Virology

Mosaic Of Tomato

Tomato mosaic can be caused by many different viruses such as TMV, Cucumber mosaic virus (CMV), Potato virus X, and Potato virus Y. the common tomato mosaic was earlier thought to be the same as TMV but now it is considered to be a distinct virus (ToMV) of the same tobamovirus group.

Symptoms Of Tomato Mosaic

The symptoms of tomato mosaic are generally influenced by temperature, day length, light intensity, plant age, virus strain, and tomato variety. In the tropic or in warm weather with long day length and high light intensity leaves show light and dark green mosaic mottle, sometimes with distortion of young leaves. Green area are sunken giving the leaf a rough appearance. In winter, with low light intensity, short day length, and temperature not above 20 C, plants often are severely stunted and leaves distorted to “fern-leaf” or tendril-shape but mottling may be slight. Seedling infection may kill the plants. Fruits are fewer, undersized, and often deformed. In some cases there is necrosis of stem, petioles, leaves and fruits.

The Causal Organism

The virus causing the common mosaic of tomato is known as Tomato mosaic virus (ToMV, tobamovirus group). There are many strains of the virus producing different symptoms and often these have been described as different diseases. These strains are tomato such a mosaic, tomato elation mosaic, yellow rings spot strain and tomato rosetted strain.

Particles of the virus are straight tubules with construction and measure 300 x 18 nm. The single stranded RNA constitutes about 5% of the particle weight. The particles occur in all the tissues including the pollen and seed but not in the embryo. The inclusion bodies appear as crystalline structures, amorphous masses, fine needles, fibrous spikes, spindle bodies, and amoeboid X – bodies.

The Disease Cycle

In tomato sap the thermal inactivation point is 85-900C, dilution end point is 1:100000 to 1:1000000. In air-dried tomato leaves the virus has been found to remain infective even after 24 years at laboratory temperatures. The sap retains infectivity for 77 days or more at laboratory temperatures and for several years at 0° to 2°C. The virus, like the TMV, is easily sap transmissible and is mainly transmitted by man through contact during cultivation. No natural vector are known. Seed transmission also occurs mainly as external seed contamination. TMV and ToMV become established in the seeds of tomato and bell pepper, irrespective of plant growth stage, at the time of inoculation. However, concentration of the virus is high in seeds from plants inoculated early. Dodder can transmit the virus. Diseased crop debris is also a source of primary inoculum.

Control

Use of virus-free seedlings is the most important step for control of tomato mosaic. To produce healthy seedlings the seedbeds should be those in which no solanaceous crop susceptible to TMV had been grown for the last 4-6 months. Soil sterilization by heat is also recommended. Seeds should be treated in hot water at 50°C for 25 min or with 20% trisodium phosphate solution. There should be an interval of at least 5 months between susceptible crops in the same filed. Field workers should avoid use of tobacco products while working in the field. If they remove a diseased plant they should wash their hands in soap solution.

Reference: Text Book Of Microbiology

Nihal Sharma

Assistant Editor

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