VIRAL DISEASES

Viral Diseases

LEAF CURL
Symptoms:
  • The major symptoms include chlorosis of leaflets and reduction in their size accompanied by curling inwards.
  • Significant reduction of nodes and internodal lengths occurs giving the plant a bushy appearance.
  • In advanced stages of infection severe stunting and partial to complete sterility occurs. Infected plants bear few or no fruits.
Pathogen:
  • The disease is caused by Tobacco yellow leaf curl virus (TYLCV) which belongs to the Gemini Virus group.
  • The particles are geminate, non-enveloped, 18 nm in diameter and genome consists of single stranded DNA.
Disease cycle and epidemiology:
  • The virus is transmitted through grafting but not through sap, contacts between plants or seeds. The sole agency of its transmission is the white fly Bemisia tabaci that transmits the virus in a persistent manner.
  • The virus has a wide host range including cultivated species, weed hosts and ornamentals.

TOMATO MOSAIC
Symptoms:
  • Mosaic is characterized by presence of dark and light green patches on the leaflets which may get distorted and puckered (Plate -3).
  • Malformation and reduction in leaf size may also occur. In case of early infection the plants are reduced in size and remain stunted.

3.3

Pathogen:
  • Tomato mosaic virus (ToMV) is a member of Tobamo virus group.
  • ToMV is an RNA-containing virus with straight rod particles about 300 x 18 nm.
Disease cycle and Epidemiology:
  • Virus transmission in the field either occurs through contact between plants and field implements.
  • The other modes of transmission are through sap and grafting but there is no vector involved.
  • The virus is externally seed borne. The infection occurs during transplanting and the virus has a wide host range.

SPOTTED WILT
Symptoms:
  • Plants show bronzing, curling, necrotic streaks and spots on leaves (Plate-4a).
  • Dark brown streaks also appear on leaf petioles, stems and growing tips.
  • The plants remain small and stunted.
  • The ripe fruit show pale or red or yellow areas on the skin (Plate-4b).
  • Sometimes affected plants are killed by severe necrosis.

3.4a


3.4b

Pathogen:
  • The disease is caused by Tomato Spotted Wilt Virus (TSWV), Tospo virus.
  • The virus particles are isometric, enveloped, and 85 nm in diameter.
  • The genome consists of single stranded RNA.
Disease cycle and epidemiology:
  • The virus is transmitted by thrips in persistent manner.
  • The virus is acquired only by intensive feeding of the larvae and transmitted by adults.
  • The virus has wide host range including bell pepper, lettuce, pea, tobacco, potato and large number of ornamental plant species.
Management:
  • The viral diseases of tomato can be managed effectively through use of virus free seed (ToMV).
  • Remove alternate hosts (TLCV, TSWV).
  • Use insecticides against the vectors and cultivation of resistant varieties / hybrids.
  • Seedling bed should be in isolation from ornamental plants and susceptible crops and the surrounding areas should be kept free from weeds.
  • Fine mesh netting may be useful for excluding thrips and other insect vectors.
  • Aqueous extracts of neem (3-5ml/litre water) are known to possess strong antiviral substances against ToMV.
Last modified: Friday, 2 March 2012, 6:16 AM