Date Published: May 11, 2017
Publisher: Public Library of Science
Author(s): Rebecca S. Bart, Nigel J. Taylor, Cyril Zipfel.
The two viral diseases, CMD and CBSD, have a major impact on cassava production in sub-Saharan Africa, together causing estimated losses of US$1 billion per year . A recent report from Kenya estimates US$1,300/hectare losses . Both viruses are transmitted by the whitefly vector Bemisia tabaci. CMD is caused by at least seven species of geminiviruses and is endemic across tropical Africa . Infected plants show varying degrees of leaf deformation, mosaic chlorosis, and compromised photosynthetic capacity, leading to reduced storage root yields (Fig 1). Conventional breeding programs have exploited resistance to CMD present within germplasm collections to generate varieties now available to farmers across large parts of Africa . In contrast to CMD, effective control strategies for CBSD are not widely available. Yield loss resulting from CBSD has been described as one of the world’s most important biotic threats to food security . CBSD is caused by two species of potyvirus and results in necrotic lesions within the storage roots, rendering them inedible and unmarketable  (Fig 1). In contrast to CMD, no varieties with robust CBSD resistance are available to farmers at this time . Cassava is also susceptible to CBB, caused by the gram-negative bacterial pathogen Xanthomonas axonopodis pv. manihotis (Xam). CBB occurs in Central and South America, across Africa and Asia—everywhere cassava is grown . Disease severity varies from water-soaked lesions on the lower leaves to complete wilting and plant death (Fig 1). It is unknown what triggers periodic severe outbreaks of CBB, although fluctuations in environmental conditions are considered a major factor . To promote disease and suppress host defense responses, Xam injects type III effector proteins into host cells. To date, only a subset of these virulence proteins have been studied [11, 12]. As with CBSD, no robust CBB control strategies are available to farmers currently, despite being recognized as an important bacterial pathogen of cassava [13–15].
Major investments over the previous decade have significantly advanced biotechnological capacities in crops such as cassava. Challenges remain, however, if the resulting products are to reach and benefit small-holder farmers. Some are biological, but others are associated with regulation, approval, and perception of crops enhanced through the use of biotechnology.
Recent funding focused on cassava has enabled engagement by an expanding and dedicated group of international plant science researchers. As a result, plant scientists are poised to take advantage of powerful and rapidly developing biotechnologies to engineer disease resistance in cassava and to deliver improved planting materials to African small-holder farmers. In addition to tackling cassava’s weaknesses, an additional noteworthy opportunity exists to use biotechnology to elucidate the molecular and genetic basis of cassava’s drought tolerance and high productivity on marginal lands. This mechanistic insight may then inform efforts to improve the hardiness of other crop species.