Tolerance trials with young cocoa trees infected with cocoa swollen‐shoot virus

Abstract

SUMMARYIn three short‐term field trials, 24 cocoa progenies were assessed for sensitivity to infection with a severe Ghanaian strain of cocoa swollen‐shoot virus. Some of the progenies were derived from tolerant parents, others had themselves shown mild symptoms as young seedlings. Stem diameters, diameter increments, canopy scores and latent periods (time between inoculation and expression of symptoms) of plants infected when 12 to 22 months old were compared with similar measurements of healthy plants.During the period between 6 and 18 months after inoculation, infection reduced the stem diameter increment of the least sensitive progeny in each trial by 52, 25 and 7%, the last value increasing to 38% during the period between 24 and 30 months after inoculation. Within trials, the effects of virus infection on stem diameter increments and canopy scores were well correlated but latent periods were not significantly correlated with any other variate.In one trial, pod yield was more closely related to canopy condition at the time of cropping than to stem diameter increment 2 yr earlier. Weak correlations between pre‐ and post‐bearing increases in diameters in these trials indicated that tolerance was not directly related to vigour. None of the progenies is considered suitable for direct use as tolerant varieties in Ghana.