Field experiments in Cameroon on cassava mosaic virus disease and the reversion phenomenon in susceptible and resistant cassava cultivars

Cassava is mainly propagated through stem-cuttings. There is evidence that some cuttings propagated from plants infected by cassava mosaic geminivirus es may produce virus-free plants. Field experiments were conducted under co ntrasting ecological conditions at two sites in southern Cameroon to study the reversion phenomenon, in particular the extent to which the virus causi ng cassava mosaic disease (CMD) was absent from cuttings collected from CMD -affected plants. Cuttings were taken from the basal, middle and upper port ions of the stems of diseased plants of susceptible and partially resistant cassava cultivars. The proportions of disease-free shoots developing from the different batches of cuttings were recorded to indicate the extent of r eversion. This was significantly less frequent in cv. Red Local (susceptibl e) (5-43%) than in the susceptible cv. Bambui Local (65%) and cv. Improved (70%), which is partially resistant to infection. Reversion was significant ly higher with cuttings collected from the youngest portions of the stem co mpared with older portions and with short cuttings (5, 10 cm) compared with those 20 or 25 cm long. There was a significant negative correlation betwe en symptom severity in different cultivars and the extent of reversion. Thi s was greater at the lowland Mile 17 site, near Buea (ca 400 masI) than at the mid-altitude site at Dschang (ca 1300 masI) where conditions were coole r, suggesting an influence of temperature. In order to assess the effect of CMD in cassava the Disease Index (DI) was developed. The result of DI anal yses showed that the DI could be used to identify cultivars on the basis of their ability to revert from CMD and the severity of CMD on the non-revert ed plants.