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

Abstract

Cassava is mainly propagated through stem-cuttings. There is evidence that some cuttings propagated from plants infected by cassava mosaic geminiviruses may produce virus-free plants. Field experiments were conducted under contrasting ecological conditions at two sites in southern Cameroon to study the reversion phenomenon, in particular the extent to which the virus causing cassava mosaic disease (CMD) was absent from cuttings collected from CMD-affected plants. Cuttings were taken from the basal, middle and upper portions 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 reversion. This was significantly less frequent in cv. Red Local (susceptible) (5-43%) than in the susceptible cv. Bambui Local (65%) and cv. Improved (70%), which is partially resistant to infection. Reversion was significantly higher with cuttings collected from the youngest portions of the stem compared with older portions and with short cuttings (5, 10 cm) compared with those 20 or 25 cm long. There was a significant negative correlation between symptom severity in different cultivars and the extent of reversion. This was greater at the lowland Mile 17 site, near Buea (ca 400 masl) than at the mid-altitude site at Dschang (ca 1300 masl) where conditions were cooler, 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 analyses 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-reverted plants.