SUMMARY
The resistance and tolerance to infection with cocoa swollen‐shoot virus (virulent strain A) of forty‐five and thirty‐eight cocoa progenies respectively was estimated using a manual inoculation method. Resistance was estimated from the proportion of inoculated seedlings which developed symptoms, and tolerance from the severity of these symptoms during a 4‐month period. Resistance was found among Nanay, Iquitos and Scavina, all Upper Amazon types; and an inter‐Nanay cross was outstandingly resistant and offers scope for improvement by breeding. Some Amazon‐Amazon and Trinitario‐Amazon progenies snowed tolerance, especially those with Trinitario T9/21 as a parent. Some progenies seem sufficiently promising for field testing to select varieties suitable for planting in those areas in Ghana where swollen‐shoot disease is epidemic.
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 2540% 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.
SUMMARY
Strains A and Anibil of cocoa swollen‐shoot virus are serologically closely related. They are more distantly related to strain M and cocoa mottle‐leaf virus (Kpeve isolate), which are distantly related to one another. Strains A and Anibil protected against one another in cocoa seedlings, but no protection was found with any other combination between these four virus isolates. All mealybug‐transmitted cocoa viruses in West Africa are probably best considered as strains of cocoa swollen‐shoot virus.
Sterculiaceae and Bombacaceae were tested for susceptibility to three virus isolates from naturally infected Cola chlamydantha trees and three from Adansonia digitata trees in Ghana. Seven species of Pseudococcidae were tested as vectors of the six isolates. These studies indicate that the Cola isolates should be classified with cocoa swollen shoot virus and those from Adansonia with cocoa mottle leaf virus.
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