Effect of the nursery application of bioorganic fertilizer on spatial distribution of Fusarium oxysporum f. sp. niveum and its antagonistic bacterium in the rhizosphere of watermelon

Abstract

We previously developed a method for the application of bioorganic fertilizer to improve the biocontrol efficacy of Fusarium wilt (Ling et al., 2010). However, the detailed behavior of Fusarium oxysporum f. sp. niveum (FON) and its antagonistic bacterium, Paenibacillus polymyxa SQR-21 (SQR-21), in the rhizosphere of watermelon after application of the bioorganic fertilizer had not been elucidated. A rhizobox experiment was carried out to investigate the distribution of FON and its antagonistic bacterium, SQR-21, in the soil at different distances from the watermelon rhizoplane by real-time PCR. The results confirmed that the bioorganic fertilizer (BIO) significantly suppressed Fusarium wilt and promoted the growth of the watermelon plants. Furthermore, we found that the application of BIO significantly enhanced the abundance of SQR-21 and reduced the population of FON in the soil near the rhizoplane as compared to the control. A maximal quantity (7.14log10copies/g soil) of P. polymyxa SQR-21 was obtained in the rhizosphere under the BIO treatment, and this number was decreased with increasing distance from the rhizoplane. The population of FON in the rhizosphere of the control was 2 orders of magnitude higher than under the BIO treatment, and with increasing distance from the rhizoplane, the FON abundance increased in the SQR-21 inoculated treatments and decreased in the treatments without SQR-21. In the soil far from the rhizoplane (bulk soil), both FON and SQR-21 reached a level that was not statistically different between the treatments, which might be attributed to the background level of the infested soil. The localization and pattern of colonization of SQR-21 on the root surface was examined in an in vitro experiment by following green fluorescent protein (gfp)-labeled SQR-21. The colonization of P. polymyxa SQR-21 was found to occur at any site on the roots, yet it was preferentially found in defined regions of the root tip and root bend. These results revealed that the BIO applied with the developed method suppressed Fusarium wilt by the promotion of SQR-21 colonization in the rhizosphere, which largely reduced the population of the pathogen. The findings also indicated that the method of nursery application of BIO satisfies the ecological requirements of the biocontrol products.