Germination and morphological changes in the germ tube of germinating sporangium of Phytophthora palmivora in the presence of some fungicides, oxalic acid and culture metabolites of Aspergillus niger

Black-pod disease of cocoa caused by Phytophthora palmivora is a national problem that takes a heavy toll on cocoa pods in the field. In direct germination involving germination by discharge of zoospores from sporangia of P. palmivora 20-22 deg C for 6 h and direct germination by production of germ-tubes arising from the sporangium at 30 deg C for 6 h were assessed in liquid media containing appropriate dilutions (Field application rate, FR, 1/2FR, 1/4FR, and 1/10FR) of commonly used copper-based fungicides (Kocide-101, Caocobre-Sandoz, Ridomil plus 72 and Champion) and a biological control agent using culture filtrate of Aspergillus niger (undiluted, 1:1, 1:2, 1:5 and 1:10 v/v dilutions) raised on V-8 broth at 35 deg C for 4 days. For the assessement of direct germination the liquid 2 % yeast extract solutions were amended with the stated varying dilutions of the fungicides or the cultural filtrate of A. niger. As oxalic acid is one of the components of the metabolites of A. niger, the effect of low concentrations of pure oxalic acid solutions (1.0x10-3, 2.0x10-3, 5.0x10-3, 6.0x10-3 and 7.0x10-3M) on indirect germination of P. palmivora sporangia was also investigated. The fungicides variably depressed indirect germination. Caocobre, Champion, Ridomil applied at the field application rate prevented direct germination. Further dilution up to 1/10FR permitted up to 50 indirect germination depending on the fungicide. Kocide - 101 was as potent as the culture filtrate of A. niger as a concentration tested completely prevented indirect germination at 20-22 deg C. Similarly, all concentrations of oxalic acid tested (1.0-7.0 x 10-3 M) completely suppressed direct zoospore discharge by the sporangia of P. palmivora. The culture filtrate of A. niger (undiluted, 1:1 -1:5v/v) completely suppressed direct germination whereas 1:10v/v dilution of the same culture filtrate permitted 15 % direct germination in 6h. The test fungicides were all less potent as their lowest concentrations used (1/10FR) permitted 20-40 % direct germination. None of the sporangia in 2 % yeast extract amended with all concentrations of oxalic acid germinated by forming germ tubes. It was observed that the morphology of the sporangia which germinated directly in the varying dilutions of the fungicides was affected. The number of emerging germ tubes produced by the germinating sporangia in the presence of the fungicides was generally fewer in the medium containing higher concentrations of the fungicides. The germ tubes produced by sporangia germinating in Caocobre and Kocide were remarkably more irregular in outline and swollen conspicuously at certain sections