Sensitivity of some nitrogen fixers and the target pest Fusarium oxysporum to fungicide thiram
This study was carried out to investigate the toxic effects of the fungicide thiram (TMTD) against five nitrogen fixers and the thiram target pest Fusarium oxysporum under laboratory conditions. Nitrogen fixing bacteria Falvobacterium showed the highest values of LD50 and proved to be the most resistant to the fungicide followed by Fusarium oxysporum, while Pseudomonas aurentiaca was the most affected microorganism. LD50 values for these microorganisms were in 2-5 orders of magnitude lower in comparison with LD50 value for Fusarium oxysporum. Thiram was most toxic to Pseudomonas aurentiaca followed by Azospirillum. The lowest toxicity index was recorded for Fusarium oxysporum and Flavobacterium. The slope of the curve for Azomonas, Fusarium oxysporum and Flavobacterium is more steep than that of the other curves, suggesting that even a slight increase of the dose of the fungicide can cause a very strong negative effect. Thiram was more selective to Pseudomonas aurentiaca followed by Azospirillum, Rhizobium meliloti and Azomonas. The lowest selectivity index of the fungicide was recorded for Falvobacterium followed by Fusarium oxysporum. The highest safety coefficient of the fungicide was assigned for Flavobacterium, while Pseudomonas aurentiaca showed the lowest value.
Six laboratory experiments were carried out to investigate the effect of the fungicide Benomyl on pure cultures of some plant growth promoting bacteria (PGPB) and some fungi. The highest LD50 was recorded for Bacillus circulans and proved to be the most resistant to the fungicide, followed by Azospirillum braziliense, while Penicillium sp. was the most affected microorganism. LD50 values for the affected microorganisms were in 21-240 orders of magnitude lower in comparison with the LD50 value for Azospirillum braziliense. The results indicate a strong selectivity for Benomyl against Rhizobium meliloti and Penicillium sp. when compared to other microorganisms tested. The highest safety coefficient was recorded for Bacillus circulans followed by Azospirillum braziliense, while Rhizobium meliloti, showed the lowest safety coefficient value compared to other bacteria. The lowest toxicity index was recorded for Bacillus circulans and Azospirillum braziliense. The slope of the curves for Bacillus sp. and Rhizobium meliloti was steeper than that of the other curves, suggesting that even a slight increase of the dose of the fungicide can cause a very strong negative effect. In conclusion, Benomyl could be applied without restriction when using inocula based on growth promoting bacteria such as symbiotic nitrogen fixers (Rhizobium meliloti), non-symbiotic nitrogen fixers (Azospirillum braziliense) or potassium solibilizers (Bacillus circulans), given that the fungicide is applied within the range of the recommended field dose.