ABSTRACT

A screenhouse study was conducted to evaluate the effects of soil treatments with a systemic fungicide (benomyl) and a compound fertilizer (NPK) on the response of cassava (Manihot esculenta Crantz) to diseases, pests and mycorrhizal symbiosis. Soil drenches of benomyl (Methyl -1- [butylcarbomyl] -2- benzimidazole carbamate) were applied at the rates of  0, 25, 100, 200, 500 and 1000 µg/g in polyethylene bags. Similarly, soil drenches of NPK (15-15-15, N – P₂ O₅ - K₂O) were applied at the rates of 0.083 g (10 kg/ha) and 0.832 g (100 kg/ha) per 4.2 kg soil. Mycorrhizal spores in rhizosphere soil and root colonization of cassava by vesicular-arbuscular mycorrhiza (VAM) were estimated at 5½ months using mycorrhizal spore recovering and stained root segment techniques. Shoot height and girth (cm) and leaf numbers  were assessed at 5½ months. At 5½ months,  viable fungal and bacterial propagules in soil samples from the rhizosphere of plants were evaluated, on potato dextrose agar (PDA) and nutrient agar (NA), respectively. Foliar analysis for N, P, K⁺, Ca²⁺ and Mg²⁺  concentrations at 5½ months  was performed after samples were first digested, using the perchloric acid and wet oxidation methods. Incidences of the cassava green spider mite (Mononychellus tanajoa Bonder) (CGM) and cassava mealybug  (Phenaccocus manihoti Matt-Ferr.) (CM) infestations were rated on a scale from 1 (no symptom) to 5 (severe symptom) with intermediates. African cassava mosaic disease (ACMD) incidence was rated bi-monthly at 3½ to 5½ months on a scale from  0 - 5. Incidence of Cercospora leaf spot disease (CLSD) was assessed based on the number of leaf spots per plant, and infection index calculated. Phytotoxicity from benomyl or NPK effects was evaluated on the basis of percentage foliar damage. The experiment was repeated once.

Cassava in soils treated with 200, 500 or 1000 µg benomyl per g soil died from severe phytotoxicity symptoms. No data were obtained from such soils or plants. Benomyl effects significantly (P = 0.05) diminished viable fungal propagules in cassava rhizosphere and CLSD infections on test plants, while depressing highly significantly (P = 0.01) VAM spores and root colonization by endophytes. On the contrary, soil treatment with the fungicide enhanced plants’ vigour (foliation) highly significantly; but no significant effects were detected for viable heterotrophic bacterial numbers in soil, CGM, CM infestations, ACMD infections and on N, P, K⁺ and Mg ²⁺ foliar concentrations. NPK fertilizer soil treatments highly significantly reduced VAM spores and root colonization, CM and CGM, and N and K titres in leaves and ACMD, but with no significant effects on CLSD infections, phytotoxicity, P, Ca²⁺ and Mg²⁺ foliar concentrations.

Reduction of VAM spores in cassava rhizosphere and root colonization by endophytes in benomyl - treated soil is attributed to mitosis inhibition in endophytes by the degradation product of benomyl. Enhanced vigour of cassava by benomyl is unexpected; but this may be due to reduction of a mycoflora competitive for nutrients. Beneficial effects of NPK on cassava vigour are similar to results of other studies. However, its depressing effects on VAM spores and colonization are traceable to P-induced permeability factors in cassava root membranes.