Effect of NPK Fertilizer on Diseases, Pests and Mycorrhizal Symbiosis in Cassava
A screen house study was conducted to evaluate the effects of a compound fertilizer (NPK) on the response of cassava (Manihot esculenta Crantz) to diseases, pests and mycorrhizal symbiosis. Soil drenches of NPK (15 - 15 - 15, N - P2O5- K 2O) were applied at the rates of 0.083 g (10 kg ha-1) and 0.832 g (100 kg ha-1) per 4.2 kg soil. Mycorrhizal spores in rhizosphere soil and root colonization of cassava by arbuscular mycorrhizal (AM) fungi were estimated at 5.5 months. Shoot heights and girths (cm) were assessed at 5.5 months. Incidence of pests-cassava green spider mites (Mononychellus tanajoa Bondar) (CGM), cassava mealybug (Phenaccocus manihoti Matt-Ferr.) (CM), African cassava mosaic disease (ACMD) and cercospora leaf spot disease (CLSD) were rated on varying scales. Mycorrhizal spores and root colonization were significantly higher in the control experiments (P< 0.01) with percentage decreases of 67 and 83% per g/soil of spores and root colonization of 61 and 83%, in the 10 and 100 kg ha-1, respectively. NPK fertilizer significantly increased plant vigour (stem & girth) (P<0.01), with increases by 16.82 to 36.45% for height and 15.20 to 25.45% for girth. Mycorrhizal symbiosis was lowest in plants amended with the highest fertilizer dosage. NPK concentrations diminished CGM, CM (P< 0.01) and ACMD (P< 0.05). However, no significant effects on CLSD infection were obtained.