Combatting Cocoa Swollen Shoot Virus Disease: What do we know?

Summary

Cocoa Swollen Shoot Virus Disease (CSSVD) is one of the major factors limiting cocoa (Theobroma cacao L.) productivity in West Africa. The only cure for CSSVD is to cut out visibly infected trees and the official eradication campaign in Ghana has cut out more than 200 million trees since 1946. 80 years of research on preventive control measures have mainly focused on resistance breeding, mild strain cross-protection (inoculation of cocoa seedlings with a mild strain of the virus to protect against the severe strain) and control of mealybug vectors. Meanwhile, diversification measures such as agroforestry (for shading) or barrier (strip) cropping have received less attention. Despite promising results, CSSVD is more prevalent in the field than ever before. The large body of knowledge on preventive control measures for CSSVD is fragmented and many publications are not easily accessible. Furthermore, the literature has never been systematically evaluated and quantitatively assessed. Hence, we consolidated this knowledge with an extensive literature review followed by meta-analysis to identify the pertinent research gaps. Out of 423 publications on CSSVD-related issues, we selected 34 studies, which contained 52 datasets on seven different preventive control measures. Results showed that resistance breeding and mild strain crossprotection may reduce CSSVD infection by 30 percent, while the potential of diversification measures (shading/agroforestry and barrier (strip) cropping) seems to be considerably higher (40 and 85 percent, respectively). However, there is a lack of evidence because of a low number of studies about diversification measures, indicating that our results have to be interpreted with care and calling for more research in this area. Future testing is needed to evaluate the efficacy of barrier (strip) cropping to reduce CSSVD, and address the effect of shade on CSSVD symptom severity. Furthermore, the practical relevance of different preventive control measures for farmers needs to be assessed, and shade should be considered in current breeding programs for CSSVD resistance.