Studies on black pod disease of cocoa: Evaluation of cocoa-based media for culturing Phytophthora Palmivora and Investigations into the seed borne nature and seed to seedling transmission of the pathogen

To demonstrate seed borne nature of Phytophthora and seed to seedling transmission of the Phytophthora fungus, a medium that would facilitate detection of the fungus in cocoa seeds and seedling tissues was needed. Six agar media, viz green cocoa pod husk agar (GCPA), ripe cocoa pod husk agar (RCPA), green cocoa bean agar (GCBA), ripe cocoa bean agar (RCBA), green cocoa mucilage agar (GCMA) and ripe cocoa mucilage agar (RCMA) were prepared from scratch and evaluated for clarity and capacity to support mycelia growth and sporulation of Phytophthora palmivora. Oatmeal agar (OMA), potato-dextrose agar (PDA), vegetable 8-juice agar (V8JA) and pineapple crown agar (PCA) were included as controls. Based on media characteristics and the fungal growth parameters studied, the GCMA was adjudged the best. Formulation of some cocoa-based media into dehydrated readily utilizable forms was attempted. Dehydrated media were prepared from the placenta and seed mucilage of green mature pods by oven-drying pieces at 70 ° C for 6 h, milling the resulting flakes and sieving to obtain a fine powder. Four powder concentrations viz 0, 1, 2 and 3% in water (wt: vol) were prepared and agar and calcium carbonate respectively added at 2% (wt: vol), and 2.5% (wt: wt). The media were autoclaved and evaluated in Petri plates for clarity and for capacity to support growth and sporulation of P. palmivora. At the same powder concentration, media prepared from the placenta supported significantly higher (P 0.05) radial growth of the test fungus (5.59-7.10mm/day) than media prepared from mucilage powder (4.55-6.18 mm/day). Aerial mycelia growth improved slightly as powder concentration increased but media clarity decreased. Media incorporating 2% placenta powder, 2,5% calcium carbonate and 2% agar prepared by boiling and filtering with cheese cloth before autoclaving gave the best result and is recommended of culturing P. palmivora. The seed borne nature of Phytophthora in cocoa was studied by assaying seeds from healthy and diseased cocoa pods on GCMA. Seed transmission was studied by artificially inoculating visually healthy seeds with P. palmivora, and sowing in 200m1 plastic cups. Parts of the resulting seedlings were assayed for the fungus on GCMA. Mature seedlings from a farmer’s nursery and seedlings from seeds of naturally diseased pods were also assayed. All 1028 seeds from healthy pods were free of Phytophthora. In contrast, 63% of seeds from diseased pods harboured the fungus. Detection of Phytophthora from the various components of infected seeds was highest for the mucilage/testa (57% detection frequency) followed by the embryo (38%) and then the cotyledon (37%). Detection of the fungus decreased when seeds were obtained further away from the symptomatic portion of the pod. Phytophthora palmivora was detected in rhizosphere soils, roots, stem bases, undetached cotyledons and testas of undetached cotyledons of seedlings from artificially infected seeds. It was absent from the middle and upper portions of stems as well as from leaves of such seedlings. Similar results were obtained with naturally infected seeds. The fungus was neither detected in farmer seedlings nor in seedlings of uninoculated seeds. It is concluded that systemic transmission of Phytophthora from the cocoa seed to the leaves of seedlings is impossible. It appears that infected seeds, when sown, release inocula of Phytophthora into the soil and these soil inocula then infect the roots and the stem base of the emerging seedling through contact (non-systemic transmission). This, however, is unlikely to occur at the farm level since farmers in Ghana plant seeds from visually clean pods.

Source: KNUST