Studies on Effect of Decomposing Products of Leaf Of Cocoyam (Xanthosoma Mafaffa Schott.) On Phytophthora Palmlvora (Butl.) Butl.

Abstract

Germination of P.palmivora sporangia and zoospores, growth of the hypha and formation of the sporangia in the presence of decomposing leaves of cocoyam were investigated. Cocoyam leaves buried in soil, from an experimental cocoa plot at the Cocoa Research Institute at Tafo, were rapidly decomposed and they completely rotted in 12 days. Fungal species belonging to the genera Alternaria, Aspergillus, Cladosporium, Curvularia, Fusarium, Penicillium, Rhizopus and Trichoderma were isolated from the decomposing cocoyam leaves. Two of the fast growing species, Rhizopua oryzae and Rhizopus stolonifer, were selected and used as agents for decomposition of the cocoyam leaves under sterile conditions.

Decomposition of the cocoyam leaves in the laboratory was carried out at 25°C and 30°C, the respective optimum temperatures for growth of R.oryzae and R.stolonifer. Growth by both species in Cassava-dextrose broth at the optimum temperature caused a shift in the pH of the medium from pH 5.30 to around pH 2.30. Culture filtrates of these fungi were therefore adjusted to neutral pH in experiments in which they were used. Direct germination of P.palmivora sporangia was greatly stimulated, by about 400 per cent, by both Rhizopus culture filtrates. Large number of germ tubes emerged from sporangia growing in the culture filtrates of the Rhizopus species. Zoospore germination was also improved by at least 30 per cent by culture filtrates of both R.oryzae and R.stolonifer. R.stolonifer greatly stimulated vegetative growth of P.palmivora, whilst R.oryzae slightly suppressed it.

Direct germination of P.palmivora sporangia was stimulated by exudate of undecomposed dry cocoyam leaves; germination in the exudate and in distilled water was 54.9 and 15.3 per cent, respectively. Indirect germination and zoospore germination were, however, inhibited by about 80 and 50 per cent, respectively, by the leaf exudate. Vegetative growth on Cassava-dextrose agar containing different concentrations was slightly improved whilst sporangial formation was depressed. . Direct germination of P.palmivora sporangia at 30°C Was inhibited by extracts of Rhizopus-decomposed cocoyam leaves. Amendment of the R.oryzae-decomposed leaf extract with Fructose Glucose, Maltoso or Sucrose negated the inhibitive influence. ' sodium nitrite, Asparag1ne and Ammonium tartrate also improved direct germination in the R.oryzae-decomposed leaf extract, but addition of asparagine caused further inhibition.

The Rhizopus-decomposed cocoyam leaf extract slightly inhibited indirect germination of P.palmivora sporangia at 20°C. With the exception of Fructose at 1.0 percent with improved germination there was greater depression of germination in R.oryzae-decomposed cocoyam leaf extracts amended with Glucose, Maltose and Sucrose of concentrations of 1.0, 2.0, 3.0 and 4.0 per cent w/v or with Ammonium nitrate, Peptone and Sodium nitrate at 0.5, 1.0 and 1.5 per cent. Asparagine improved indirect germination only at 0.5 per cent concentration, but slightly suppressed it at higher concentrations of 1.0 and 1.5 per cent.

Zoospore germination was slightly inhibited by Rhizopus - decomposed cocoyam leaf extract. The inhibition was annulled by Fructose, Glucose, Maltose and Sucrose. Higher concentrations of Peptone and Sodium nitrate, at 1.5 per cent, supported even poorer germination than the unamended extract, but germination at the lower concentrations of 0.5 and 1.0 per cent in Ammonium tartrate and Peptone-amended extract was superior.

The zoospores of P.palmivora diSintegrated in extract of R.oryzae-decomposed cocoyam leaf extract of pH 4.0. Germination improved gradually from 40.9 to 79.3 per cent on raising the pH of the extract from pH 5.0 through 9.0. Optimum sporangial germination occurred at pH 7.0 and gradually decreased as the pH moved away from the optimum to both the alkaline and acid ends of the pH range. There was germination at the extremes, pH 4 and pH 9, used. Extracts of leaves decomposed by either Rhizopus species slightly improved vegetative growth of P.paluivora on Cassava dextrose agar but depressed sporulation with increase in concentration, from 5.0 to 20.0 per cent.

An aqueous extract of soil from the same plot at the Cocoa Research,Institute Tafo only slightly reduced direct sporangial germination at 30 C and zoospore germination but did not affect indirect germination at 20˚C nor vegetative growth and sporangial formation in P.palmivora. However, extract of the soil containing decomposed cocoyam leaves substantially decreased zoospore germination from 14.6 per cent in the control of distilled water to 2.9 per cent., Direct sporangial gemination was similarly reduced from 40.2 in distilled water to 7.7 per cent in the extract. Vegetative growth remained unaffected but sporulation was reduced slightly at higher concentrations (15.0 and 20.0 per cent v/v) of the extract.

The decomposition products of cocoyam leaves contained very little nutrients. Although Glucose, Fructose, Mannitol and Sorbitol were identified, by paper chromatography, in the exudate of dry cocoyam leaves, at concentrations of 5.0 - 7.0, 6.0 - 7.0, 2.0 - 3.0 and less than 1.0 microgram per gram of dry leaf, respectively, only Trehaloso,at 18.0 - 20.0 microgram per gram, was identified in the Rhizopus-decomposed leaf extract.

The amino acids, Alanine, Amino-butyric acid and Asparagine were also identified, by paper chromatography, at concentrations of 1.0 - 2.0, less than 1.0 and 2.0 - 3.0 microgram per gram, respectively, in the exudate of dry cocoyam leaves but none was detected in the Rhizopua-decomposed leaf extract.