Witches' broom disease investigations, 3: Notes on the occurrence of witches' broom disease of cacao at River Estate 1939-1942

1. Observations on witches' broom disease made at River Estate, Diego Martin, Trinidad, from 1939-1942 are discussed. 2. During the three years of the investigation the disease has rapidly increased in severity, and although by 1942 certain trees had begun to show signs of considerable damage, there is no reason to believe that it has reached its maximum intensity. 3. Variation in the number of brooms produced per tree per annum is very great, but it was established that trees which produce many brooms one year are likely to do so again the next year and that trees which produce only a few brooms are likely to behave similarly the next year. This applies both to fan- and cushion-brooms. 4. Cushion-brooms and flower production are closely correlated, but there is no such relationship between fan-brooms and flush; nor are cushion-brooms and fan-brooms per tree closely correlated. It is suggested that these figures indicate that if there is any form of resistance to the disease to be found in Trinidad cacao it is not likely to be an inherent protoplasmic one but more probably some form of disease-avoidance in the vegetative tissues of certain trees. It does not follow that a tree which shows resistance to fan-brooms will show any comparable resistance to cushion-brooms or to pod infection.