Diseases of fruit plantation medicinal and aromatic crops (2+1)

Mahali/koleroga

Causal organsim: Phytophthora meadii (=P. arecae)

In areas receiving heavy rain fall, mahali (heavy devastation) or koleroga (kole=rotting, roga =disease) is most dreaded disease. Butler in 1906 first recorded this disease from Mysore. Lesli,C.Coleman (1910’s) who worked extensively on this disease and developed management practices to manage the disease which are still working satisfactorily.

Symptoms

• Characteristic symptoms include rotting and excessive shedding of immature nuts from the trees.
• The first sign of the disease is on the nuts, on which water soaked lesions usually develop towards the base. Because of this watery rot, the disease is locally called as “Neerugole”.
• Later leads to discoloration, discoloration starts browning then leads to deep browning, later dropping of nuts takes place..
• The fallen nuts show the felty, white mass of mycelium of the fungus which soon envelops the entire surface. Infected nuts loose their luster. This type of symptoms with boost like growth of the fungus , as also locally called as “Busurugole”.
• The disease gradually spreads among the bunch ultimately covering the entire bunch wherein they rot and shed from the bunches.
• Fruit stalks and rachis of inflorescence are also affected. They are lighter in weight which deteriorate and become unsuitable for chewing.
• Very often, the top of the affected trees may also dry resulting in withering of leaves and bunches.


Etiology

• The mycelium is inter or intra - cellular, coenocytic but forms septa in older stages.
• Haustoria are finger like, occasionally branched and sparsely produced.
• The sporangiophores are irregularly branched and the sporangia are pyriform to elliptical.
• Oospores are the sexual spores which are also act as dormant spores that survive for about 4-5 years.
  
Mode of spread and survival

• Rain and wind plays an important role in the initiation and spread of the disease since low temperature and high humidity are favorable for the growth of the fungus.
• Intermittent rains with alternative sunshine are more conductive to a rapid spread of the disease.
• The close plantation of trees and plant grown in valleys gives ideal conditions for spread. Formation of oospores in the diseased nuts at the end of the season and dormant mycelia present in cracks and crevices of the tree helps the pathogen to perpetuate from season to season.
• The fungus also infects potato tubers, apple and cocoa fruits, fruits of Citrus nobilis, Artocarpus incisa and Agave wightii.
  
Epidemiology

• The optimum temperature favourable for the fungus is 18-22^oC
• Relative Humidity of more than 95% is congenial for the development of disease.
• Sporangia of the fungus have been caught successfully in aeroscope slides at a height of 10-11m .
• The fungus infects the nuts readily but not the older nuts. Heavy rainfall, wind, low temp ,alternate sunshine and rainfall favours the disease.
Life cycle
Management

• Cleanliness and field sanitation including destruction of diseased tree tops and plant parts are the foremost important steps in the management of the disease. Fallen infected nuts should be collected and burnt/destroyed.
• Spraying with Bordeaux mixture (1.0%) is recommended. Vegetable oils from ground nut, sesame, coconut or sunflower added to Bordeaux mixture before spraying also protect the palm from this disease.
• Prophylactic sprays with Bordeaux mixture (1%) spray before the onset of south west monsoon and a second application 40-45 days later has been recommended with Metalaxyl MZ (0.2%).
• Cover the fresh bunches after first spray with polythene bags to avoid new/further infections.
• While spraying, the whole crown region of the tree should be covered.