Vascular streak dieback is a serious threat in cocoa cultivation and cause cent per cent loss if left unprotected.
The disease is caused by air-borne fungal pathogen. The fungus ejects the spores into the air during night time and spread through wind.
The spores grow on fallen unhardened leaves through epidermis, mesophylls in the vein and grow down into the stem; the incubation period is 3 - 5 months before the first symptoms are visible.
Wetting of the surface by rain is essential and infection occurs only in wet weather condition. Lack of wind and high humidity limits the disease spread.
The first indication of the disease is yellowing of one or two leaves on second or third flush behind the growing tip. The diseased leaves fall prematurely within few days of turning yellow.
The youngest and oldest leaves remain intact while all the middle leaves have fallen the affected shoot is eventually dried and if unchecked the disease will extend back along the main branch resulting in progressive death of the plant.
The diseased stem, when split longitudinally a brown streaking of the wood with presence of fungus in the xylem vessels is very common. Streaking of the wood prior to death is the perfect diagnostic character of the fungus.
– Identification of severely diseased or dead plants uprooting and burning them immediately to avoid spread of the pathogenic spores.
–Prune the diseased branches about 30 – 40 cm below the end of visible streaking which delay the disease build up and remove the sources of fresh infection besides preventing further growth of the fungus on infected trees.
–Judicious within canopy pruning, improve aeration, sunlight penetration and reduce the disease spread.
– Apply bordeaux paste on the pruned area and spray 1 per cent Bordeaux mixture or copper oxychloride 0.25 per cent twice i.e. in May – June and again October as a prophylactic measure.
–Soil drenching with 0.25 per cent copper oxychloride (2.5g/ litre of water) around the plant.
M. Kannan & M. Ananthan
Horticultural Research Station, ThadiyankudisaiPerumparai, Dindigul, Tamil Nadu