Natural occurrence and distribution of stem cankers caused by Phytophthora megakarya and Phytophthora palmivoraon cocoa

Abstract

Epidemiological studies were conducted in five cocoa growing districts in the Eastern Region of Ghana solely infected by Phytophthora palmivora and five districts in the Ashanti and Brong Ahafo Regions prevalently infected by Phytophthora megakarya to determine the natural incidence, the vertical distribution on trees and the probable sources of stem canker infections, and to isolate and identify the causal pathogens. The incidence of canker in the solely P. palmivora infected area was higher (between 0% and 16.0%) than in the area mainly infected with P. megakarya (0.5–8.0%). Differences were found in the natural height distribution of cankers in the two areas, whilst the areas solely infected with P. palmivora showed a near normal curve, those prevalently infected with P. megakarya were positively skewed. Most of the cankers caused by P. megakarya were found at the base or near the base of the tree trunks (1–40 cm above ground level), while those of P. palmivora were concentrated between 41 and 100 cm from the ground level. The majority (71.8%) of cankers in the solely P. palmivora infected area were cushion-borne, followed by 24.3% from unknown sources and only 3.9% from the soil. In contrast, a significantly large proportion (32.6%) of the cankers in the prevalently P. megakarya infected area were soil-borne, although cushion-borne cankers formed the majority (48.4%) due to the presence of P. palmivora infection whilst those of unknown sources constituted 19.0%. Phytophthora megakarya was frequently isolated from all the three sources of canker infections, indicating P. megakarya readily causes stem canker on cocoa. These results emphasise the importance of different reservoirs as sources of primary inoculum for diseases caused by the two Phytophthora species particularly pod rot infection on cocoa.