Diseases of Fruit Plantation Medicinal and Aromatic Crops

Lethal yellowing

Causal organism:Phytoplasma Like Organism

• Lethal yellowing is a pandemic disease of coconut palm which has destroyed thousand of palms in South Florida earlier and in recent years, this malady has become most common in' Jamaica.
  
• The rate of spread of 'Lethal Yellowing'' (LY) in Jamaica is similar to that of LY in Florida (McCoy, 1976).
  
• The term 'Lethal Yellowing' was first used my Nutman et al., (1955), to denote a specific disease of coconut palms in Jamaica and subsequently applied to diseases of identical symptomology in other countries.
  
• Parthasarathy (1974) found mycoplasma-like-organisms (MLO) in phloem tissues in Electron microscopic examination.
Symptoms

• The first symptom of lethal yellowing in mature coconut palms is the premature dropping of most of the nuts regardless in size.
  
• Next symptom is the necrosis of new inflorescence with blackening of tips and most of the male flowers are dead and thereby no fruit set on such flower stalks.
• Next, the lower fronds turn yellow which spreads gradually to younger leaves.
  
• However diseased yellowed leaves are found turgid and not flaccid as in case of root wilt disease.
  
• These leaves become yellow, ultimately turn brown, desiccate and hang down.
• Finally, the newly emerged spear leaf collapses and death of the terminal bud occurs resulting in fall of the top of the palm within 6 months after appearance of the symptoms. Lethal Yellowing spreads not only but it kills rapidly.


Epidemiology

• Neglected orchard
• Sucseptable host
• Summer season
• High vector population
• Source of inoculum
• Primary source of inoculum- Infected leaf and plants
• Secondary source of inoculum- Phytoplasma transmitted by leaf hoppers.
Linking MLO and Lethal Yellowing

• The presence of MLO within the phloem vascular tissue of coconut palms infected by LY was reported in 1972 by many workers.
  
• Firstly, VILO have consistently been detected in diseased but not in healthy palm tissue.
  
• Secondly, linking MLO to LY is the antibiotic response of diseased palms, particularly tetracycline.
  
• Thus, antibiotic treatments were developed as a disease management tool.
Management

• Rapid removal of infected palms will show the apparent rate of disease spread;
• Imported Malayan Dwarf palm, which has been found to be a resistant cultivar, can be replanted in diseased gardens.
• Stem injection of tetracycline group of anti-biotic would suppress symptoms development, as mycoplasma are known to be sensitive to tetracycline.
• Spraying of insecticides like diazinon or dimethoate twice in a week may reduce the vector (M. crudus) population substantially and thereby control LY to a great extent.