Impact of environmental factors, chemical fungicide and biological control on cacao pod production dynamics and black pod disease ( Phytophthora megakarya) in Cameroon

Abstract

The impact of environmental factors and microbial and chemical control methods on cacao pod production dynamics and spread of black pod disease caused by Phytophthora megakarya was assessed over three consecutive years in a smallholder’s plantation. Significant positive correlations were found between rainfall records and pod rot incidence when assessed after a 1-week lag. Disease distribution across various pod developmental stages showed that immature pods were the most susceptible to P. megakarya attack. Weekly observations of the pod distribution and disease progression at various developmental stages on cacao trees sprayed with fungicide Ridomil, Trichoderma asperellum biocontrol agent (strain PR11), or untreated control trees indicated that the total pod production and the incidence of black pod rot was significantly different between the treatments. The disease rates were 1.73%, 47.1% and 71.23% in the plots treated with fungicide, PR11 and untreated, respectively, in 2004, and 0.67%, 11.35% and 34.04% in the same plots in 2005. A significant carry over effect of the biological agent was noticed up to 1 year after the sprays with strain PR11 were halted. The disease incidence averaged 16.67% in the plots previously sprayed with PR11 and 35.88% in the control plot, although both the plots were sprayed with the same contact fungicide, Kocide. Monitoring through enumeration and molecular typing revealed that Trichoderma propagules remained consistently more abundant in flower cushions of cacao trees than in soil or bark. This study showed that microbial control of black pod disease with PR11 was promising but not as effective as the chemical methods under the current high disease pressure, and therefore needs to be integrated with other control methods to establish a sustainable management system for black pod disease in Cameroon.