Resistance of cocoa to swollen‐shoot virus in Ghana. I. Field trials

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Abstract
SUMMARY: In Ghana, field resistance to cocoa swollen‐shoot virus (CSSV) in young, bearing trees was assessed by monitoring natural spread by mealybug vectors from adjacent graft‐inoculated line sources. Apparent rates of virus spread were estimated by regressing proportions of infected trees on time from inoculation. In some hybrids between parents derived from Upper Amazonian material, spread was only 25–40% that in currently recommended varieties. This resistance is the most effective, feasible measure to reduce economic losses and the resistant hybrids could be made available quickly for large scale use.The results from trials in which virus spread most rapidly were the easiest to interpret. Spread of CSSV is erratic, necessitating large experiments, and in trials with equal replication the largest plots gave the greatest precision. Neighbouring plot adjustment proved unsatisfactory as it improved precision but appeared to reduce accuracy.