Corynespora cassiicola (Berk. and Curt.)

Distribution

IRRDB surveys indicate that the disease is a significant, endemic problem needing regular control measures in Indonesia, Sri Lanka and Thailand. Sporadic, controllable outbreaks have been reported in India, Malaysia and one region in Brazil.

An outbreak of the disease occurred in Sri Lanka in 1986 which prompted the eradication of clone RRIC 103 which was at that time the most susceptible widely planted clone. The same clone planted on an experimental scale in Indonesia, Malaysia and Thailand was also seriously infected with the disease. The 1998 survey indicates that the disease is very serious in Cameroon, India, Indonesia, Malaysia, Sri Lanka. For results of 1998 Survey.

Symptoms (for greater detail)

C. cassiicola infects rubber leaves of various ages and the symptoms developed depend on the age of the leaves. On young yellowish green leaves, brownish small circular lesions with papery centres and surrounded by yellow halos are more prominent. The characteristic symptoms of the disease are seen on mature leaves whereby the lesions are large with dead papery centres often situated along or beside the main veins or smaller veins which are discoloured and form the railway track or fishbone appearance.

Leaf tissues surrounding the lesions turn yellowish to brownish red and the leaflets will drop-off. A leaflet can be infected by one or multiple lesions but one lesion, especially if it is sited on the main vein close to the leaf base, is sufficient to cause leaf defoliation. Young and old leaves can be defoliated. C. cassiicola also infects young shoots and leaf petioles causing brownish elongated bark splitting wounds and shoot dieback. The canopies of trees severely infected by Corynespora are sparse and the trees are stunted with dead twigs and branches. Trees suffering these protracted conditions will die.

Spread

Corynespora produces spores which are spread by wind or rainsplash. The conidia are found throughout the year. In Malaysia, the maximum number of spores are trapped during August to September with maximum liberation at about noon. A hot humid climate favours the disease.

Control

In laboratory tests, several fungicides (benomyl, prochloraz, chlorothalonil, propineb, mancozeb, captan) have been reported to inhibit growth of C. cassiicola. Limited field trials indicate that benomyl is the most effective. Good control is achieved only if the weekly spraying is extended for several months, as leaves of all ages are susceptible to the disease.

Chemical control would be economically prohibitive if repeated spraying rounds are necessary. The best strategy to manage the disease is to plant resistant clones. Clones which are known to be susceptible to the disease should not be planted in regions where the climate is suitable. The three most susceptible clones to Corynespora in Indonesia, Malaysia, Sri Lanka and Thailand are RRIC 103, KRS 21 AND RRIM 725. Other susceptible clones are: PPN 2058, PPN 2444, PPN 2447, RRIM 600, Gt.1, IAN 717, IAN 873, Fx 25.