Date Published: February 22, 2019
Publisher: Public Library of Science
Author(s): Nami Minato, Sophearith Sok, Songbi Chen, Erik Delaquis, Iv Phirun, Vi Xuan Le, Dharani D. Burra, Jonathan C. Newby, Kris A. G. Wyckhuys, Stef de Haan, Peng Zhang.
Cassava mosaic disease, one of the ten most economically important crop viral diseases in the world, was first reported in Southeast Asia from a single plantation in Cambodia in 2015. To determine the presence and incidence of Sri Lankan cassava mosaic virus (SLCMV) one year after first detection, a total of 6,480 samples from 419 fields were systematically collected from cassava production areas across Cambodia (3,840 samples; 240 fields) and Vietnam (2,640samples; 179 fields) in the 2016 cropping season. Using PCR-based diagnostics, we identified 49 SLCMV-infected plants from nine fields, representing 2% of the total number of fields sampled. Infected fields were geographically restricted to two provinces of Eastern Cambodia, while no infection was detected from any of the other sampled sites in either country. Symptom expression patterns in infected plants suggested that SLCMV may have been transmitted both through infected planting materials, and by Bemisia tabaci, the known whitefly vector of SLCMV. In addition, 14% of virus infected plants did not express typical symptoms of cassava mosaic disease on their leaves, highlighting that molecular-based validation is needed to confirm the presence of SLCMV in the field. None of the owners of the SLCMV-infected fields indicated acquired planting materials from the plantation in Ratanakiri where SLCMV was first reported. The surveillance baseline data generated for both countries is discussed in light of future options to control and manage cassava mosaic disease.
In 2016 Sri Lankan cassava mosaic virus (SLCMV) was reported for the first time infecting cassava in Southeast Asia . This report consisted of positive virus detection from a single commercial plantation with symptomatic plants in Ratanakiri province in Eastern Cambodia in May, 2015. Prior to this positive identification, Southeast Asia had been considered free of cassava mosaic disease (CMD). Given the negative effects on production and economic returns of CMD in other settings [2–6], an alert to notify the presence of the disease in the region was warranted . At the time a window of opportunity for effective disease control through eradication or quarantine seemed apparent, as presence of the disease was assumed to be restricted to a limited geographic area.
Our study provides the first systematic baseline assessment of SLCMV presence and incidence for both Cambodia and Vietnam after the virus was first positively detected in Cambodia. We show that SLCMV is not only present in the province where it was first detected, but also in the neighboring province of Stung Treng. In fact, levels of incidence observed in Stung Treng were much higher than Ratanakiri in terms of number of infected fields and within-field incidence rates (Fig 1; Table 2). While the exact mechanism and first location of introduction of SLCMV to Southeast Asia remains unclear, positive detections in our study remained confined to Eastern Cambodia. The range of distribution of the virus was up to 70 km away from the 2015 initial detection site reported by Wang et al. . However, this doesn’t necessarily imply that the disease has spread from this point, because the original study only sampled a single location based on symptomatic observations, and did not attempt to ascertain the geographic extent of the infection. Our findings indicate that at the time of the study, SLCMV was still geographically restricted to a relatively confined region of Cambodia, thus improving the potential for preventative measures to limit its further spread. Such measures could include quarantine, eradication, or restrictions on plant movement.
We report on a baseline-level systematic bi-national survey of SLCMV presence and incidence in Cambodia and Vietnam one year after the disease was first reported in Eastern Cambodia. Since Southeast Asia contributes over 95% of global cassava exports , the potential negative impacts of SLCMV on cassava-based production systems are a major regional and global concern. The potential impacts of SLCMV threaten the precarious livelihoods of millions of smallholder farmers across Southeast Asia. At the time of our bi-national surveillance exercise, the 2016 range distribution of SLCMV remained restricted to Eastern Cambodia. Our finding of highest SLCMV levels in Stung Treng province, even when compared to the neighboring province of Ratanakiri, where the disease was first detected in 2015, suggest that the range of the outbreak is already beyond the province of initial detection.