Abstract:

Cyanogenic plants, such as cassava exhibit genetic resistant to diseases and pest which is associated
associated with a general defence signalling mechanism that responds to stress situations. Cyanogens
in cassava vary diversely with cultivar^ stress, location, environment and plant age. Factors that
induce plant stress often result in increased total cyanogens. Interestingly though, increased total
cyanogens often associate with increased resistance to stress. This overlap between plant defence
systems and plant stress is not coincidentaL The infection of cassava with diseases and pests not only
affects yields, but also influences the level of total cyanogens, which are presumed to be a part of
the plant defence system. To qualify the extent to which total cyanogens are influenced by cassava
mosaic disease (CMD), the effects of CMD status, cultivar resistance and plant vigour on total
cyanogens accumulation in cassava at different growth stages of the crop was investigated, A total
of436 cassava plants were studied in the field and greenhouse, representing three dijferent CMD
status categories i.e., resistant^ moderately susceptible and highly susceptible genotypes. Associated
cyanogenic potential (CNp) levels >vere determined using standard protocols. CMD and CNp were
shown to be highly associated (r = 0.90, P d" 0.001 for variety; Pd" 0.003 for CMD) suggesting
a role of CNp in the defence mechanism against CMD infection. Notably, lower cyanogen levels
occurred in vigorous highly susceptible cassava than in the non-vigorous highly susceptible (most
stressed), suggesting a positive correlation between stress level and CNp content. The results of the
greenhouse studies matched with those from the field further supporting the observed association
between CMD and CNp (P d” 0.001), although, CNp levels were lower than in field trials (by 19,3
fold for roots and 6.3 fold for leaves) possibly due to temperature and nutrition effects. Future
studies including understanding of the genetic link between CNp and CMD, interaction between
vigour x variety x CMD status, ejfects of seasonality on CNp accumulation, temperature regimes
and nutrition on CNp yield, together may further our knowledge and capacity to control CMD