Effect of Water Activity and Plant Extracts on the Growth of Phytophthora palmivora (Black Pod of Cocoa) in South Western Nigeria

Abstrakt

Laboratory studies was conducted to evaluate the effect of water activity and hot water extracts of African eggplant (Solanum macrocarpon), salt and oil tree (Cleistopholis patens) and the composite mixture on growth, conidia germination and sporulation of Phytophthora
Palmivora invitro. The extracts and the composite mixture were applied in situ at three concentrations (15, 45 and 60%) by mixing 1 ml of the different concentrations with 10 ml of molten PDA. Unmodified potato Dextrose Agar (PDA) media with water activity (aw = 0.995 ) and PDA modified with non-ionic solute glycerol (aw = 0.97, 0.964 and 0.91) were poured into 9 cm petri- dishes and inoculated at the center with one microliter of standardized conidial suspension for evaluation of growth and conidial density of the fungus. Result from the study shows that the growth rate and conidiation varied significantly at different water activity levels being highest (4.34mm day-1) at (aw = 0.995) when water was freely available and decreased with water stress (aw = 0.97 – 0.91). The conidial density was highest under the greatest water stress condition (aw= 0.91). The extracts applied at all the concentrations reduced mycelia growth and conidial germination. The composite mixture of the extracts was more effective in reducing mycelia growth (3.15mm day-1) compared to the control (4.34mm day-1). Similarly, conidial germination were reduced significantly by the composite mixture values being (38, 25 and 16%) at 60, 45 and 15% concentration respectively. There was no significant effect of the extracts on sporulation at the tested concentrations. The study therefore shows that extracts of the plants and water activity significantly affect the growth and conidiation of Phytophthora palmivora and the information obtained can be used in the management of the disease on the field.