STUDIES ON THE BLAST DISEASE OF SEEDLINGS OF THE OIL PALM (ELAEIS GUINEENSIS JACQ.)

Abstract

The series of investigations carried out into the blast disease of oil palm seedlings, a disease of high economic importance, showed that the disease was widespread in nurseries throughout south-western Nigeria, The disease was prevalent between October of the year of planting and March of the following year and appeared to be severest in November and December. The incidence of the disease was lower among seedlings planted into the nursery at the beginning of the rainy season in April than, among seedlings planted in June, July or August. Coconut (Cocos nucifera Linn.) and other palms of economic and ornamental importance, for example, Are ca catechu Linn., Ptychosperma elegans Blume, and Roystonea regia (H. B. K.) Cook also showed heavy blast infection. Pythium splendens Braun (Accession No. IMI 149554) and Rhizoctonia lamellifera Small (Accession No, IMI 149556) were consistently isolated from infected roots. Pathogenicity tests with these fungi showed that the blast disease developed as a result of the co-infection of oil palm seedlings by both fungi, P. splendens was found to be more pathogenic than R. lamellifera under experimental conditions. The infective propagules of P. splendens were found to be more abundant at the ploughline depth of 9 inches than at depths of 1, 3, 6, 12, 15, 18, 24 and 36 inches. The recovery of the fungus from nursery soil by the root baiting technique was high in July-October and low in November-January. These variations in the recovery of the fungus from soil were found to correspond with variations in soil moisture but not with soil temperature. The cultural practice of incorporating organic manure with soil increased while air-drying of soil decreased the recovery value of the fungus from soil. In vitro studies on P. splendens and R. lamellifera showed that the linear growth, sporangial and sclerotial production and germination were better in agar media rich in simple sugars than those deficient in them. The best temperature range for the linear growth, sporangial and sclerotial production and germination was found to be 25-30°C. Longevity of sclerotia of R. lamellifera was highest at a relative humidity value of 95% and at a low temperature of 20°C. Soil temperature in polythene bag planting was found to be positively correlated (r = 0.7758) with the blast disease at the 1% level of significance. There was no statistically significant correlation between the disease and soil temperature or soil moisture in ground bed planting. The incidence of the blast disease was found to be higher in the subsoil and ground bed planting than in the topsoil and polythene bag planting respectively. Shading of nursery seedlings in the dry season particularly from October to December was found to be more effective than Chemical soil treatment with Femasan or Benlate in reducing the incidence of the blast disease. Soil treatment with pentachloro-nitrobenzene appeared to aggravate the disease. Extension work seedlings produced at NIFOR were found to be more resistant to the blast disease than grove palm seedlings.