PATH 372 Diseases of Horticultural Crops
PATH 372 Diseases of Horticultural Crops
PATH 372 Diseases of Horticultural Crops
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Tristeza affected trees look chlorotic and sickly in the early stages. Gradually the<br />
leaves drop and the defoliated twigs show die-back. The declining trees die<br />
gradually but sometimes apparently healthy trees die suddenly.<br />
Vein clearing or vein flecks (elongated translucent area) in young leaves <strong>of</strong> acid<br />
lime is seen intermittently when viewed against light (characteristic symptom).<br />
In sweet orange, the specific symptom <strong>of</strong> tristeza is honeycombing, a fine pitting<br />
<strong>of</strong> inner face <strong>of</strong> bark in the portion <strong>of</strong> trunk below the bud union. In acid lime<br />
which is highly susceptible to the disease, vermiform or linear pits appear in the<br />
woody cylinder.<br />
Tristeza infected citrus trees on sour orange rootstocks cause phloem necrosis at<br />
the graft union<br />
Diseased trees usually blossom heavily. Trees with stem pitting are stunted and<br />
set less fruits. The fruits are <strong>of</strong> smaller size and <strong>of</strong> poor quality (insipid fruits). As<br />
the fruits develop, the tree wilts partly or completely.<br />
Grapefruit and acid lime are susceptible irrespective <strong>of</strong> root stock.<br />
Pathogen: Tristeza is a highly flexuous filamentous virus 2000nm long and 10-12nm<br />
in width. The virus is restricted to only phloem tissue and is a member <strong>of</strong><br />
Closterovirus group. CTV has ss +ve sense RNA <strong>of</strong> 20 K nucleotides as its genome.<br />
Mode <strong>of</strong> survival and spread<br />
The disease primarily spreads through grafting and budding. Under field conditions, it<br />
is transmitted by the black citrus aphid, Toxoptera citricida. The virus is not seed<br />
borne.<br />
Management:<br />
Strict quarantine measures to be enforced<br />
Use certified budwood free <strong>of</strong> CTV<br />
Remove all diseased trees as and when the disease is noticed. Fresh plantings<br />
to be taken with virus free materials on tolerant rootstocks. For sweet orange and<br />
mandarin avoid susceptible root stocks.<br />
For Andhra Pradesh, Maharashtra and Karnataka, Rangapur lime is<br />
recommended as a root stock resistant to Tristeza. For the Punjab region,<br />
Jattikhatti, Cleopatra mandarin and sweet orange are recommended as resistant<br />
root stocks.<br />
For acid lime, use seedling preimmunised with mild strain <strong>of</strong> tristeza virus (Cross<br />
protection).<br />
Periodic sprays <strong>of</strong> insecticides like Monochrotophos 0.05 % reduce s the<br />
secondary spread <strong>of</strong> the disease in the orchard.<br />
7) Greening or Huanglongbin: Candidatus Liberibacter asiaticus (Fastidious Phloem<br />
limited Bacterium), Obligate gram negative bacterium<br />
7