Hosam EAF BayoumiHamuda*
Rhizobial populations introduced into legume crop fields as biofertilizer become non-target microbiomes for the fungicides applied in the field. An in vitro experiment was carried out to evaluate the response of four Rhizobium leguminosarum strains originated from different soil types to twenty-five fungicides using four different techniques (paper disk, hole, streak, and microfermentor techniques). Fungicides were applied at five concentrations (0.0, 0.1, 1.0, 10, and 100 mg/l). The results from this study provide critical baseline information for further examinations to optimize the growth of these symbiotic N2 -fixing bacteria, the microfermentor technique was carried out as following the range of concentrations was tested in 5 ml yeast extract mannitol (YEM) broth medium inoculated with 125 µl bacterial suspension of cell density (106 CFU/ml) and the growth was measured spectrophotometrically at 550 nm after 48 h incubation in rotary shaker (150 rev./min.) at 28°C. The results were presented as relative growth rate of control tubes. Results showed that the sensitivity of investigated Rhizobium strains to the fungicides were in agreement accordance with those found earlier. Comparing the results obtained from agar and liquid cultures, it was found that the effect of fungicides on the growth of the tested Rhizobium strains at each concentration were similar in both cultures. The degree of growth inhibition zone had the same tendency to the investigated concentrations of each fungicide similar to the degree of turbidity obtained by spectrophotometry. Strain of Lóbab Z (isolated in Hungary) however, proved to have the high tolerance to most of tested fungicides followed by Bükköny 75/4 (Hungarian Strain), HB-3841 (isolated in Libya) and E1012 strain (U.K. origin) was the most sensitive strain. When the fungicides were compared, Zineb-80, Baytan, Agrocit, and Maneb-80 were the least toxic fungicides, but Orthocide was the most toxic one, followed by Dithane DG, PangenR, and Captan. Cober, Ortho-phaltan, Plantvax, Mycozol, Cobox, Thiram, Terreneb, Benlate, Brassicol, Dithane M-45, Ipam, Dithane FL, were moderately affecting the survival and growth of the rhizobial strains. MIC was also calculated. Finally, the degree of fungicide toxicity is associated with the Rhizobium strain. More investigation can be done with different rhizobial species.
Published Date: 2020-07-10; Received Date: 2020-06-17