

Organism - Fusarium oxysporum f.sp. vasinfectum & Verticillium dahlia

 

Vascular wilts, such as Fusarium wilt and Verticillium wilt, are capable of significantly reducing cotton yields, and negatively impacting fiber quality. Properly diagnosing these diseases is critical in developing a management system. While subtle differences can be observed in the field, laboratory examinations are often required to differentiate the two diseases. Variety selection is the most effective factor in reducing wilt associated losses.

Fusarium wilt

Causal agent: Fusarium oxysporum f. sp. vasinfectum (Fov)

Disease development depends on host susceptibility and pathogen - infection by Fov is more if root-knot nematode (Meloidogyne incognita)  infection is present.  Disease occurs in warmer temperatures and sandy soils.

Fusarium can cause severe symptoms in susceptible cotton varieties. These include a general wilt and yellowing and necrosis of lower leaf margins. The vascular system of infected plants is discolored brown in affected portions of the tissue. This is most apparent in the lower stem and upper taproot. The discoloration starts in the taproot, spreads into the stem, and is generally continuous in contrast to the speckling nature of the discoloration in plants affected by Verticillium wilt.

Symptoms

occur throughout the growing season

• Initial symptoms consist of chlorosis and wilting on the margin of the leaf

• Infected seedlings often die, and may be confused with seedling disease. In seedlings and young plants, cotyledons and leaves wilt, may turn necrotic, and even fall off the plant, resulting in bare stems.

• Diseased plants exhibit a continuous discoloration of the vascular tissue

· Seedlings of susceptible varieties often die and in mildly affected plants, lower leaves develop symptoms but plants survive, but with reduced vigor and noticeable stunting.

• Fusarium wilt occur more if root-knot nematode infection is present. Certain strains of the causal fungus only cause symptoms when plants are also infected with the root knot nematode. In those cases, galls are usually prevalent on lateral roots.

 

 

Transmission

·       The fungus sustains itself on the outer surface of roots of many crops and weeds and survives indefinitely in soil.

·       The pathogen is seed-borne in cotton, causing long-distance spread,

·      Is also spread whenever infested soil is transported on boots, farm equipment, in flood irrigation, etc.

 

Verticillium wilt

Organism: Verticillium dahliae

Disease development depends on host: variety, developmental stage, and plant density, pathogen: defoliating vs. non- defoliating type, aggressiveness, and inoculum density. Occurs in  cool, wet environments

Symptoms

• Initial root infections occur early; symptoms are more evident post-bloom

• Infected plants may be wilted or stunted

• Chlorosis or necrosis of leaf margins and interveinal areas may be

• Light to dark brown discoloration (speckled) is prominent in the vascular system

• Severe defoliation can result, leading to reductions in yield and fiber quality

        

Management of cotton wilt

• Proper identification of pathogen is required. Sample infected plants for presence of  Fov / Verticillium

• Use pathogen-free seed produced in disease-free fields at all times

• Assay soil for root-knot nematodes- Nematode risk level will influence management options-  Use of nematicides will indirectly lead to less Fusarium wilt damage. Control of the root knot nematode is important to managing Fusarium wilt caused by most genotypes of the causal fungus. Nematicides, root knot nematode-resistant varieties or both are often necessary in fields infested with the nematode

• Rotation to any crop other than cotton prevents an increase in the soil population of Fusarium but may not significantly reduce the number of spores in the soil. The fungus will sustain itself on the roots of most plants, including weeds (without causing any symptoms), and cannot be eliminated by crop rotation alone.

• Remove infected plants. Use disease resistant varieties/cultivars for several seasons to reduce wilt severity in subsequent years

• Any field operation that moves soil from one location to another can spread spores of the fungus and introduce it to other fields. Washing soil from equipment with pressurized water will help limit the spread of Fusarium

• Other containment options include restricting traffic in affected patches, destroying affected plants and surrounding non-symptomatic plants, and stopping irrigation of affected patches in order to prevent movement of infested soil.