ABSTRACT
The oil palm (Elaeis guineensis Jacq.) is a tropical tree crop mainly grown for the production of vegetable (palm) oil. The oil palm neck bending disease caused by a fungus, Thielaviopsis paradoxa, is a recent emerging disease outbreak in Nigeria. Thielaviopsis paradoxa, is highly diverse in nature and has been reported in other regions of the world to cause neck bending either individually or in combination with other pathogens. The research was aimed at characterizing the causative pathogen(s) of the neck bending disease in Nigerian oil palm plantation and to establish the efficacy of different agrochemicals and mineral elements in the management of the pathogen. Oil palm plantations from five locations namely: Need Produce Farms in Osun state, IkoleEkiti in Ekiti state, E farms, Uzanu and Okomu in Edo state were evaluated for neck bending disease incidence rate. Fungal isolates were obtained from diseased oil palm trunk and soil samples. Pathogenicity tests were conducted on 6 months old oil palm seedlings from Okomu plantation. Morphological characterization of fungal isolates was conducted. Various fungicides (Mancozeb 80%, Carbofuran 3%, Cabopuran 3% and Pyraclostrobin 40g/l) were tested in vitro, for antifungal activity against two identified isolates of T. paradoxa at different concentrations (5, 10, 15 and 20 ppm). In vivo experiment, using ring spraying and center spraying methods, was conducted to ascertain the influence of the most effective fungicide on oil palm yield. The effects of different minerals (ZnSO4, MnSO4 NH4Cl, KCl, CuSO4, CaNO3 and MgSO4) were tested on the growth response of T. paradoxa. In addition, the effect of NPK 13:11:21 + 2% MgO fertilizer application on the management of palms infected with of T. paradoxa was tested. Results showed that the disease incidence rate ranged from 0.03% to 4.93% in Okomu plantations and E farms, respectively. A high palm infection rate of 550 palms was observed in Okomu plantations compared to a low rate of 5 palms in Need Produce farms. Result from pathogenicity test showed disease symptoms on the base and inner tissue of the palm (center spear). Symptom manifestation included lesion and fungal colonization at the base of the palm followed by a crack in the inoculated parts of the fronds within three days of inoculation. The inner tissue was characterized by gradual invasion and necrosis and finally fracture and collapse of the spear fronds resulting in the neck bending disease of the oil palm. Amongst the fungicides tested, Carbofuran 3% was the most effective against the pathogen at a concentration of 15 ppm. The ring spray method significantly improved the yield in all the parameters assessed from the in vivo antifungal studies, especially the inflorescence and fruit bunch of the oil palm compared to the center spear spraying method. T. paradoxa cultured on glucose broth showed the best mycelial growth. For palms not treated with fertilizer, the rate of infection progressed rapidly with a 100% rate of infection at week 4. However, treated palms showed stabilized infection rate of 12.5 % after two weeks with total recovery at week 3. This study showed that Carbofuran 3% and NPK 13:11:21 + 2% MgO fertilizer were best selective control measures in the management of T. paradoxa in oil palm.
