PATH 372 Diseases of Horticultural Crops

Tristeza affected trees look chlorotic and sickly in the early stages. Gradually the
leaves drop and the defoliated twigs show die-back. The declining trees die
gradually but sometimes apparently healthy trees die suddenly.
Vein clearing or vein flecks (elongated translucent area) in young leaves of acid
lime is seen intermittently when viewed against light (characteristic symptom).
In sweet orange, the specific symptom of tristeza is honeycombing, a fine pitting
of inner face of bark in the portion of trunk below the bud union. In acid lime
which is highly susceptible to the disease, vermiform or linear pits appear in the
woody cylinder.
Tristeza infected citrus trees on sour orange rootstocks cause phloem necrosis at
the graft union
Diseased trees usually blossom heavily. Trees with stem pitting are stunted and
set less fruits. The fruits are of smaller size and of poor quality (insipid fruits). As
the fruits develop, the tree wilts partly or completely.
Grapefruit and acid lime are susceptible irrespective of root stock.
Pathogen: Tristeza is a highly flexuous filamentous virus 2000nm long and 10-12nm
in width. The virus is restricted to only phloem tissue and is a member of
Closterovirus group. CTV has ss +ve sense RNA of 20 K nucleotides as its genome.
Mode of survival and spread
The disease primarily spreads through grafting and budding. Under field conditions, it
is transmitted by the black citrus aphid, Toxoptera citricida. The virus is not seed
borne.
Management:
Strict quarantine measures to be enforced
Use certified budwood free of CTV
Remove all diseased trees as and when the disease is noticed. Fresh plantings
to be taken with virus free materials on tolerant rootstocks. For sweet orange and
mandarin avoid susceptible root stocks.
For Andhra Pradesh, Maharashtra and Karnataka, Rangapur lime is
recommended as a root stock resistant to Tristeza. For the Punjab region,
Jattikhatti, Cleopatra mandarin and sweet orange are recommended as resistant
root stocks.
For acid lime, use seedling preimmunised with mild strain of tristeza virus (Cross
protection).
Periodic sprays of insecticides like Monochrotophos 0.05 % reduce s the
secondary spread of the disease in the orchard.
7) Greening or Huanglongbin: Candidatus Liberibacter asiaticus (Fastidious Phloem
limited Bacterium), Obligate gram negative bacterium
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