Phytophthora megakarya and P. palmivora on Theobroma cacao: Aspects of Virulence and the Effects of Temperature on Growth and Resistance to Fungicides

The effect of climate change is not only expected to cause a decline in areas of suitable crop production, but also impact the occurrence and severity of agricultural diseases. Theobroma cacao, the plant behind the global chocolate industry, and the source of livelihood of an estimated 40 – 50 million people is threatened by this phenomenon. In West Africa, Phytophthora palmivora and P. megakarya are the two causal agents of black pod disease, the most destructive of cocoa diseases, with P. megakarya described to be highly virulent. The aim of this study was to ascertain the current dynamics of cocoa black pod disease caused by the two Phytophthora species in Ghana. Sixteen Phytophthora isolates collected from the six cocoa growing regions were used in this study. Using inoculation methods and microscopic analysis, it was found that P. palmivora isolates were more aggressive, virulent and produced many zoospores and chlamydospores than P. megakarya. These results contradict earlier reports, a clear indication of an emerging trend in black pod disease levels caused by the two Phytophthora species. Temperature has considerable effects on the growth of Phytophthora isolates, with optimum temperatures of highest mycelia growth in P. megakarya being 25 ˚C while that of P. palmivora ranges from 25 ˚C to 28 ˚C. P. megakarya was found to be more sensitive to the higher temperatures and recorded no mycelial growth at 31 ˚C and 34 ˚C. On resistance to fungicides, Ridomil Gold (6 % metalaxyl M + 60% Cupper (I) oxide) applied at the recommended rate (3.3g/L), was very effective against Phytophthora isolates in vitro at varied temperatures with 100% inhibition of mycelia growth. The interactive effect of higher temperatures and fungicide (Ridomil Gold) on P. palmivora and P. megakarya in vitro is appreciable, and should be further investigated in order to establish minimum Ridomil Gold dosage for effective and safe disease control in the future. The findings of the present study provide new insights of P. palmivora and P. megakarya infections on cocoa that require further investigations for effective disease management in Ghana.