Toxicological Effect of some commonly used Pesticides (Herbicides, Insecticides and Fungicides) on Soil Fungi

Abstract

Pesticides are considered to be effective crop protection chemicals in modern agriculture. However, they can also disrupt microbial processes that are essential for continued soil fertility. The present study was carried out to evaluate the toxicological effect of three pesticides namely; an insecticide known as Lara force (lambd-cyhalothrin 2.5% EC)), an herbicide known as Amin seal (720g/l Dimethylamine salt as an aqueous solution) and a fungicide know as Ridomil Gold (66wp metanaxin). Soil samples were collected from the depth of about 5-20 cm into a sterile plastic container for laboratory analysis. Potato dextrose agar (PDP) was used for enumeration and isolation of fungi. Incubation was at 25°C for about 3 to 7 days. The soil pesticides were separately applied to soil samples in the following doses: Control (soil without pesticide), Normal dose recommended by the manufactures, double of the dose and half of the recommended dose. Serial single dilutions of the soil were prepared using one gram of the soil sample. After which the cultural characteristics and the microscopic features were used to identify the various fungal isolates. The result showed that, fungal isolates such as Aspergillus niger, Aspergillus oryzae, Rhizopus nigricans, Aspergillus tamari, Cladosporium sp., Penicillium citrinum were isolated from the soil samples. The different doses of the pesticides inhibited the growth of the fungal species at various degrees. Hence, the greatest changes were induced by the double dose with percentage frequency occurrence of fungal isolates ranging from 0 to 4% for soil treated with insecticide, 0 to 20 % for soil treated herbicide and 0 % was recorded for soil treated with fungicide. The results of the study also indicate that there were differences in the values of the population of the fungal species in the treated and untreated soil samples when they were subjected to different exposure periods with the control (Untreated soil) having a significantly larger population of fungal isolates (p<0.05) than those isolated from the untreated soil samples. The findings of the study show that the fungicide exhibited higher adverse (p<0.05) effect on the fungal species than the insecticides and herbicides at the same duration of exposure. While the trend of inhibition on growth of fungi was observed from the initial effects of the different pesticides until 10 DAT, rise in fugal population was observed at 15 DAT, The present study showed that the pesticides treatments significantly affected the percentage occurrence of the fungal species and their populations in the soil, and the degree of inhibition was closely related to doses and exposure periods of pesticides applications and also varied with the type of pesticides. Thus, the presence of pesticides residues in soil which could have direct impacts on soil fungi is a matter of great concern.