CASSAVA CYANOGENIC POTENTIAL AND RESISTANCE TO PESTS AND DISEASES

Cyanogenesis, the ability to generate hydrogen cyanide (HCN), is taxonomically widespread in the plant kingdom; over 1000 plant species are reported to produce cyanogenic glucosides in variable concentrations. Cassava (Manihot esculenta Crantz), a highly cyanogenic species, originated in the Neotropics, where many farmers and indigenous peoples prefer to cultivate varieties with high rather than low cyanogenic potential. There are several hypotheses that might explain this preference, one of which asserts that HCN released by the plants is a defense mechanism against pathogens, arthropods and mammalian pests. Evidence of HCN as a feeding deterrent to arthropods has been demonstrated in plants such as sorghum, white clover and peach. Two cassava pests appear to be deterred from feeding due to cassava leaf and root cyanogen content. It has been shown that growing cassava is an unacceptable food for the grasshopper, Zonocerus variegatus, because of the relatively large amounts of cyanogens produced in the leaves. Cassava roots contain variable amounts of cyanogens and research has shown that the burrowing bug, Cyrtomenus bergi, prefers feeding on roots with low rather than high cyanogen content. Nymphal mortality was greater on a cassava clone with high cyanogenic potential, while adult insects lived longer and produced more eggs on a clone with low cyanogenic potential. In field trials, considerably more feeding damage was observed on clones with low cyanogenic potential, and in free choice tests, maize was a preferred host. High nymphal mortality was noted particularly during the first two instars.