## 4 Discussion  
Arbuscular mycorrhiza is the most widespread type of mycorrhizal symbiosis, a mutualistic association between plants and fungi. AMF are found ubiquitously in soils wherever their plant hosts are available. In one of our previous surveys carried out during 2009 to 2011, we found AMF in rhizosphere soil of _S. milliorrhiza_ belonged to _Glomus_, _Gigaspora_, _Scutellospora_, and _Entrophospora_, and _Glomus_ was dominant. This is compatible in a certain degree to [PERSON]'s study on host specificity of AMF, which indicated _Glomus hoi_ consistently occupied a large proportion of root systems and out-performed the other fungi [37]. Along with continuous cropping, the colonization rate of AMF in rhizosphere soil of _S. milliorrhiza_ dropped markedly. In the soil continuously cropped with _S. milliorrhiza_ for one to two years, the colonization rate of AMF was more than 80%, but in that cropped for three years, the colonization rate was only 40% (unpublished data). In this study, AMF infection rate of control plants was 46%. On the other hand, in the 2009-2011 survey we also found that an increase in the number of years of continuous cropping was associated with an increased incidence of Fusarium wilt, which could reach 60-70% after continuous cropping for three years, a rate significantly higher than that following continuously cropping for one or two years (10-20%). The causative agent of Fusarium wilt in _S. milliorrhiza_ is _Fusarium oxysporum (F. oxysporum)_[16]. Fusarium wilt decreased the plant yield and reduced the effective components content [15]. _Glomus moseae_ inoculation can improve resistance against root and other pathogens. In tomato pre-inoculated with _G. moseae_ increased tolerance against either _Phytophthora nicotiana_ var_ parasitic_[38] or _Rhizoctonia solani_[39] was observed. In the present study, we evaluated the effects of AMF _G. moseae_ on continuous cropping of _S. milliorrhiza_. Several studies suggested that an early establishment of AMF symbiosis with the host plants reduced the time for the host plants to receive benefits from this mutualistic relationship to result in improved growth and reduced diseases [40; 41], and plants pre-inoculated with mycorrhiza performed better in the field than those inoculated simultaneously with a pathogen and AMF [40]. We thus decided to sow the seeds in AMF affected soils first, and then transplant healthy seedlings into the field when the AMF root colonization rate reached over 80%. We found in 50-day-old _S. milliorrhiza_ seedlings, the root colonization by _G. moseae_ reached approximately 96%; the seedlings were suitable for field transplantation. Our results showed inoculation with _G. moseae_ can improve the growth of continuously cultivated _S. milliorrhiza_. The inoculation significantly decreased dry seedlings caused by Fusarium wilt; the dried seedling rate in the control plants was 52.6%, while that in the inoculated plants was only a little more than 10%. It was reported by [PERSON] et al. that _G. moseae_ could control tomato soil borne diseases caused by _Fusarium oxysporum_ f.sp. _Iyocpersici_; root exudates played an important role in plant-microbe interactions in the rhizosphere, and citrate and chlorogenic acid could be identified as possible candidates for the reduction in _Fusarium_ spore germination [30]. Moreover, AMF also can inhibit the secretion of mycotoxin such as trichothecenes by _Fusarium_ species, which cause serious disease on infected plants. In _Fusarium sambucinium_ infected potato plants, _Glomus irregulare_ inhibited the growth of _F. sambucinium_ and significantly reduced the production of the trichothecene 4,15-diacetoxysicrenpol in roots and tubers, thus alleviating black dry rot of the potato tuber [42]. _G. moseae_ inoculation also improved biomass yield of continuously cropped _S. milliorrhiza_. The inoculated plants showed notable increase in shoot and root dry weight, crown width, stem diameter, plant height, and root length (\(p<0.05\)). This is in accord with many reports [43; 44].