The components and deployment of resistance to cassava mosaic virus disease

Abstract

Cassava mosaic virus disease (CMD) is prevalent and causes serious losses in cassava (Manihot esculenta) in southern India and in many parts of sub-Saharan Africa. The disease is caused by viruses of the Geminiviridae that are transmitted by the whitefly Bemisia tabaci and disseminated in the stem cuttings used routinely for propagation. The main approach to control is through the use of virus-resistant varieties, but suitable ones are not always available and susceptible varieties are still widely grown. This explains why CMD continues to be a problem in many areas. CMD-resistant varieties have several features which are considered in this review:- • They are not readily infected, even when exposed to large amounts of vector-borne inoculum. • When infected they develop symptoms that tend to be inconspicuous and not associated with obvious deleterious effects on growth or yield. Moreover, the symptoms become even less conspicuous as growth proceeds and plants may eventually recover and become symptomless. • Infected plants support a low virus content and they are likely to be a poor source of inoculum from which further spread can occur. • Virus is not fully systemic within infected plants which can be a source of uninfected planting material when stem cuttings are collected for further propagation. This phenomenon is referred to as ‘reversion’ and it has an important ‘cleansing effect’ in restricting or preventing the progressive deterioration in health status that would otherwise occur during successive cycles of vegetative propagation. The available information on the different components of resistance is discussed and it is concluded that they are inter-related features of the same basic mechanism that restricts virus entry, replication and movement within the host. It is argued that the effectiveness and durability of virus-resistant varieties are likely to be influenced by the way in which they are deployed. However, this topic has received little attention from researchers and there is continuing uncertainty on the effects of CMD on the yield of resistant varieties and on the role of phytosanitation. This involves the use of virus-free planting material and the removal (roguing) of any additional diseased plants that occur. Some consider that these procedures complement the use of virus-resistant varieties and should be adopted, whereas others argue that they are unnecessary and inappropriate. It is concluded that there is considerable scope for utilizing resistant varieties more widely and more effectively than at present, but in doing so it is important to avoid eroding the genetic diversity that is currently such a marked feature of cassava cultivation in Africa.