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.