Diseases Caused by Virus in Plants (With Control Measures)

The following points highlight the four main diseases caused by virus in plants. The diseases are: 1. Tobacco Mosaic 2. Cucumber Mosaic 3. Yellow Vein Mosaic of Bhindi 4. Little Leaf of Brinjal.

Disease # 1. Tobacco Mosaic:

This plant disease is caused by Tobacco Mosaic Virus (TMV). It is known to occur in all tobacco growing countries of the world. It was first described in detail by Adolph Mayer in 1886. Tobacco Mosaic Virus specially infects tobacco and members of family Solanaceae. The virus may affect more than 150 genera of herbaceous dicotyledonous plants including many vegetables, flowers and weeds.

Symptoms:

It produces mosaic like symptoms on plants. The symptoms on the healthy plant appear after ten days of infection. The first symptom is light green coloration between the veins of the young leaves. This is quickly followed by the development of mosaic or mottled pattern of light and dark green areas in the leaves (Fig. 1).

Leaves on infected plants are often small, curled and puckered. Symptoms on plants include chlorosis, curling, mottling, dwarfing, distortion and blistering of leaves. Mosaic does not result in plant death, but if infection occur early in the season, plants are stunted.

Etiology:

Causal organism

Genus: Tobamovirus symptoms.

Species: TMV

Species TMV named after Tobacco Mosaic Virus.

It is the first virus to be discovered by Iwanowski in 1892. It is a rod shaped virus. The virion measures 18 x 300 nanometers and weighs 39 million Daltons. The capsid is made from 2130 protein subunits (each subunit consists of 158 amino acids) and one molecule of genomic single stranded RNA. The ssRNA consists of 6,400 nucleotides.

Disease Cycle:

It is the most persistent plant virus. It has been known to survive 50 years in dried plant parts. The most common sources of virus inoculum for TMV are the debris of infected plants that remain in the soil and certain infected tobacco products which contaminate workers hand.

It is also transmitted mechanically by vegetative propagation of plants, grafting, seeds, pollens and being carried on the mouth parts of chewing insects. Once the virus enters the host, it begins to multiply by inducing cells to form new virus.

It takes over the metabolic cell processes resulting in abnormal cell function. Its RNA direct the synthesis of viral protein. After the RNA and the protein is produced, they undergo self assembly. After the assembly they are released outside the cell after its lysis.

Control Measures:

There are no known chemical treatment used under field conditions that eliminate viral infection from plant tissues once it occurs.

However, some important control measures to check the infection are:

i. Discarding infected plants.

ii. Growing virus free plants.

iii. Propagate plants via seeds rather than vegetatively.

iv. Crop rotation.

v. Growing of resistant strains.

vi. Removing of all weeds during and after the growing season.

vii. Disinfecting tools by placing them in boiling water for 5 minutes and washing with a strong soap or detergent solution.

viii. Discouraging use of tobacco by workers.

ix. Encouraging the practice of washing hands by workers with soap and water before and after handling plants.

Disease # 2. Cucumber Mosaic:

This disease is known to occur world wide in both temperate and tropical regions. The virus was first found in cucurbits (cucumis sativus) showing mosaic symptoms and since then it is called as cucumber mosaic. The Cucumber Mosaic Virus (CMV) has a wide range of hosts. It attacks approximately 1200 species of 100 families and causes significant losses in many vegetable and horticultural crops.

Symptoms:

Symptoms occur when the plants are about 6 weeks old and growing vigorously. Seedlings are seldom attacked. Four to five days after infection, the young leaves become mottled, distorted, wrinkled and then edges begin to curl downward forming rossette like clump near the ground (dwarfed appearance).

Older leaves develop chlorotic and then necrotic areas along the margins which spread laterally over the entire leaf. Fruits (cucumbers) develop pale green or white areas intermingled with green bumpy areas (often called white pickle). Infected fruits are oddly shaped and appear gray. They often have bitter taste and after picking become soft and spongy.

Etiology:

Causal organism:

Genus: Cucumovirus

Species: Cucumber Mosaic Virus

It is a linear positive sense single stranded RNA (ssRNA) virus. The CMV genome consists of three viral strands messenger-sense RNA molecules designated as RNA1, RNA2 and RNA3. Each RNA is enclosed with a protective protein coat with each being a distinct, single, spherical shaped particle.

Disease Cycle:

Perennial, biennial and annual weeds harbour CMV in roots, tubers and underground parts. These are important sources of virus transmission. The virus is also transmitted from infected plants to healthy plants by aphids (Aphis gossyii, Myzus persicae etc.), mechanical methods, sap, seeds, clothes and hands of the workers. It enters the host cell and replicates in the cytoplasm. The movement through cell to cell occurs by plasmodesmata or phloem.

Control Measures:

1. Development of genetic resistance in cucumber species.

2. Eradicate of infected plants and perennial weeds.

3. Using trap crop method (growing resistant varieties around the perimeter of the field and to place the susceptible plants in the middle).

4. Use of insecticides to control aphid population.

5. Crop rotation.

Disease # 3. Yellow Vein Mosaic of Bhindi:

Yellow vein mosaic of bhindi (Abelomoschus esculents) or vein clearing of bhindi most devastating disease in all the bhindi growing regions of India. In case the plants get infected at early stages of development it causes 80% of crop loss.

Symptoms:

The diseased plants can be recognised from a distance due to the yellowing of entire network of veins Fig. 2. The characteristic symptoms of the disease are the homologous network of yellow veins enclosing islands of green tissue within. In severe cases entire leaf become chlorotic.

Infected plants stunted and bear very deformed and small, yellow green fruits. Distortion of leaf stalks and stem occur at the advance stage of infection. The disease cause heavy loss in yield, if the plants get infected within 20 days after germination.

Etiology:

Causal organism: Begomovirus

Or

Bhindi yellow Vein Mosaic Virus

Disease Cycle:

The disease is transmitted by white fly Bemisia tabaci. The population is high during hot summer months, the crop is seriously affected then. The virus also survives on various weeds growing along the roadside for e.g., Croton sparsifolia, Ageratum etc.

Control Measures:

i. The vectors that are responsible for the spread of virus need to be controlled by spraying dimethoate 0.03 percent or monocrotophos 0.05 at 10 days intervals, (spraying must be done at late hours).

ii. Foliar spray of 5 to 10 ml neem oil in a litre of water at weekly intervals.

iii. Removing and destroying disease affected plants from crop fields to avoid secondary spread.

iv. To destroy the host weeds such as croton.

v. Crop rotation.

vi. Use seeds collected from disease free plants.

vii. Growing resistant varieties for e.g., Akra anamia, Akra Abhay. Punjab Padmini etc.

Disease # 4. Little Leaf of Brinjal:

This disease of brinjal was first reported in India in 1938. It was first considered a disease caused by virus. However, in 1969, it was attributed to a Mycoplasma-like organism. As far as known this disease occurs in India and Sri Lanka only. It is a serious disease of brinjal and can cause 100% yield loss in diseased plants.

Symptoms:

The characteristic symptom of the disease is the smallness of the leaves. Petioles get shortened. Lamina of the leaves becomes soft and pale yellow. Internodes of the top branches are shortened resulting in a bushy appearance of the affected plant. Mostly there is no flowering but if flowers are formed they remain green. Fruiting is rare in affected plants. However, if any fruit is formed, it becomes hard, tough and fails to mature.

Etiology:

Pathogen: Phytoplasma:

Mycoplasma like bodies (MLB) have been detected in phloem cells of roots of brinjal plant affected with little leaf disease. The size of MLBs varies from 230 nm to 770 nm. Each MLB contains ribosomes and nuclear material suspended by 16.5 nm wide tripple layered unit membrane.

Disease Cycle:

It is a sap transmissible disease. It occurs in nature of Datura fastuosa, Vinca rosea etc. The disease is transmitted by the insects (leaf hoppers), Cestius phycytis (Eutettix phycites) and Empoosco devostolls. The pathogen also survives on weed hosts.

Control Measures:

1. Fort-nightly spray of the insecticides such as Ekalox or Folidol till the fruit set helps to check the spread of disease.

2. The insect vectors can also be controlled by spraying the crop by Dimethoate (Rogor-30 EC) or Oxydemiton methyl (Metasystox 25 EG) or Monocrotophos (monocil) @ 1 ml per litre of water.

3. Disease can be controlled when phorate @ 1.0 kg/ha is applied to the seed beds and seedlings are dipped in acqueous solution of 0.05% tetracyline along with 0.05% monocrotophos.

4. Growing of disease resistant varieties such as Brinjal round, Black beauty, Pusa purple cluster etc.

5. Use of barriers of trap crops.

6. Sowing can be adjusted to avoid main flights of the insects.

7. Crop rotation.

8. Early removal of infected plants.